“Efficacy” is how well a treatment works in ideal circumstances, such as in a carefully contrived scientic test. Unfortunately, real life is rarely ideal! (You may have noticed.) “Effectiveness” is how well the same thing works in typical clinical settings and patients’ lives. Which is what matters to most patients.
A classic example of efficacy versus effectiveness is strong calorie restriction for losing weight: anyone who substantially restricts calorie intake over long periods will lose weight, but it’s a serious error to assume that it’s just a matter of discipline. Low calorie diets are so difficult to sustain for so many reasons, many of them out of our control, that most people will gain back any weight they do manage to lose. They are efficacious, but also notoriously ineffective.
A recent editorial in the British Journal of Sports Medicine explains that exercise (in a physical therapy context) is well-known to be efficacious, but may not be effective. That is, it works well when tested in the lab, but not so much for real patients. Again, effectiveness is what matters to patients! If effectiveness is low, only a few lucky and/or disciplined people can realistically expect to benefit.
For example, I recently shared a perfect example of this problem: Ylinen et al found fairly good evidence that strengthening is probably efficacious for many people with chronic neck pain … but maybe only if they stick to it for as long as a year. That takes a lot of discipline, and many people will fail at it. (And some will fail even if they stick to it, because not all cases respond to strengthening.) So even if it does work in a scientific test, can we say “it works”? It’s ambiguous.
I think there’s a large grey zone here: some exercise therapies are much more difficult and impractical than others. When they are truly efficacious, they are an opportunity for determined patients with frustrating chronic pain: an intervention that might really work, but onlyif you’re up to a substantial challenge! Many people aren’t … but you can be some will be. And for those patients only, “it works.”
See Kenny Venere’s more detailed discussion of effectiveness and efficacy.