full article 9500 words
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.
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. No scientific basis for this idea was ever presented: it was just one of those things that everybody “knew,” a folk remedy justified by the generations of wise old wives and bathers. The physiology of it certainly wasn’t explained. No evidence for the efficacy of Epsom salts was ever presented, nor did it seem necessary to talk about it.
Occasionally someone made a vague reference 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 sulphate.”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 almost never actually know which specific toxins they are talking about.4 I have thoroughly studied the subject of post-exercise muscle soreness — which is 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 — I decided it was time for a reality check.
Does an Epsom salt bath do anything?
Epsom salts are magnesium sulphate heptahydrate, usually shortened just to magnesium sulphate (note also the American spelling “sulfate”). It was originally obtained by boiling down mineral waters at Epsom, England.
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.”
My search for scientific evidence concerning Epsom salt baths was disappointing. I was unable to find even a single scientific paper studying their effect on body pain. Folk remedies are often generally 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?
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 uses of magnesium sulphate include the treatment of irregular heart rhythm, low blood magnesium,9 eclampsia,10 and severe tetanus.11
But there appears to be simply nothing at all published about alleviating aches and pains or “detoxification.” Apparently, researchers just aren’t interested, or (more likely) they simply can’t get funding for the work.It seems like researchers just aren’t interested in studying the effect of Epsom salts on muscle pain.
Strangely, 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 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. Could they possibly have just neglected it? Or is it more likely that Epsom salt baths simply have no (clear, known) medical usage?
In the perfect absence of any testing, all we can do is speculate about the possible mechanisms of action. 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?
“(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 work, it’s important to understand that the words “detoxification” and “osmosis” are virtually the only explanations offered — and they are both hopelessly misleading. Usage of these terms mostly just reveals a poor understanding of both toxins and osmosis. If we are to understand Epsom salt, we need to get past this and understand it for the right reasons.
The osmosis/detoxification explanation is never actually clarified in any detail by the people tossing the words around. Presumably it is intended to mean — roughly — that either osmosis is actually a mechanism of detoxification (getting something nasty out of the body through the skin), or that osmosis is the mechanism by which something in Epsom salts can get into the body and then have a detoxifying effect.
People often mistakenly believe that osmosis refers to the movement of substances — ions and molecules — across a membrane. Alas, that is simply wrong by definition. Osmosis does not move particles. Osmosis refers to the movement of solvents only across thin membranes, towards higher concentrations of the substances dissolved in them.12 Take it from the Osmosis Cats!show cats13
You can demonstrate this clearly by soaking a potato in salty water: the water is “sucked” osmotically out of the cells, they lose their plumpness, and the potato goes limp. Poor little potato. It’s the water that moves around. (Or cats.)
Reader Dorrie B. pointed out something interesting: Epsom salts might be an effective treatment for topical skin infections, as salt is certainly inhospitable to microganisms. Osmosis may not be able (by definition) to transport salt ions across the skin, but it certainly isn’t kind to bacteria in this equation: it sucks them dry.
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.14
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, resulting in higher vulnerability to infection.15 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.
So, however Epsom salts get into the body, if they do, it’s not by osmosis. Osmosis doesn’t work through the skin. 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 course.16 But Epsom salts do not increase that.17)
The top layer of the skin, the stratum corneum, consists of dead cells stuffed with a kind of embalming substance, keratin, a fibrous protein. Water can go through them, and it can around them either, thanks to a microscopic “uniquely structured fatty layer” between them, which no one knew about until surprisingly recently (2012).18 Plus we have glands that coat the skin in waterproofing oils. When those oils wash off, the dead skin cells can soak up a little water and swell a bit, like soaked beans. (This is not the cause of skin pruning — that’s an old myth.19)
The swelling of the superficial skin cells is not due to osmosis, but rather to a limited “capillary action” in which water molecules flow into small spaces. (Paper towels absorb liquid the same way.) The stratum corneum is mostly waterproof precisely because osmosis is not a significant factor here. Those cells are dead, containing mostly just dry keratin, and surrounded by lipids — not fluid containing dissolved substances which could osmotically “suck” water through the skin. The presence of lipids and their arrangement results in “exceedingly low permeability.”20
Furthermore, the stratum corneum is generally an effective barrier to diffusion: ions and molecules dissolved in water mostly cannot pass through the stratum corneum, again because there is minimal water in the outer layers of skin for them to diffuse through. This is not to say that nothing gets past the skin, just not much, and definitely not water.
“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.”21
Booze molecules, for instance. You can’t get drunk through your skin, alas. Contrary to the Danish myth. As proven by Danish researchers in late 2010.22 It’s funny, but it’s not a joke.
The point is that it’s not exactly obvious to people what substances can 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, contact poisons and other examples that some things clearly do get past that dead outer layer to interact with the living cells beneath, or even into the interstitial fluids and blood stream. The skin keeps water and much else out, but some things get through. How?
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 corneum23 — the 500 Dalton rule.
Magnesium ions are way smaller than 500 Daltons, at a barely-there atomic mass of just 24 Daltons. (Water molecules also extremely tiny — just 18 Daltons — but recall from above that the skin is also built specifically to keep those molecules out.)
So size does matter, but it’s definitely not the only thing that matters. There are almost certainly other mechanisms involved. For instance, cells in the living layer of the skin take an active role in managing the passage of some substances. Topical allergic reactions are an obvious demonstration of this: the immune system over-reacts to an “invader.” A complex and imperfect system, obviously.
The 500 Dalton rule means that it’s plausible in principle that magnesium ions can diffuse through the stratum corneum, but ions are not necessarily going to easily diffuse through the fairly arid and water-proof stratum corneum — smallness may be necessary to drift across, but that doesn’t mean it’s enough. We still don’t know if they are actually going pass from bath to bloodstream in numbers that are potentially interesting and useful. There are just too many ways the messy details of biology might surprise us. The only way to find that out is to set aside the speculation about what’s possible in principle, and do some proper before-after testing of magnesium absorption…
This quote from a book by a doctor24 was submitted to me by a reader as an “authoritative” opinion on absorption:
Regularly bathing in hot water to which Epsom salts have been added can help draw out toxins from the skin.
This is not an authoritative opinion: it’s a vague and unsupported one. That anyone would mistake it for authoritative is rather depressing.25 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 depressing. Hat tip to reader Bryan B. who found an interesting 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.”26 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 I agree it’s pretty good evidence that absorption is minimal or nil.
And that contradicts virtually the only other science on this topic.
In 2006, Rosemary Waring, a British biochemist at the University of Birmingham, did a nice science experiment with Epsom salts.27 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 sulphate 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 sulphate in their blood after the baths than they did before the baths.28 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.”
Better still, Dr. Waring told me that a colleague of hers in London has done another experiment which showed that “magnesium sulphate can cross human skin using pieces of excised human skin in a special apparatus.”
Now eleven years later, neither experiment has yet actually been published,29 and that’s a significant reason for caution.30 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 — so sorry.)
Meanwhile, what can we make of Dr. Waring’s results? One thing only so far …
It’s not much, but it’s important. And when I found the new evidence, I embraced its implications. (That’s how science works. That’s why being anti-science is like being anti-honesty.31 I happily admit that Dr. Waring’s results might mean that I was simply dead wrong about the absorption question originally.)
How would it work, this crossing of the skin, if it’s happening? 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.
When you encounter surprising results in science, don’t just settle on the first explanation that comes to mind — it could easily be wrong. Dr. Waring says that she “assumed that it simply diffuses across the stratum corneum,” and that certainly is possible. But what other explanation could there be for the results? How else could magnesium sulphate have gotten into the bloodstream in her experiment? 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 plausible. 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 smaller membrane? It’s a small target! And I shudder to think of the measures required to test this hypothesis! 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.32 But it has to be pretty much injected. (Live a little: click that footnote!)
A number of 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 amused female 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!
Perhaps 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.33 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.
If Epsom salts do get across the skin … so what? Is it any good to have a few extra ions of magnesium and sulphate kicking around your bloodstream? The rest of this article continues to mostly cast doubt on the possible therapeutic effects.
There might well be a therapeutic effect, but we have no information about what it is, how it works, what it works for, how strong the effects are, what side effects there might be, and so on. The increased levels of these ions shown by Dr. Waring’s experiment are small, about a 10% increase on average (and none in some subjects, remember). The concentrations could also be quite different in the fluids between cells — she didn’t measure that. It is still completely unclear what effects these ions could have on your tissues when they arrive.
There is no doubt that magnesium sulphate has effects on physiology. Several of those effects are reasonably well known, including a few common medical applications mentioned earlier. There are also unpleasant effects. But, judging from the established medical uses of Epsom salt, there is definitely no particular reason so far to believe that having more magnesium or sulphate in your blood is going to be much use to you — unless you have eclampsia or tetanus or autism.34
The closest thing there is to a relevant science experiment is one study of injected magnesium sulphate which found that it “did not reduce muscle pain” and caused “unpleasant side effects.”35 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.
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.36
While it’s certainly conceivable that increasing levels of magnesium and/or sulphate 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. 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.
Generally speaking, explanations for the benefits of Epsom salts are really vague, as discussed above: “osmosis and detoxification.” Once in a blue moon, you’ll see Epsom salts (or magnesium in particular) more exactingly 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 scale37 — 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. I don’t think, as a rule, that people on calcium channel blockers are walking around feeling no pain, like a superpower.
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.I don’t think that people on calcium channel blockers are walking around feeling no pain.
There is some limited evidence that magnesium (just that ion) may reduce pain, perhaps because it is a “calcium channel blocker and N-methyl-D-aspartate antagonist,” as in a 2009 experiment.38 However, this is uncertain science. Several studies have been done, with conflicting results. Most were reviewed in 2007:39 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!
I asked Dr. Waring to speculate about the therapeutic effects. She pointed out that patients with rheumatoid arthritis are known to have low sulphate levels. Molecules produced by the inflamed tissues in these patients may interfere with the production of a protein that is used to produce sulphate from another molecule (cysteine), thus lowering sulphate levels.40
However, low sulphate 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.
Obviously non-salty baths have some benefits of their own. Epsom salts routinely get the credit for these benefits. It goes like this:
Tricksy, the human mind is.
Dr. Waring’s results are irrelevant to the popular idea of “osmosis,” which refers to the movement of water, not molecules. Nor do her results imply anything about that other popular concept associated with Epsom salts, “detoxification.” And yet, 99% of the time, “osmosis” and “detoxification” are the concepts presented as the justification for bathing in Epsom salts. Can you trust advice that simplistic?
The detoxification claim implies either that Epsom salts somehow “suck” toxic substances out of your muscle tissues, or that Epsom salts get into your system and then somehow “clean up” some toxic substances that they encounter. There is no scientific evidence at all for either of those basic detoxification scenarios, and both involve some seriously optimistic assumptions, leaps of logic, avoidance of detail … all made by people who are usually trying to sell the stuff.
Epsom salt bath prescriptions are invariably brief, and are often accompanied by 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 sulphate 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 sulphate was added to the diet as a nutrition [sic].41
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.
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.Most people agree that Epsom salts make water feel “smoother.”
And it makes you floatier! But only ever so slightly. 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.42 However, any salt in your bath — Epsom or otherwise — is going to make you at least a little bit lighter in the water.
The purpose of floatation therapy is primarily to reap the benefits of deep relaxation, which are noteworthy.43 It sounds lovely to me — 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.
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 sulphate 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 sulphate 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!”
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.
This is a new section that I’ve just started in the fall of 2012. It will grow.
Industry protectionism for baths? A reader asks, “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.” 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.
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? Yes, I believe it is “the same,” at least insofar as it is “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.
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. 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.
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):
Other interesting reading:
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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 oldest and most regularly updated articles on PainScience.com, though no updates were logged for at least the first four years of its lifespan. But it has always been popular, and has always attracted lots of feedback, including some great comments that have inspired many corrections and improvements.
— Expanded on the discussion of inhalation as a vector for salt magnesium absorption.
— Correction about the size of water molecules, and some clarification about how skin waterproofing works thanks to some interesting science.
— Small but worthwhile clarifications of the 500 Dalton rule and the “Osmosis mistake” (which I continue to get too much ignorant email about).
— New section, “What does get across the skin? Obviously some things do.” Discussion of 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.
— 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.
— Fixed incorrect information about skin pruning and added the actual (and much more interesting) explanation; cited to substantiate the impermeability of the stratum corneum.
— 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.
— Added the first piece of evidence against the absorption of Epsom salt.
— Added a particularly awful example of a bad article about Epsom salts. See immediately above in the Further Reading section.
— Added picture of cats demonstrating osmosis.
Some updates missed.
— Added information about the effect of Epsom salt on bacteria on the skin.
— Corrected several typographic errors.
Many earlier updates unlogged.
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 focussed 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 Should You 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, Delayed Onset Muscle Soreness (DOMS): The biological mysteries of “muscle fever,” nature’s little tax on exercise. Basically what it boils down to is that the top 5 effective treatments for muscle pain after exercise are:
“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
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
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
They argued it in three ways:
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- virtually all common contact allergens are under 500 Dalton, 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 Dalton;
- all known topical drugs used in transdermal drug-delivery systems are under 500 Dalton.
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
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In fit unpremedicated patients undergoing minor operations and who were ambulant on the afternoon of the operations, pretreatment with magnesium sulphate given intravenously did not reduce the incidence of suxamethonium induced myalgia below that in a similar series who received no prophylactic therapy. The injection of magnesium in conscious patients is followed by unpleasant side effects.
A beautiful animated tool for visualizing the scale of cells.BACK TO TEXT