Pediatrician, author, and medical school professor Dr Aaron Carroll surveys the evidence that cardiovascular stents, which are intended to help keep clogged heart arteries open, may be no better than treatment with drugs alone. One reason this is surprising is that patients who get stents typically have a marked reduction in chest pain, but the research suggests this might simply be a placebo effect.
Placebos -- sugar pills or sham treatments -- are especially good at reducing all sorts of pain, at least for a substantial majority of patients. A clever drug dealer might try selling fake opioids, which could benefit a lot of people with chronic pain, and if they get addicted to a placebo, that's not so bad. Actually, I've seen homeopathic drops sold for ear pain in name-brand pharmacies, which amounts to the same sort of thing. (Homeopathic remedies are substances thought to cause the symptoms they're supposed to treat, but diluted to such an extreme that often not one molecule of the original substance remains. There are people who really believe in this despite both hard evidence and common sense.)