A cup or two of Epsom salt in a bath supposedly relieves pain — specifically, muscle pain from over-exertion (delayed-onset muscle soreness), conditions like myofascial pain syndrome (“trigger points”) and fibromyalgia — and speeds healing1 from minor injuries such as muscle strains and tendinitis. It is touted by a few as being a good source of magnesium, better than taking pills; nearly everyone thinks it’s “detoxifying.”
Claims and recommendations of this nature can be found by the thousands online. Bags and cartons of Epsom salts are available at any drugstore. Why, I have a package right here.2 It says:
Dissolve desired amount (1–2 cups) of crystals in a hot bath to produce a mineral water treatment to aid in the relief of muscular aches and pains.
When I went to my super-duper advanced massage therapy college, my instructors suggested Epsom salt baths as a good thing to prescribe to our clients.
Mostly my colleagues and mentors just made vague references to “detoxifying” the muscles, perhaps “by osmosis.” Nothing more exact was ever discussed because, frankly, I am sure that not one person in the building could have even named the compound “magnesium sulfate.”3 Me included.
I have wondered ever since if there was anything to it.
I am strongly skeptical of all health-related claims involving “toxins,” mostly because people who toss that word around never seem to know which toxins they are talking about.4 I have thoroughly studied the subject of post-exercise muscle soreness — probably the leading cause of hot baths — only to discover that it’s basically been proven that there are no known remedies for it.5 And after a long, hot Epsom salt bath of my own one night — which had no apparent effect on my unusually sore muscles, as usual — I decided it was time for a proper reality check. I started studying this topic (and I’ve never really stopped).
Does an Epsom salt bath actually do anything? Does brining yourself like a turkey do any good? Can you pickle your pain away? Is there any plausible way that Epsom salts could have an effect on your sore muscle tissue, or on the healing of injuries?
Epsom salts are magnesium sulfate heptahydrate, usually shortened just to magnesium sulfate (note also the British spelling “sulphate”). It was originally obtained by boiling down mineral waters at Epsom, England. The magnesium specifically is often considered the active ingredient.
It is quaintly referred to in the plural — Epsom salts instead of Epsom salt — but it’s just one kind of salt, and other than tradition there’s no more reason to say “salts” than there is to say “please pass the table salts.” It’s just an odd affectation, and both the singular and plural form are fine.
Other than tradition there’s no more reason to say Epsom “salts” — plural — than there is to say “please pass the table salts.”
The chemical structure of Epsom salts … so that you know this is a serious article.
A search for scientific evidence concerning Epsom salt baths is disappointing. There are basically no studies of their effect on body pain. There weren’t when I first tackled this topic in the early 2000s, and there still aren’t any as of 2019. There may never be.
Folk remedies are often neglected by researchers, but not usually so completely. There are usually at least a few experiments testing popular remedies kicking around. Why wouldn’t the use of Epsom salts for muscle soreness be similarly blessed?
A crystal of magnesium sulfate heptahydrate — Epsom salt.
There is plenty of research relevant to other medical uses of Epsom salts.6 For instance, on my package of Epsom salts, instructions are also given for internal usage as a laxative — which does work78 and is actually FDA approved and probably the most common and generally known medical usage. Other (internal) uses of magnesium sulfate include the treatment of irregular heart rhythm, low blood magnesium,9 eclampsia,10 and severe tetanus.11
There are also some incredibly bogus and crazy (dangerous) uses of Epsom salts. For instance, naturopaths may use it to try to dissolve gallstones — which doesn’t work, and can cause serious poisoning.
But there appears to be simply nothing at all published about alleviating aches and pains by any means.
It’s not just researchers who’ve neglected it. For instance, Epsom salt baths do not even rate a mention in Home Remedies: Hydrotherapy, massage, charcoal, and other simple treatments, a large and credibly referenced compendium of traditional remedies assembled by a pair of doctors. They describe five other medicated baths — alkaline (soda) baths, starch baths, oatmeal baths, peroxide baths, and sulfur baths — for conditions ranging from poison ivy rashes to diabetic gangrene (!), but they never mention Epsom salt baths.
In the near perfect absence of directly relevant science, all we can do is speculate scientifically about the possible mechanisms of action. And that I will now do, at absurd length.
“(With) a grain of salt,” (or “a pinch of salt”) in modern English, is an idiom which means to view something with skepticism, or to not take it literally.
Regardless of whether Epsom salts baths might work, it’s important to understand that the words “detoxification” and “osmosis” — the ideas most commonly invoked to explain the point of Epsom salts baths — are hopelessly misleading and explain nothing.
A poison is literally any harmful substance, and even something safe in typical doses becomes a poison in overdose (so you can be poisoned by either by lots of water or a minuscule amount of lead). Toxins are technically poisons produced by living things, like venom or metabolic wastes, but informally the word is synonymous with poison.
There’s a staggering variety of poisons and toxins, but the two major categories that are probably what most regular people hope they can “detox” are pollutants and metabolic “wastes”:
Pollutants are things that get into us from the outside. The best specific candidates would be the persistent organic pollutants like pesticides, flame retardants, and polychlorinated biphenyls (PCBs, now banned, but formerly ubiquitous in many plastics). Lead is also an alarmingly common environmental poison (and much in the news lately). All of these are indeed found in our environment and our bodies, where they mostly get trapped in fat and otherwise sequestered. We definitely would like to get rid these, if only we could.
Metabolic “wastes” is a much murkier category, because most of them aren’t actually wastes at all. Cellular chemistry produces a lot of molecules, with many fates. Technically these are toxins because they are biologically produced and they would be harmful in abnormal concentrations… but they are normal products of biology, and so most of them are either safely excreted, or actually re-used and re-cycled. As in the rest of nature, not much in cellular chemistry is wasted. Lactic acid is the ultimate example: misunderstood for decades, even by many people who should know better, lactic acid is not a persistent waste product and you wouldn’t want to “flush” it or “suck” it out of your muscles even if you could.12 If the mythology and misconceptions about lactic acid are this bad, it’s unlikely that there’s some other metabolic waste that is a better candidate for something that is regularly poisoning us and needs to be purged.
Why does osmosis even come up for this topic? It seems to be the layman’s notion of how Epsom salts detoxify. I guess they visualize toxins being sucked out of the body through the skin. But even if those nasty toxins are in there and need out-sucking, that is not how osmosis works.
Many people get osmosis bass-ackwards: they believe it refers to the movement of things floating in water across a membrane, but that is wrong by definition. It’s actually the water itself that moves. Osmosis refers to the movement of water only across thin membranes, towards the side that is “thicker” with dissolved particles.14 Take it from the Osmosis Cats!show cats15
You can demonstrate this clearly by soaking a potato in salty water. The water is clearly “sucked” osmotically out of the potato’s cells: it loses its plumpness and goes limp. Poor little potato. It’s the water that’s moves around. (Or cats.)
And so, by definition, Epsom salts baths cannot suck the toxins out of anyone or anything (or suck magnesium ions into anyone or anything).
A reader spelled out a couple of other commonly paired ideas about how Epsom salts baths might detoxify:
I always thought epsom salt baths were supposed to detoxify by opening your pores, and then the salts (ionized) help to supplement your body’s own detox processes (sweating toxins out through pores) by drawing the toxins out through the pores. It’s not osmosis, although the scientifically uninitiated might confuse it with such. It’s just toxins coming out through sweat pores, like they’re supposed to do, only with a little boost because of an ionic attraction.
The “scientifically uninitiated” might also think we sweat out toxins. This is one of the most obviously ridiculous of all ideas about detox. Sweating is for cooling, not taking out the trash — it’s not a significant excretory pathway.16
No one has ever cured anything but stress in a steam room. A sweat lodge has never saved anyone from any kind of poisoning. And even if sweat-to-detox was a thing, you’d still have to explain how cooler, non-sweaty salt soaks are supposed to detox.17
Ionic attraction is just some icing on this quackery cake. It’s an awful idea, most widely known from “ionic foot baths,” one of the of all snake oils.
Ionic bonding is electrostatic stickiness, the atomic scale equivalent of rubbing a balloon and then sticking it to your hair. It doesn’t reach out and grab things like a tractor beam, let alone across a multicellular structure like a sweat gland; it’s more like velcro, bonds forming only when ions actually touch each other. Even if sweat glands did squirt out toxins, ionic attraction can’t help out by reaching across the gland any more than a charged balloon is going to scoot all the way across a room to get to your hair. Sweat glands are a lot bigger than ions. To an ion, a gland might as well be a giant train station.
And now back to osmosis …
If Epsom salts get into the body, it’s definitely not by osmosis. Osmosis doesn’t work through the skin, because skin is almost perfectly waterproof. If it weren’t, you would dehydrate like an earthworm on a sunny sidewalk. (You do dehydrate significantly by sweating in a bath, of course20… but Epsom salts do not boost that (see last section).
The top layer of the skin, the stratum corneum, consists of dead, dry cells stuffed with a kind of embalming substance, keratin, a fibrous protein. Water can’t go through or around the keratin, thanks to a microscopic “uniquely structured fatty layer” between them, which no one knew existed until 2012.21 The presence and arrangement of lipids betwixt the keratin molecules results in “exceedingly low permeability.”22
Plus we have glands that coat the skin in waterproofing oils! So there’s that too. When those oils wash off, the dead skin cells can soak up a little water and swell a bit, like soaked beans. (Fun fact: that’s not the cause of skin pruning. Skin pruning is actually an active process for improving grip in wet conditions.23)
What is waterproof is osmosis-proof by definition. The skin is an effective barrier to diffusion of water molecules and therefore of osmosis. This is not to say that nothing gets past the skin, just not much, and definitely not water.
Skin: also a booze barrier
“Human skin has unique properties of which functioning as a physicochemical barrier is one of the most apparent. The human integument is able to resist the penetration of many molecules.”24
Alcohol molecules, for instance. You can’t get drunk through your skin, alas. Contrary to the Danish myth. As amusingly proven by Danish researchers in late 2010.25 It’s funny, but it’s not a joke.
The point is that it’s not exactly obvious to people what substances can or cannot get through the skin. Ask your friends: most of them will guess that some alcohol probably does get through the skin — maybe not enough to get drunk (or booze baths would be a more popular practice), but some. In fact, none gets across. Probably because alcohol molecules are just too dang big …
The skin is not a perfect barrier to all substances, which is obvious because of medicinal patches and creams, allergic reactions, and contact poisons. Some things do indeed get past that fibrous, fatty outer layer to interact with the living cells beneath, or even into the interstitial fluids and blood stream. How do they do it?
Size matters! If molecules are small enough, they can slip through the skin, like a small fish through a loose net. In 2000, Bos and Meinardi argued that a teensy enough molecule, smaller than 500 Daltons, can drift through the corneum26 — the 500-Dalton rule. And the magnesium ions in an Epsom salt bath are way smaller than 500 Daltons, at a barely-there atomic mass of just 24 Daltons.
I’m sure we can just stop there. There are probably no other obscure, technical chemistry considerations. The magnesium is small enough to get through, case closed.
Ha ha, just kidding!
Magnesium ions have some special properties that are highly relevant to their absorption. Bizarrely, they may swell dramatically when wet, like tapioca. In fact, this has been the conventional wisdom for some time, and one of the main reasons that many experts have dismissed the possibility of magnesium absorption.27
So … now is the case closed? Wet magnesium gets too fat for absorption? Still no! It turns out this rabbit hole goes way deeper than any rabbit would ever care to burrow. The conventional wisdom about Mg ion swelling has been challenged by some recent research. And I’m going to get into it, because whee, science is fun! But I’ve already gone far enough to make the really important point here: no one bloody knows how this actually works, and if you think you can guess whether or not magnesium ions get through the skin, please give your head a shake. Biology and chemistry is mind-bogglingly complex and the details are truly, madly, deeply non-guessable.
Science: Hey, can I have that skin off your belly? You don’t need it right?
Tummy Tuck Patient: Um, sure …
Science: Thanks! Got to find out if magnesium ions can get through that.
This section is all about one odd experiment28 that involves a number of strange rituals performed on skin samples, like gluing hair follicles shut with super glue. It almost answers the tricky scientific question of whether magnesium ions can be absorbed through the skin, but it still falls short.
It does convincingly show that magnesium ions can diffuse through the stratum corneum, and that hair follicles probably facilitate that movement, but it does not establish that they do so in clinically meaningful numbers, especially in the conditions of a typical Epsom salts bath. Regardless, it’s neato science.
- They tested the skin as-is, but for comparison they also tested skin that was damaged by applying tape and ripping it off, 30 times!
- And they tested skin with hair follicles that they actually plugged with tiny little microscopic dabs of super glue (the details are a bit technical, but basically that’s what they were up to).
- And they used a fluorescent dye that binds to magnesium ions, so they could track its progress through the skin.
Like I said: an odd experiment.
They tested 5, 15, and 60-minute exposures of two concentrations of magnesium solution, medium and strong, corresponding to ocean water and the Dead Sea respectively. (Note that these concentrations are much greater than the concentration of salt in a typical Epsom salt bath. Which matters.) Their key findings:
- “topically applied magnesium permeates through human stratum corneum”
- “magnesium permeability varies based on concentration and time of exposure”
- “hair follicles have significant contribution towards magnesium penetration through skin”
Magnesium ions diffusing through the stratum corneum. The brighter the warm-toned pixels here, the more magnesium.
So, voila, absorption! But there are some major caveats, of course.
- It’s clear from their imagery that a lot of Mg ions remain “stuck” in the most superficial layer of the stratum corneum, like hair in a drain filter.
- Even at concentrations much higher than anyone would normally put in their bath, “a significant increase was only observed at 60 minutes.” Almost nobody is taking super salty hour-long baths (except at “float spas,” more on this below). At 15 minutes, the medium concentration exposure is barely different from none at all.
- Although they claim a “significant” contribution of hair follicles, this is an irritatingly poor word choice. They mean statistically significant, not “important” or “large”… and it’s right on the very margin even of statistical significance (p <0.05).
I got into the details of this science not because it proves that magnesium soaks through skin, but because it’s charmingly weird, and because it proves that the whole problem is so absurdly complicated that we absolutely cannot guess the solution. Not even an extremely educated guess. Walter White couldn’t guess it. There are just too many ways the messy details of biology might surprise us. The only way to actually find out is to set aside the speculation about what’s possible in principle, and just do some proper before-after testing of magnesium absorption, in actual medicinal situations …
This quote from a book by Dr. Kenneth B. Singleton30 was submitted to me by a reader as an allegedly “authoritative” opinion on absorption:
Regularly bathing in hot water to which Epsom salts have been added can help draw out toxins from the skin.
Actually, that is not an authoritative opinion: it’s a vague and silly one. That anyone would mistake it for authoritative is rather depressing.31 The only thing that can determine whether magnesium heptahydrate is absorbed from a bath is careful, thorough testing — opinion is irrelevant, even from a real expert.
Fortunately, not all my mail is so depressingly gullible. Hat tip to reader Bryan B. who found an interesting Israeli military study and sent it to me. (I love it when readers do that.) It’s a safety study of a lotion developed “to improve protection against chemical warfare agents.”32 Like suntan lotion, but for chemical burns. Yikes.
This lotion had rather a lot of magnesium in it. And soldiers were not poisoned by the magnesium. Indeed, it didn’t appear to cross the skin at all: “there were no significant differences in magnesium levels between the placebo and the study groups in any of the applications.” The delivery system — lotion — could be quite different than soaking in water with dissolved magnesium sulfate. But it is pretty noteworthy evidence that absorption is minimal or nil when putting high concentrations of Mg on the skin.
That information is not necessarily correct, but it is certainly more authoritative and worth bearing in mind, than the opinion of Dr. Singleton.
In 2006, Rosemary Waring, a British biochemist at the University of Birmingham, did a nice science experiment with Epsom salts.33 She did more or less exactly what any curious person would do if she wanted to know whether or not Epsom salts can get past skin: she measured magnesium and sulfate in the blood and urine both before and after people bathed in Epsom salts.
She found them to be higher after the baths! 16 out of 19 people had more magnesium and sulfate in their blood after the baths than they did before the baths.34 Fascinating!
Dr. Waring’s results are straightforward. No therapeutic effects of Epsom salt were studied or claimed — she just studied absorption, and did not try to make any more of it, showing the restraint of a pro. What could be simpler?
I was so interested in these results (although still a bit skeptical) that I contacted Dr. Waring by email. “I agree that it is a bit surprising,” she replied, “but the results are certainly there and in fact there are hints in the past literature that this could happen.”
And how would it work, I asked, this crossing of the skin? Dr. Waring:
I don’t have any evidence as to how magnesium sulphate crosses the skin, though I have always assumed that it simply diffuses across the stratum corneum, helped by the fact that it’s in a hot bath.
Possibly not the best science ever done
Dr. Waring’s study has been
cited thrown in my face by countless people over the years who read this article only just far enough to get angry enough about my skepticism to send an email. None of them read far enough to see her name in a heading, her photo, or the full section of analysis. And, shocker, none of seemed to be aware of the potential problems with Dr. Waring’s research. And there are some.
Unfortunately thirteen years later, her tiny experiment has never actually been published,35 and that’s a major reason for caution.36 It is a basic rule of science that evidence can’t really be taken too seriously until it has been exposed to peer review and repeated by other scientists. Just because experimental results haven’t been replicated yet doesn’t mean we ignore them, but it does mean that we have to take them with a grain of salt. (That pun was simply unavoidable.) Also, Dr. Waring also has a relevant bias: she’s interested in magnesium supplementation as an autism treatment.37
I also think it’s notable that Dr. Waring’s speculation about mechanism is extremely basic. People assume because of her unpublished study that she’s a big time salt absorption expert, but obviously she wasn’t considering any of the chemical complexities discussed above … or below. She assumed, like most people, that the heat of a bath probably increases the permeability of the skin. But that’s not a safe (or expert) assumption.
Enough of it sure does. But probably not bath heat. Speaking of studies that get thrown in my face, someone haughtily hurled this one at me as if it was the last word, absolute proof that a hot bath boosts Mg absorption. Let’s look …
A 2008 experiment study showed that brief, intense heating of the skin can dramatically increase its permeability.38 Park et al tested short burts of heat: 5 milliseconds to 5 seconds at 100 to 315 degrees Celcius. With more heat, dramatically more molecules could cross (the duration of exposure had less effect). Skin permeability was increased by a few multiples in the low end of the range, all the way up to three orders of magnitude at the most extreme temperatures. Wow.
The mechanism is fascinating:
Is any of this applicable to baths and the absorption of magnesium sulfate? Maybe, but it’s unlikely.
The highest comfortable bath temperature for most humans is around just 40˚C (104˚F), which is less than half the lowest temperature studied in this experiment. The effect studied mostly depends on actually damaging the skin. It is conceivable that permeability starts increasing at lower temperatures with longer exposures … but sixty degrees lower? For the duration of a bath? Probably not for most substances. Skin probably evolved to be a good barrier across the range of temperatures humans are exposed to, which would certainly include 40˚C.
Also, not all substances will respond the same way to heat. If you studied transdermal delivery of many different substances at 100˚, you would probably see a wide range of effects. The only way to know if the skin is more permeable to magnesium at 100˚ or 40˚than at room temperature (as with a cream) is to check — and no one has, to the best of my knowledge.
And so this study definitely does not actually show that the heat of a bath enhances magnesium absorption … and neither does any other study I’m aware of.
How else could magnesium sulfate possibly get into the bloodstream? If it does, as Dr. Waring’s experiment seemed to show? Reader Adrian J. had an unusual idea:
Is it possible that the salt diffuses across the epithelium in the anus if the rectum relaxes to some degree in the warm water?
Wow, that’s some awesome lateral thinking! And I think it’s actually plausible, because now we’re talking about a “mucus membrane,” a completely different and much more permeable layer than skin. For what it’s worth, we know that alcohol absorbs quite handily through the rectum — rather too well, in fact, so do not try at home.39 But it has to be pretty much injected. (Live a little: click that footnote!)
But I find myself uncomfortably wondering … just how much do I relax in a hot bath? That much? And how much salt could diffuse across that more permeable but much much smaller membrane? It’s a tiny target!
I shudder to think of the methods required to test this.
The up-yer-bum hypothesis was surprising, but you’d be even more surprised by how many readers have asked if the vagina might be an absorption route. A fair question, but this has the same problem as anal absorption: too small and too tight. After quizzing several bemused femme-friends about it, I am confident that it would be highly irregular for any respectable quantity of bath water to percolate into one’s ya-ya.
And you thought an article about salt baths would be boring! No wonder this the most popular Epsom salts analysis on the internet!
How about inhalation? Another possibility for mucus membrane absorption
Maybe salt can be inhaled with steam. There’s actually a therapy (“halotherapy”) based on this, but it’s a weak idea in general and impossible as a salt delivery system: when water evaporates, it leaves most solutes behind, salt in particular.40 Many substances evaporate with water and “contaminate” steam — lots of volatile compounds and assorted tiny particles basically just getting thrown around, which is why we can smell a bubbling pot of soup — but these occur only in trace amounts, mostly nowhere near enough to be plausibly medicinal. Human olfaction, despite being shabby by animal kingdom standards, can still get a nice rich scent from a mind-bogglingly small number of molecules. Water from a soup is still remarkably pure despite the odour, and definitely has no salt in it.
Another related possibility is that we might inhale tiny droplets of water (aerosols of salt water) that float over the surface of a bath. Such droplets would contain dissolved salts at the same concentration as the bath, but these are nearly microscopic tiny water droplets. Remember that most people can swallow an entire magnesium pill with no obvious effect, which contains insanely more magnesium than you could ever absorb from the air over an Epsom salt bath, assuming there’s any at all, most of which would never even come close to a mucus membrane. Again, not really a plausible source of medicinal absorption.
The Epsom Salt Council claims, without detail or nuance, that “magnesium sulfate is absorbed through your skin,” citing the opinion of the notorious Dr. Christine Northrup to support the point, without so much as a link to substantiate that this is in fact her opinion. But it probably is: Dr. Northrup has earned extensive, harsh criticism for the way she “famously pushes woo in the cause of women’s health.” We’re talking about someone who believes in chakras, astrology, angels, mysticism, feng shui, and Tarot cards … and magnesium absorption. The same page is littered with similar examples of shallow reporting and appeals to dubious “authority.” For instance, further along, they also cite Carolyn Dean on this topic … a former MD who had her Canadian license to practice medicine revoked. And so on.
That’s the kind of quality scholarship The Epsom Salt Council has brought to the conversation!
I’ve presented several lines of evidence, demonstrating that the absorption question is surprisingly complicated and interesting … and inconclusive. We will have to live with the mystery.
Meanwhile, it is obviously reasonable to be skeptical, as many experts are. A thorough 2017 scientific review of both the evidence and rationale for transdermal absorption of magnesium41 makes a critical point: although there may now be adequate evidence to suggest that transdermal evidence is possible in the right conditions, that evidence is not nearly strong enough to support claims that it is superior to oral supplementation. Gröber et al conclude that they “cannot yet recommend the application of transdermal magnesium.”
It’s not just a matter of whether Epsom salts can be absorbed … it also has to be a better way of getting magnesium into the body than simply swallowing it. And that’s a much higher bar to clear.
And then there’s the question of whether or not it ever matters, which finally brings us to the second major part of the article …
If Epsom salts do get across the skin, so what? Is it any good to have some extra ions of magnesium and sulfate kicking around your bloodstream? Why did the ions cross the skin anyway? What’s the point?
Magnesium deficiency may be a real problem, and it may also be related to pain, supplementation might make sense, and soaking in the stuff could be a way of getting some magnesium… but probably not as good as just eating it.
To a much lesser degree, and much less clear, sulphate deficiency could also be thing, but it’s a much more obscure topic (very little science).
There is little doubt that magnesium sulfate probably has some effects on physiology in some contexts. Several of those effects are well known, including a few common medical applications mentioned earlier. There are also unpleasant effects, like diarrhea. But, judging from the established medical uses/effects of Epsom salt, there is definitely no particular reason so far to believe that having more magnesium or sulfate in your blood is going to be much use to you — unless you have eclampsia, tetanus, or a confirmed magnesium deficiency.
I’m aware of only one science experiment that seems to be directly relevant to the effect of more magnesium in the bloodsteam on pain. It was a study of the acute effects of injected magnesium sulfate, which is presumably very similar to absorbing it, just faster. The injections “did not reduce muscle pain” and caused “unpleasant side effects.”42 Yuck! Not exactly encouraging!
So there’s not really any particular reason to believe anything about the therapeutic effects of Epsom salts for aches and pain. We can really only speculate. And speculating about basic biology is really difficult. It’s a great way to be wrong.
Magnesium is a mainstream electrolyte, a famous molecule, and many people are supplementing magnesiun, for better or worse. But sulfate is obscure, and sulfate supplementation is quite rare. The magnesium information below is resting on fairly firm foundations, plenty of science; sulfate is an information desert by comparison. You can find a few popular articles about sulfate deficiency… most of which actually have very little to say about sulfate and quickly veer off into magnesium. There’s almost no science at all about sulfate deficiency.
I asked Dr. Waring to speculate about the therapeutic effects of Epsom salt, and she asserted that patients with rheumatoid arthritis are known to have “low sulfate levels,” but this is not really established science.43
And that’s really all I have — all anyone has — on the topic of sulfate supplementation. For the rest of the article, I’ll just be focusing on magnesium, which is on much firmer ground conceptually…
Magnesium is one of the essential minerals. And we use a lot of it: it’s one of the big three, along with sodium and potassium. If it’s missing, blood pressure rises, glucose tolerance drops, and our nerves get just a wee bit trigger happy. Fixing a deficiency is actually sedative! Which could certainly be relevant to chronic pain.44
Despite its biological importance, deficiencies may be common and related to pain. It’s in roughly the same category as vitamin D deficiency and pain: it might matter to a lot of people, especially pain patients, and supplementing is quite safe.45 In the generally scammy world of supplements and nutraceuticals, vitamin D and magnesium both stand out as being less lame (especially for chronic pain patients).
The other big idea about magnesium is that it prevents cramps. This idea is mostly inspired by the myth that cramps are caused by dehydration and a shortage of electrolytes like Mg46 — they aren’t, and so adding Mg probably doesn’t prevent cramps.47
Availability: magnesium is easy to get
Regardless of its biological effects, magnesium supplementation probably isn’t necessary: just eat right! Unlike with vitamin D, which is a bit tricky for a lot of people to get, it’s downright easy to get adequate magnesium from a reasonably healthy diet. Despite this, Mg deficiency is common even in wealthy places because grains are a poor source, and good sources are not nearly as popular (salad, basically — leafy greens and nuts are especially good). But
So while some people might need it, and it might be related to pain, it’s easy to fix with food and even easier to fix by taking pills. There’s truly no need to bathe in the stuff. But I’ll get into it anyway, because that’s what this is all about.
For Epsom salts baths to help common aches and pains because of magnesium, at least three things need to be established:
- Magnesium deficiency has to actually be linked to a common aches and pains — not more serious/chronic pain.48
- Magnesium supplementation has to actually be an effective treatment. Even if a lack of the stuff is a problem, simply oversupplying the system will not necessarily help.49
- An Epsom salts bath specifically has to be actually better than dietary sources or oral supplementation — otherwise, just eat the stuff!
None of these things has actually been established. The absence of any one of them is a deal-breaker. But the last one is the most obvious: it’s extremely unlikely that bathing is the best way to supplement magnesium, especially when there are such obviously viable, convenient alternatives. The increased levels of magnesium ions shown by Dr. Waring’s experiment — the most absorption-friendly evidence available — are small, about a 10% increase on average (and none at all in some subjects, remember). Consistent dosing would just be impossible.
This is a classic problem with all kinds of supposedly amazing pain cures: pain has too many different causes for one medicine to be really effective.
There are many types of muscle and joint pain that have little or nothing at all in common with each other physiologically. For instance, the pain of fibromyalgia originates in dysfunction of the central nervous system, which is completely different from the pain caused by exercise, which in turn is completely different than the physiology of trigger points. Even “basic” muscle pain is incredibly complex and has many flavours.50
While it’s certainly conceivable that increasing levels of magnesium ions in the bloodstream could help with some pain problems, it’s extremely unlikely that it would help enough different sorts of pain to be generally “good for” pain, especially the kind of acute soreness that we all occasionally suffer from.
This is an important logical problem: in principle, nothing can possibly help all types of pain, or even more than a couple of them.
Similarly, Epsom salts probably cannot simultaneously perform the two tricks most often touted: “relieve pain” and “speed healing.” Those are completely different things.
They might even be mutually exclusive. For instance, the primary source of injury pain is inflammation — a complex and painful physiological process intended to … wait for it … speed healing. Indeed, the only known mechanism by which you could recover faster from an injury would be to increase inflammation. If bathing in Epsom salts did that, it would make you hurt more, not less. Of course, there could be other ways to speed up healing — in an “anything’s possible” kind of way — but it’s still pretty far-fetched that a single molecule could pull off both that miracle and reduce pain at the same time.
The point here is just that the conventional wisdom is pretty murky.
Good for “brain troubles”
Salt has been used for well, just about everything. Like these effervescent brain salts. Cory Doctorow: “The best thing about effervescent brain salt is that it’s not immediately clear whether it’s salt to make effervescent brains even more delicious, or salt to give you an effervescent brain, or effervescent salt for brains. Also, it appears to come in a Tabasco bottle & everything that comes in a Tabasco bottle is always awesome.”
Once in a blue moon, you’ll see Epsom salts (or magnesium in particular) described as a “calcium channel blocker” with the implication that this is obviously good for pain. Unsurprisingly, this is another misleading oversimplification. Although it’s more specific and impressive sounding, it’s not a heck of a lot more meaningful than “detoxification.”
Calcium channels are itsy bitsy — molecular scale51 — holes in cell walls that let calcium in and out as a trigger for a bunch of biochemical business. They exist primarily in muscle tissue (including the heart), blood vessels, and neurons. There are a number of druggy ways to interfere with them, including magnesium. Calcium channel blockage is a reasonably well understood bit of physiology, and the main clinical usage of calcium channel blockers is to decrease blood pressure by reducing the strength of muscle contraction in the heart and blood vessels. Although other effects undoubtedly exist, there is no particular reason to believe that they have any potent effect on any flavour of pain.
Lots of people are walking around with calcium blockers in their blood. Calcium blockers aren’t rare drugs. Since there are numerous drugs that block calcium channels in various ways, it’s a bit implausible that there would be some kind of powerful pain-killing effect that no one’s noticed.
Yes, it is possible that magnesium absorbed through the skin does something different, something good, for certain kinds of pain. After all, different calcium blocker drugs have different effects! But there’s not a shred of good, direct evidence of it. So it really boggles the mind that anyone would toss this idea around with any confidence. Seriously, they’re pretty much making it up as they go — wild speculation.
There is some limited evidence that magnesium may reduce pain, perhaps because it is a “calcium channel blocker and N-methyl-D-aspartate antagonist,” as in a 2009 experiment.52 However, this is uncertain science. Several studies have been done, with conflicting results. Most were reviewed in 2007:53 four showed a positive effect, seven showed no effect greater than a placebo, and in one experiment the subjects actually experienced more pain (ouch).
And so, although “the biological basis for its [magnesium’s] potential antinociceptive effect is promising,” the authors actually concluded that no pain-killing effect could be found. So much for the miracle of calcium channel blockage: it fails the “impress me” test. It seems unlikely that magnesium would fail to relieve pain in those tests, and yet somehow succeed when absorbed from Epsom salts baths.
Clearly, this mystery is not solved yet. While there is a plausible mechanism for magnesium ions reducing pain, it is clearly neither well understood nor reliable. Do you suppose the picture is any clearer for Epsom salts in your bath? Don’t bet on it!
Reader Dorrie B. pointed out something interesting: Epsom salts might be an effective treatment for topical skin infections, as salt is certainly inhospitable to many microganisms. It can suck them dry, like the potato example I just mentioned. This may be the explanation for some rare prescribing practices I’ve heard about, such as recommending Epsom salts baths for anal fissures (yes, my email inbox has anal fissures in it).
An Epsom salt bath definitely cannot disinfect a puncture wound (as one of my readers was told). A strong salt solution is anti-bacterial, but the problem with rusty nails is the risk of deep injection of Clostridium tetani — beyond the reach of any soak.54
But this is also a great example of how complex these questions can be, because salt bathing might also damage populations of other bacteria on the skin, and/or make the skin more habitable for bacteria (moister), resulting in higher vulnerability to infection.55 Do people who bathe or swim in salt water regularly suffer any ill effects? Are they more susceptible to new infections? They might well be: even a 10% or 20% difference would not be obvious to the victims, but would nevertheless be clinically significant and biologically interesting.
Likely that research hasn’t been done, but my point is just that it’s really surprisingly difficult to say whether or not a given biological effect is “good” — it’s almost never that simple, and it’s a good thing to bear in mind throughout this article.
Obviously non-salty baths have some benefits of their own. Epsom salts routinely get the credit for these benefits. It goes like this:
- Patient has a problem and tries non-salty hot baths or soaking. However, because it’s just a bath and expectations are low, this effort is never particular diligent. This is key to the setup: the patient has never really given non-salty soaking a good try.
- Patient gets the idea to try Epsom salts! This seems much more promising.
- Thus inspired, the patient proceeds to soak quite diligently — much more diligently than ever before.
- When some benefit is then observed, patient attributes this to the salt — of course. Maybe it is, but maybe it’s just the unusual regularity of the nice soaking. The point is that we obviously can’t know … but the patient is now officially biased.
- If the benefits are at all notable, this person will usually start proclaiming to anyone who will listen that they "know" that Epsom salts work.
- When challenged (“It might be just the hot bath, eh?”), they will almost certainly object and claim (correctly!) that they have tried simple hot soaking without results. They have indeed. But it was never actually tried well enough to really know.
Tricksy, the human mind is.
Is there any other reason to put Epsom salts in your bath? Well, Epsom salts dissolved in your bath does make the water feel nice. 😉 No research is required to prove that: just try it! Most people agree that the water feels smoother, slicker, silkier.
And then there’s the bouyancy, which is the basis of an industry.
Salt it makes you floatier! Infinitesimally floatier. High concentrations of Epsom salt in your bath will increase the water’s specific gravity (density) to the point where you will start to float — just like in the Dead Sea, or Utah’s Great Salt Lake — because the body is, on average, much less dense than salty water. The concentrations of salt required for floatation therapy are much higher than Epsom salt packaging recommends, by the way.56 However, any salt in your bath — Epsom or otherwise — is going to make you at least a little bit lighter in the water.
Most people don't bathe in high concentration of Epsom salts for long periods — probably 99.9% of all Epsom salts baths are home baths with very low concentrations.
But that’s at home! You can also pay to bathe in much, much higher concentrations. Floatation baths and tanks are quite popular, and I have tried them and enjoyed them. Although the floating industry is not new — sensory deprivation tanks have been around for decades — it has definitely been surging. There are several new float spas here in Vancouver in the last couple years. In some places, the floatation industry is selling frequent and long salt baths, up to two hours of soaking and floating at a time. That much time is a luxury very few people can afford! Shorter floats are much more common.
It’s plausible that bathing in such high concentrations has different effects than home Epsom salt bathing, but no one is studying that.
The main purpose of floatation therapy is to reap the benefits of deep relaxation, which are noteworthy.57 It’s a lovely experience, but irrelevant to the relief of muscle aches and pains except via the straightforward (and perfectly legit) mechanism of relaxation.
For more detailed discussion of floatation and immersion therapies, see Get in the Pool for Pain: Aquatic therapy, aquajogging, water yoga, floating and other water-based treatment and injury rehab options.
98% of the time, “osmosis” and “detoxification” are the concepts presented as the justification for bathing in Epsom salts. Don’t trust advice that obviously simplistic. It involves some huge leaps of logic and detail avoidance … all made by people who are usually trying to sell the stuff. Consider the source.
Epsom salt bath prescriptions are often decorate with some really strange claims of healing powers. For instance, I found one website that recommended taking Epsom salts internally as well as bathing in them:
Researchers in nutrition, through controlled experimentation, have found that Magnesium sulfate accelerates the body’s healing time by 30%. As an example, if an injury required three weeks to heal under normal or standard conditions, it would only require two weeks to heal if Magnesium sulfate was added to the diet as a nutrition [sic].58
That’s really ludicrous. Accelerated healing time is a comic book concept — something Wolverine does — not an even remotely legitimate medical concept. And imagine the unpleasant surprise of the hapless reader who takes this advice when they discover the laxative effects of ingesting Epsom salts! Naturally, no source for this alleged experiment was given.
Epsom salts bathing is often recommended carelessly and overconfidently, without any genuine knowledge of the physiology or science (or lack thereof). Those who claim to “know” that Epsom salts work cannot seem to demonstrate that they also “know” much about physiology or science. While it certainly remains possible that there is a therapeutic effect, it’s pretty clear that we shouldn’t take their word for it.
Q. Why would the FDA allow studies to be published that show Epsom salts are more effective than their muscle relaxers and pain pills? For $1/lb of epsom salt this would kill their market and profits.
A. Industry protectionism for baths? Preposterous! The FDA has literally nothing to do magnesium sulfate research. They cannot regulate it. Any Big Pharma/FDA conspiracy against Epsom salt is clearly failing, because the stuff is available literally everywhere, and the FDA actually approves it for use as a laxative and a variety of external uses. Approval seems like kind of a funny way of implementing an anti-salt agenda. In short, this question is just knee-jerk anti-mainstream medicine paranoia based on major misunderstandings of what the FDA is and how it works. Nevertheless, it is a concern I’ve often see expressed, so I decided it was time to address it here.
Ironically, if there is any relevant commercial bias, it is one in favour of Epsom salts. For instance, the Epsom Salt Council exists to promote the industry and is “eager to let everyone know the benefits of our product and … spread the word about the wonder that is Epsom salt.” They prominently publish uncritical and unequivocal claims of medical benefit on their website.
Q. Ok so you don’t believe that Epsom salt will do anything in a bath. So how about sea salt? You believe it’s the same? Useless for muscle aches and stuff like that?
A. Yes, I believe it is “the same,” at least insofar as it is probably “useless muscle aches and stuff like that.” Of course there is much more chemistry going on in sea water than Epsom salts, but not in any way that seems to make any practical difference. In fact, it’s pretty clear that people who swim in the ocean a lot are not enjoying impressive pain-killing benefits the rest of us are missing out on — which is yet another example of how the skin is a pretty effective barrier.
Do not try at home
Oddly, there are a lot of electric baths in medical history. Electricity was a wonderful way to make health ideas based on vitalism seem more real & science-y.
Q. Maybe there’s a “bio-electric” function to Epsom salts in water. There’s more and more being discovered about small electric/magnetic fields and how we are affected by them.
A. Er, no, I think not. It’s not inconceivable, but it is pretty far-fetched. It’s generally true that biology ingeniously exploits most properties of nature to get things done, including electromagnetism, and we likely still have things to learn about that (that’s what the book The Body Electric was about, and despite its age and flaws it’s a darned interesting read). But whatever those systems might be, it’s super unlikely that they have any meaningful interaction with a slightly salty bath, let alone one that’s relevant to aches and pains. It’s even less likely that any such effect wouldn’t be much more obvious in, say, sea water. Even if salty baths just bestowed a vague feeling of well-being and vitality, like mountain air, that would be biologically remarkable … but still well short of a useful medical effect. And in fact salty baths do not have an obvious mountain-air like goodness.
Q. Why does the bag of epsom salts (and other random places online) warn that diabetics should not take baths with epsom salts?
A. That’s a bit of a stumper. As far as I can tell, it's just about the fact that diabetic feet are vulnerable, and soaking increases the risks of infection that they might miss due to impaired sensation. The foot has very thick and often damaged superficial layer of the skin (stratum corneum), which might make it a little more vulnerable to damage from soaking, but that’s any kind of foot soaking: I see no obvious reason for singling out Epsom salt soaks, except that it’s popular. Maybe diabetics, with their many foot problems, are somewhat more likely to want to soak their feet. Ironically, one of the few good medical benefits of Epsom salts may actually be infection control. It's a bit of a topical antibiotic.
Q. Is Epsom salt good for a hangover? Something to do with electrolytes …
A. Electrolytes are ions (like magnesium) in an electricty-conducting solution (which includes blood), and they have countless roles in our physiology. Getting drunk can deplete and/or disturb electrolytes (electrolyte chaos is a thing with alcoholics). So that’s the kernel of truth here, but there are many problems on the way from there to Epsom salt actually being “good for a hangover.”
Let’s start with the fact that it’s fairly unlikely that soaking in Epsom salts is actually an effective way to get some magnesium into you into you. Meanwhile, food is hands-down the best form of electrolyte supplementation, and water is the best overall treatment for hangover because most of the symptoms of a hangover are due to dehydration, not electrolyte depletion. And there’s the fact that magnesium is only one of several important electrolytes. And the biggest problem: with your garden variety hangover, there is some electrolyte imbalance due to the dehydration, but not major shortages of electrolytes. The mechanism for significant electrolyte depletion is a lot of vomitting, diarrhea, and sweating, which is only going to be applicable to the very worst of all hangovers. That’s a lot of problems!
But Epsom salt is mainly reckoned to be good for a hangover for the same simplistic reasons it’s touted for everything else: detoxification and relief from aches and pains. A hangover hurts, right? And you’ve basically poisoned yourself! Since the thinking about Epsom salts never even remotely attempts to get specific about what kinds of toxins or aches and pains, really any of them are candidates.
It doesn’t work. Believe me, I’ve tried. This is the kind of “medical” advice you’re going to get only from celebrities and amateurs.
Q. I am currently pregnant and looking into ways to treat the fluid retention. Many websites say that epsom salt baths will “draw out” the excess fluids but I can’t find any information about this being studied at all. I will probably try it tonight anyway, but as a scientist I would love to know if there is any basis to it.
A. This idea about Epsom salt bathing is relatively rare, but it also much more about the idea of osmosis than the more common detoxification claim. Any talk about the “drawing out” of water is definitely about osmosis (whether the speaker knows it or not). There’s a classic science classroom demonstration of osmosis, which involves sucking the fluids out of a potato by soaking it in salt water. That’s a perfect analogy for this claim. If a potato could be “bloated,” it could take an Epsom salts bath to solve the problem, and osmosis would be the mechanism.
Unfortunately, we are not like potatoes, and this simply will not work for bloated humans, because the skin is totally impermeable to water, as discussed thoroughly in the first third of this article. Nor is there any plausible pathway for osmosis across mucus membranes — unless you’re taking a very weird bath, there simply isn’t enough contact between any mucus membrane and bathwater.
I can do no better in defense of Epsom salt bathing for aches and pains than “anything is possible.” There is no good or specific reason to believe that bathing in dissolved Epsom salts will have the slightest effect on muscle soreness or injury recovery time. Although this folk wisdom may someday prove to have a sound rationale, clearly there is none that its advocates have thought of — or even tried to think of, it seems.
There’s even decent evidence that Epsom salts can’t even get past the skin barrier — Israeli soldiers can smear on magnesium rich cream without the slightest effect on their blood levels of magnesium. That’s pretty damning.
On the other side of the evidence, thanks to Dr. Waring, we know that it’s still possible that we are a living experiment in absorbing magnesium sulfate ions every time we bathe in dissolved Epsom salts! And maybe, just maybe, they do something worthwhile once they get past the skin. And it’s very cheap, and almost certainly safe — just as no one is obviously getting any miracle cures out of Epsom salt bathing, they aren’t suffering any obvious ill effects either.
So, why not? At the very least, they’ll make your bath feel silkier! And at most? Who knows — maybe those magnesium and sulfate ions do have some healing powers. It’s certainly not impossible. Just don’t buy into all the crap about osmosis and detoxification. As the old Scottish proverb says, “Always keep your mind open — but not so open that your brains fall out!”
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About Paul Ingraham
I am a science writer, former massage therapist, and I was the assistant editor at ScienceBasedMedicine.org for several years. I have had my share of injuries and pain challenges as a runner and ultimate player. My wife and I live in downtown Vancouver, Canada. See my full bio and qualifications, or my blog, Writerly. You might run into me on Facebook or Twitter.
Still need help with myofascial pain? Now that you probably won’t be counting on Epsom salts …
If you think this article is detailed, you should see my tutorial about muscle pain and myofascial pain syndrome! This kind of exhaustively researched writing about Epsom salts is only possible because I sell some of the other articles on this website. No writer can afford to create truly good, detailed content and then just give it all away: we have to make a living somehow. Please reward my efforts by taking a look at my tutorials. Although Epsom salts seem unlikely to be a significant source of relief, there are plenty of other options for self-treatment of muscle pain. PainScience.com publishes an extremely thorough tutorial about myofascial trigger points (muscle knots):
- PS Save Yourself from Muscle Strain! — Muscle strain (pulled muscle) and muscle pain explained and discussed in great detail, plus every imaginable treatment option
- PS Post-Exercise, Delayed-Onset Muscle Soreness — The biology & treatment of “muscle fever,” the deep muscle soreness that surges 24-48 hours after an unfamiliar workout intensity
- PS Hot Baths for Injury & Pain — Tips for getting the most benefit from a hot soak, the oldest form of therapy
- For fun, see this short, excellent example of a typically terrible popular article about Epsom salt: its title asks, “Quick Cures/Quack Cures: Is Epsom Worth Its Salt?” but there is not a critical thought anywhere to be found. The writer assumes that Epsom salts are "effective, affordable therapy," and then proceeds to try to "explain" it with a not-actually-a-quote from Dr. Waring. If Dr. Waring actually said that, she was speculating beyond the reach of science, but it's more likely the journalist simply paraphrased to make her speculation sound much more definite than it was. And of course the article parrots the osmosis error: "draws foreign bodies." No no no! That's not osmosis! Grade school chemistry fail!
Other interesting reading:
- Magnesium sulfate on Wikipedia.com.
Nineteen updates have been logged for this article since publication (2006). All PainScience.com updates are logged to show a long term commitment to quality, accuracy, and currency. more
When’s the last time you read a blog post and found a list of many changes made to that page since publication? Like good footnotes, this sets PainScience.com apart from other health websites and blogs. Although footnotes are more useful, the update logs are important. They are “fine print,” but more meaningful than most of the comments that most Internet pages waste pixels on.
I log any change to articles that might be of interest to a keen reader. Complete update logging of all noteworthy improvements to all articles started in 2016. Prior to that, I only logged major updates for the most popular and controversial articles.
See the What’s New? page for updates to all recent site updates.
This is one of the very oldest and most thoroughly updated articles on PainScience.com, still changing and improving over a decade after the original version was first written in late 2006. It has always been popular, and has always attracted lots of feedback, including some great questions that have inspired many corrections, improvements, and entertaining tangents. However, after the 2018 reboot, it will probably now yield to many other writing priorities and remain relatively stable for a long time.
May — More detail: Added more information about magnesium supplementation as it relates to muscle cramping. [Section: Magnesium supplementation.]
January — Upgrade: Added table of contents, the final step in a series of major updates in 2017 and 2018, addressing many problems and limitations noted over many years. It is now ready for another decade of being constantly read by thousands of people per week!
2018 — Major expansion: Added substantial coverage of the basics of magnesium and sulfate deficiency and supplementation. Speaking of deficiency, the absence of this information was a major oversight for many years. Despite it’s great size and thoroughness, this sub-topic was strangely missing, but that deficiency has now been largely addressed. [Section: The therapeutic value of Epsom salt: If magnesium and sulfate ions can get from bath to blood, what exactly do they do when they get there?]
2018 — New section: This new section starts the process of re-focussing the article on Epsom salts bathing as a form of magnesium supplementation. [Section: Epsom salts bathing as a method of magnesium supplementation.]
2018 — New sub-topic: Answered reader question about treating bloating, and added some information about the laughably low credibility of claims published by the Epsom Salt Council. [Section: Selected questions and answers.]
2018 — Major edit: Substantial, diverse editing of all sections about absorption. [Section: Absorption: Can Epsom salts actually get through your skin in the first place?]
2018 — New section: Comparing poisons and toxins and providing examples of pollutants and metabolic “wastes.” Important context! [Section: Toxins & Osmosis: What “toxins” do Epsom salts supposedly get rid of?]
2018 — New section: No notes. Just a new section. [Section: What about sweating? And ionic attraction?]
2017 — New section: Really fun new section based on Chandrasekaran et al. I laughed out loud repeatedly writing this one. Because what’s funnier than harvested human skin, amiright? [Section: Testing magnesium absorption on harvested human skin samples.]
2017 — New section: A new section based largely on on the findings of Park et al. [Section: Hot hot hot! Does heat increase skin permeability?]
2017 — Added depth: Expanded on discussion of inhalation as a vector for salt magnesium absorption. [Section: Maybe up your bum? Or hoo hoo? Nostrils? Absorption via mucus membranes.]
2017 — Minor correction: Correction about the size of water molecules, and some clarification about how skin waterproofing works thanks to some interesting science. [Section: So, what does get across the skin? Because obviously some things do.]
2016 — Minor clarification: Small but worthwhile clarifications of the 500 Dalton rule and the “Osmosis mistake” (which I continue to get too much ignorant email about). [Section: So, what does get across the skin? Because obviously some things do.]
2016 — New section: New section discussing the small size of magnesium ions and the 500 Dalton rule of absorption. This isn’t exactly a reversal of my position on absorption, because I was always officially and openly agnostic about it. However, I also clearly thought absorption was a priori implausible, and this update reverses that opinion, which is worth emphasizing. [Section: So, what does get across the skin? Because obviously some things do.]
2015 — Correction: Fixed an incorrect premise of a minor point: water vapour is pure by definition and cannot be salty, and therefore cannot be even a slightly plausible mechanism of delivery of salt to mucous membranes. [Section: Maybe up your bum? Or hoo hoo? Nostrils? Absorption via mucus membranes.]
2015 — Correction and improvement: Fixed incorrect information about skin pruning and added the actual (and much more interesting) explanation; cited to substantiate the impermeability of the stratum corneum. [Section: Skin is definitely waterproof (and therefore also osmosis-proof).]
2013 — Improvement: Added an explanation of why it is probably a bad idea to use Epsom salts as a replacement for a tetanus shot after a dirty puncture wound. [Section: Another possible biological effects: can salt disinfect? Osmosis and all the wee beasties.]
2012 — New section: Reported on evidence against the absorption of Epsom salt. [Section: Some evidence against absorption: Israeli soldiers smear magnesium all over themselves in high concentrations and it doesn’t get inside.]
Many earlier updates unlogged. No updates were logged at all for the first several years of this article’s long lifespan (2006–2011); a few updates were logged 2012 through 2016, and then consistently after that.
- “Relieves pain” and “speeds healing” are as different from each other as a flying dream is from actual flying. Actually speeding up healing is kind of a big deal. BACK TO TEXT
- Of course! Can’t very well debunk it without trying it, can I? I’ve had many Epsom salts baths! BACK TO TEXT
- I still can’t remember it reliably, because chemical names stick in my head about as well as my cousins’ birthdays. Ambush me with the question sometime: “What’s the chemical name for Epsom salts? Schnell, schnell!” I’ll be stumped as likely as not. BACK TO TEXT
The idea of “toxins” is usually used as a tactic to scare people into buying some kind of de-toxifying snake oil. Obviously there are dangerous substances; the problem is with the kind of people who toss the idea around, the reasons they do it (fear, profit, ignorance), and because toxin claims are usually so vague that they are literally meaningless, except as a marketing message. Indeed, “detoxification” may be the single most common marketing buzzword in alternative health care.
The body deals with undesirable molecules in many ways. It eliminates some and recycles others; some are trapped in a safe place; and quite a few can’t be safely handled at all (metals). Most alleged “detox” treatments are focused on stimulating an excretion pathway, like sweating in a sauna. But it’s not like sweating is broken and the sauna is fixing it! The only truly “detoxifying” treatments help the body eliminate or disarm molecules the body cannot process on its own. A stomach pump for someone with alcohol poisoning is literally “detoxifying.” So are chelation for heavy metals, and antivenoms.
I cover the specific idea of “flushing” toxins in Why Drink Water After Massage? (Massage is wonderful for all kinds of reasons — it doesn’t need the support of the idea that it detoxifies.) For more general consumer advocacy and education about toxins, see “Detoxification” Schemes and Scams (from QuackWatch.org).BACK TO TEXT
For more detail, see another article on PainScience.com, Post-Exercise, Delayed-Onset Muscle Soreness: The biology & treatment of “muscle fever,” the deep muscle soreness that surges 24-48 hours after an unfamiliar workout intensity. Basically what it boils down to is that the top 5 effective treatments for muscle pain after exercise are:
- maaaaybe massage, but probably not
- Swain R, Kaplan-Machlis B. Magnesium for the next millennium. South Med J. 1999 Nov;92(11):1040–1047. PubMed #10586828. ❐ See also the Wikipedia article magnesium sulfate (Wikipedia). BACK TO TEXT
- Izzo AA, Gaginella TS, Capasso F. The osmotic and intrinsic mechanisms of the pharmacological laxative action of oral high doses of magnesium sulphate. Importance of the release of digestive polypeptides and nitric oxide. Magnes Res. 1996 Jun;9(2):133–138. PubMed #8878010. ❐
“A common use for high doses of oral magnesium salts is to produce a laxative effect to treat constipation,” explain the authors of this scientific paper. “In the intestinal lumen the poorly absorbable magnesium ions (and other ions such as sulphate) exert an osmotic effect and cause water to be retained in the intestinal lumen.”BACK TO TEXT
- James LP, Nichols MH, King WD. A comparison of cathartics in pediatric ingestions. Pediatrics. 1995 Aug;96(2 Pt 1):235–238. PubMed #7630676. ❐
This paper compared the effectiveness of different laxatives, showing that Epsom salts do indeed move the bowels along … but not as quickly as sorbitol.BACK TO TEXT
- As occurs with chronic diarrhea, magnesium malabsorption, alcoholism, diuretic use and a few other disorders.
BACK TO TEXT
- Eclampsia is a dangerous and fairly common complication of pregnancy.
BACK TO TEXT
- Muscle spasms caused by bacterial infection with Clostridium tetani, which produces the neurotoxin tetanospasmin.
BACK TO TEXT
Lactic acid (lactate) is the poster boy for the “waste” metabolites, but it’s not a dead-end waste product at all: it’s actually a useful molecule with a productive metabolic fate, even re-use as a fuel, and it’s not the cause of muscle pain at any time except the immediate aftermath of intense exercise, and probably not even then. Research has shown that acute muscle fatigue — the “burn” — is probably caused by calcium physiology (see Bellinger, Fredsted, Wiltshire).
Many massage therapists still think lactate can be “flushed” by massage (which may actually backfire, see Wiltshire) If people needed massage to help them “clear” lactic acid, sprinters would drop like flies without emergency massage after every race.
Lactate as a “bad” molecule is one of the most persistent silly myths in all of exercise science. See Lactic Acid Is Not Muscles' Foe, It's FuelBACK TO TEXT
- I’m deliberately over-simplifying the definition of osmosis there, just for readability. Osmosis actually involves the movement of any solvent across a membrane. And water is a solvent, of course. I referred only to water in this context because, unless you bathe in turpentine, the only solvent in your bathing-osmosis equation is going to be good ol’ H2O. BACK TO TEXT
- Full, formal definition of osmosis: “Osmosis is the spontaneous net movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, in the direction that tends to equalize the solute concentrations on the two sides. It may also be used to describe a physical process in which any solvent moves across a selectively permeable membrane (permeable to the solvent, but not the solute) separating two solutions of different concentrations” BACK TO TEXT
- A reader asked: “Isn’t that cat diffusion?” Good question, because that would be the easier visual metaphor, and you could easily caption it that way. But this caption makes it clear that the cats here are representing solvent molecules (e.g. water molecules) and not solute (which isn’t represented by anything). So the cat is a water molecule “flowing” across the “membrane.” BACK TO TEXT
- Imbeault P, Ravanelli N, Chevrier J. Can POPs be substantially popped out through sweat? Environ Int. 2018 Feb;111:131–132. PubMed #29197670. ❐ Good quality reporting on this study from National Geographic: “Fact or Fiction: Can You Really Sweat Out Toxins?” BACK TO TEXT
- Early claims about the benefits of bathing in salty water significantly predate hot baths: they were about salty bodies of water like the Dead Sea, where sweating is not a factor. If heat/sweating was actually required for the mechanism of action in an Epsom salts bath, that would leave its antecedents in need of some other mechanism. And that would definitely be at odds with the principle of Occam’s razor. BACK TO TEXT
- Sweat is already very salty. Adding salty bathwater to salt sweat is not biologically interesting. BACK TO TEXT
- Nothing moves through a sweat gland freely: it’s not an open tunnel, but more like a jet engine, full of moving parts doing extremely specific things. The gland is doing metabolically expensive active transport, working hard to pump specific molecules out of body (mostly water). A bit of salt in solution on the outside of the gland is not going to have much impact on that. Sweat glands are gonna do what they do, regardless of what’s in solution on their doorstep. BACK TO TEXT
- If you weigh yourself before and after a bath or sauna, you may find a surprising 1-5 pound weight reduction. This fairly obvious effect is presumably due to fluid loss from sweating. The amount is impressive, considering that you may well have consumed fluid at the same time that you were getting rid of it through your sweat glands. However, it’s certainly consistent with the well-known hazard of fainting in that context, which every public hot tub has very clear warnings about. So I think it’s a reasonably safe assumption that we really do sweat a lot in a hot bath! BACK TO TEXT
- Iwai I, Han H, den Hollander L, et al. The human skin barrier is organized as stacked bilayers of fully extended ceramides with cholesterol molecules associated with the ceramide sphingoid moiety. J Invest Dermatol. 2012 Sep;132(9):2215–25. PubMed #22534876. ❐ This is a really cool paper; NewScientist.com has a nice plain English translation: “Strange fat explains skin’s waterproof properties.” BACK TO TEXT
- Potts RO, Francoeur ML. The influence of stratum corneum morphology on water permeability. J Invest Dermatol. 1991 Apr;96(4):495-9. PubMed #2007788. “The stratum corneum (SC) provides the barrier to water loss for the skin of mammals. A significant body of evidence now exists suggesting that extracellular SC lipids are primarily responsible for this barrier. … These results are interpreted in terms of the unique morphology of the SC, where lipids form an extracellular continuum that is highly tortuous. Thus, the exceedingly low permeability of the SC may be due, in large part, to its unique morphology.” BACK TO TEXT
- The stratum corneum swells, but only a little, and we know it’s not the cause of pruning because it doesn’t happen when the nerve supply to the fingers is damaged, which means it’s a neurologically regulated phenomenon. We’ve known this for a long time, but we didn’t have any good idea why until quite recently. This video and this article in Nature explain in detail. Basically, “wrinkly fingers improve our grip on wet or submerged objects, working to channel away the water like the rain treads in car tyres.” The mechanism is constriction of the blood vessels under the skin: as they constrict, they pull the skin with them. And testing has confirmed that this actually improves grip on wet surfaces, which “could have helped our ancestors to gather food from wet vegetation or streams.” I’ll be darned! BACK TO TEXT
- Bos JD, Meinardi MM. The 500 Dalton rule for the skin penetration of chemical compounds and drugs. Exp Dermatol. 2000 Jun;9(3):165–9. PubMed #10839713. ❐ BACK TO TEXT
- Hansen CS, Færch LH, Kristensen PL. Testing the validity of the Danish urban myth that alcohol can be absorbed through feet: open labelled self experimental study. BMJ. 2010;341:c6812. PubMed #21156749. ❐ PainSci #54850. ❐
Can you get drunk through your skin? In this MythBusters-style experiment, three adults were “tested” in the office of a Danish hospital: specifically, their feet were submerged in a bowl containting three 700 mL bottles of vodka. It’s hard to tell if the researchers are serious about this, but they obviously had fun doing it!
However, the subjects did not become intoxicated, and their blood alcohol levels did not change. They concluded: “Our results suggest that feet are impenetrable to the alcohol component of vodka. We therefore conclude that the Danish urban myth of being able to get drunk by submerging feet in alcoholic beverages is just that; a myth. The implications of the study are many though.”
Indeed.BACK TO TEXT
- Bos 2000, op. cit.
They argued it in three ways:
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- Virtually all common contact allergens are under 500 Daltons, larger molecules are not known as contact sensitizers. They cannot penetrate and thus cannot act as allergens in man.
- The most commonly used pharmacological agents applied in topical dermatotherapy are all under 500 Daltons.
- All known topical drugs used in transdermal drug-delivery systems are under 500 Daltons.
- Jahnen-Dechent W, Ketteler M. Magnesium basics. Clin Kidney J. 2012 Feb;5(Suppl 1):i3–i14. PubMed #26069819. ❐ PainSci #52761. ❐ Chandrasekaran et al regarding this paper: “The radius of the hydrated magnesium ion has been reported to be 400 times higher than its dehydrated form, leading to the assertion that it is almost impossible for magnesium ions to pass through biological membranes.” BACK TO TEXT
- Chandrasekaran NC, Sanchez WY, Mohammed YH, et al. Permeation of topically applied magnesium ions through human skin is facilitated by hair follicles. Magnes Res. 2016 Jun;29(2):35–42. PubMed #27624531. ❐ BACK TO TEXT
For context, I’ll repeat this quote from Chandrasekaran et al: “The radius of the hydrated magnesium ion has been reported to be 400 times higher than its dehydrated form [Jahnen-Dechent et al], leading to the assertion that it is almost impossible for magnesium ions to pass through biological membranes.”
“When we re-examined this calculation, it was found that the volume of the magnesium ion is 451.76 Å3 [4 (4.76)3] in the hydrated state, but based on the ionic radii of dehydrated and hydrated magnesium ions, i.e., 0.87 Å and 4.76 Å respectively [10-13], we calculated that the hydrated radius of the ion is only 5.47 fold (4.76Å) greater than its dehydrated radius. Based on our recalculation … we postulated that the hydrated magnesium ion could potentially penetrate by bulk diffusion through the 10 Å pores formed by protein subunits in the lipid membrane [5, 14], or by other means, such as hair follicles.BACK TO TEXT
- Singleton, Kenneth B. The Lyme Disease Solution. 2008. p396.
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- Medical training credentials are not remotely a guarantee of an intelligent opinion, and this demonstrates it beautifully. All it does is show the kinds of ridiculous things that get said about Epsom salts without a shred of evidence or even an intelligible biological rationale. BACK TO TEXT
- Eisenkraft A, Krivoy A, Vidan A, et al. Phase I study of a topical skin protectant against chemical warfare agents. Mil Med. 2009 Jan;174(1):47–52. PubMed #19216298. ❐ BACK TO TEXT
- Waring RH. Report on Absorption of magnesium sulfate (Epsom salts) across the skin. Unpublished. 2006. PainSci #56301. ❐ BACK TO TEXT
- The others had increased urine levels of magnesium, implying that “the magnesium ions had crossed the skin barrier and had been excreted via the kidney, presumably because the blood levels were already optimal.” In other words, whatever magnesium was absorbed into the bloodstream was promptly removed by the body. BACK TO TEXT
- When I asked Dr. Waring about publication, she explained “we just haven’t got around to it yet. I hope to do a bit more and then publish with my London colleague.” BACK TO TEXT
- There are any number of flaws in the experiment that we can’t know about. For example, one reader sent me this interesting point by e-mail: “Dr. Waring could not possibly have measured absolute MgSO4 blood content: the only way to do that would be through dialysis or post-mortem, only relative concentration. Any degree of dehydration would have an absolute effect on relative concentration; i.e., all solutes would show increased concentration due to dehydration, including any magnesium already present in the bloodstream. Since the study is unpublished, it's not possible to know whether the Dr controlled for this.” BACK TO TEXT
- Dr. Waring has done other research claiming to show that autism is correlated with magnesium deficiency, and her primary reason for studying Epsom salt absorption through the skin was to investigate it as a possible autism treatment. This is not a serious bias, but it’s worth acknowledging, because it could certainly mean that she was hopeful when she did the experiment that Epsom salts bathing would constitute a meaningful method of supplementation … and data and its intepretation tends to get warped by wishful thinking, which is precisely why studies need to be replicated, and why it’s more of a shame that this one hasn’t even been published. BACK TO TEXT
- Park JH, Lee JW, Kim YC, Prausnitz MR. The effect of heat on skin permeability. Int J Pharm. 2008 Jul;359(1-2):94–103. PubMed #18455889. ❐ PainSci #52769. ❐ BACK TO TEXT
See 2007 Darwin Awards: The Enema Within, in which a man died from an alcohol enema. “In order to qualify for a Darwin Award, a person must remove himself from the gene pool via an ‘astounding misapplication of judgment.’ Three litres of sherry up the butt can only be described as astounding.”
See also, if you dare, this real news item about a frat boy who almost killed himself “butt chugging” — getting drunk from alcohol injected into the butt. Seriously. “The only thing more embarrassing than almost dying from allegedly butt-chugging is hiring a lawyer to deny it.” No doubt.BACK TO TEXT
- Water vapour is mostly pure H2O by definition. There is such a thing as solutes that get “dissolved in steam” and carried along with it without technically “evaporating,” but that’s a fairly exotic phenomenon. Your standard salts neither evaporate nor get carried away with evaporated water. BACK TO TEXT
- Gröber U, Werner T, Vormann J, Kisters K. Myth or reality-transdermal magnesium? Nutrients. 2017 Jul;9(8). PubMed #28788060. ❐ PainSci #52771. ❐ BACK TO TEXT
- Chestnutt WN, Dundee JW. Failure of magnesium sulphate to prevent suxamethonium induced muscle pains. Anaesthesia. 1985 May;40(5):488–490. PubMed #4014628. ❐
Muscle pain is one of the side effects of suxamethonium chloride, an anaesthetic drug used to cause short-term paralysis. In this study, injecting magnesium sulphate had no benefit compared to doing nothing, and was “followed by unpleasant side effects.“Granted, the relevance of this kind of muscle pain to the more common kinds is unknown. But it is suggestive. BACK TO TEXT
Molecules produced by the inflamed tissues in these patients may interfere with the production of a protein that is used to produce sulfate from another molecule (cysteine), thus lowering sulfate levels. In Dr. Waring’s words:
“The cytokines released in the inflammatory state actually depress the expression of cysteine dioxygenase, the rate-determining step in the conversion of cysteine to inorganic sulfate. About 80% of the in vivo requirement of sulfate goes through this pathway as sulfate is not well-absorbed from the gut.”
However, low sulfate levels are a possible result of having rheumatoid arthritis, not a cause — and thus boosting them back up again will not necessarily solve anything. And even if it did, that’s a therapeutic effect that is very particular to rheumatoid arthritis — a serious, agonizing joint disease — which probably has little or nothing to do with the kinds of pain that most people put Epsom salt in their baths for.
It would be great if Epsom salt baths helped people with rheumatoid arthritis, but good evidence of that would, in a way, pretty much shoot down the other claims of therapeutic effect, which rely on completely different ideas about how and why Epsom salt might work. But, of course, there is as yet no evidence one way or the other.BACK TO TEXT
- Chronic pain often involves a phenomenon known as sensitization: more pain with less provocation. It is broadly plausible that any sedative effect could blunt sensitization. Potent sedative drugs (like benzodiazepines) have limited efficacy and often backfire and actually cause more pain. Addressing a magnesium deficiency might be a much safer, healthier method of “sedation.” And that’s pure speculation. BACK TO TEXT
- We absorb it on an as-needed basis, so excesses are mostly just pooped out — maybe with a little cramping and diarrhea. BACK TO TEXT
- Infamously for those who have followed performance science over the last 20 years or so, cramps are not caused by electrolyte loss or dehydration, acute or chronic. See Minetto and Schwellnus. What actually does cause them is still unclear (shocker): all that really is clear is that it happens in very fatigued muscles. For a more detailed discussion of this myth, see Cramps, Spasms, Tremors & Twitches. BACK TO TEXT
- Garrison SR, Allan GM, Sekhon RK, Musini VM, Khan KM. Magnesium for skeletal muscle cramps. Cochrane Database Syst Rev. 2012 Sep;(9):CD009402. PubMed #22972143. ❐
This 2012 review of a few studies concluded that “It is unlikely that magnesium supplementation provides clinically meaningful cramp prophylaxis to older adults experiencing skeletal muscle cramps.” Amazingly, the evidence for the effect of Mg on cramps caused by exertion, disease, and pregnancy is just inadequate. It’s not really very plausible, but you’d think someone would have studied exercise-induced cramping at least, given its relevance to elite athletics.BACK TO TEXT
- While it would be a big deal if we knew that magnesium deficiency was a factor in chronic or serious pain, it wouldn’t have anything to do with the legend of Epsom salts, which are supposedly good for acute and minor pain. BACK TO TEXT
- Not all deficiencies are caused by an inadequate supply. The classic example is malabsorption of iron as a cause of iron deficiency: it doesn't matter how much you consume if you can't absorb it. BACK TO TEXT
- Consider the seminal text, Muscle Pain: Understanding its nature, diagnosis and treatment. It has nine chapters devoted to nine different kinds of muscle pain. It also doesn’t mention Epsom salts. Not once. BACK TO TEXT
- Learn.genetics.utah.edu [Internet]. Cell Size and Scale; 2010 [cited 17 Jan 21].
A beautiful animated tool for visualizing the scale of cells.BACK TO TEXT
- Kogler J. The analgesic effect of magnesium sulfate in patients undergoing thoracotomy. Acta Clin Croat. 2009 Mar;48(1):19–26. PubMed #19623867. ❐ BACK TO TEXT
- Lysakowski C, Dumont L, Czarnetzki C, Tramèr MR. Magnesium as an adjuvant to postoperative analgesia: a systematic review of randomized trials. Anesth Analg. 2007 Jun;104(6):1532–9, table of contents. PubMed #17513654. ❐ The authors of the review concluded: “These trials do not provide convincing evidence that perioperative magnesium may have favorable effects on postoperative pain intensity and analgesic requirements.” BACK TO TEXT
- There are a lot of microorganisms that might be on a rusty nail, but CT is the scary common one that can kill you very unpleasantly (a chance of death by muscle spasm, arg). The reason you get a tetanus shot in that situation is that it is a very effective just-in-case prevention, good bang for buck. The idea that anyone would recommend an Epsom salt bath as a replacement for that is quite terrifying, a fine example of dangerous ignorance. While a very strong Epsom salt solution might kill every bacteria in a shallow wound, a deep wound can put bacteria into the bloodstream that leaves the site in seconds — completely inaccessible to any topical antibacterial solution. Ironically, as mentioned above, magnesium heptahydrate is actually treatment for the muscle spasms caused by CT — but it can’t prevent the infection via a puncture wound in the first place. BACK TO TEXT
- It is now well understood that every microscopic nook and cranny of our skin — indeed, our entire body, inside and out — is thickly populated with an ecosystem of microorganisms, more diverse than any jungle (see We Are Full of Critters). It is also likely that one of the primary functions of these teensy jungles is to maintain a balance of power, where it’s difficult for any organism to dominate. If soaking in salt water kills bacteria, it might kill off the bacteria that normally live on the skin as well. BACK TO TEXT
- Which actually suggests an interesting point: if modest amounts of Epsom salts in your bath allegedly has therapeutic effects, then it is reasonable to guess that the much higher concentrations of salt used in floatation therapy or found in the famous salt lakes would have a really dramatic effect — perhaps even a toxic effect. But bathing in much higher concentrations of salt has no significant effect at all … other than making people float. BACK TO TEXT
- A UK floatation tank manufacturer’s website, floataway.com, admirably restrains itself from extravagant claims of medical benefits, discussing only the benefits of relaxation. As for Epsom salt, the website says it is used “because it raises the density of the water, making it easy to float, and because it has a silky feel which is very good for the skin.” I’m not sure what they mean by “good for,” but I’m guessing it just feels pleasant. BACK TO TEXT
- RacingSmarter.com [Internet]. Epsom Salt & Apple Cider Vinegar Treatments Nature's Healing & High Energy Bath; 2006 [cited 10 Nov 2, page defunct]. BACK TO TEXT