My evidence comes mostly from personal experience. I have more than one serious chronic illness, which have caused secondary effects like chronic depression which I have tried to treat with pharmaceuticals. Also, my attempts to treat the chronic illnesses have put my physiology into novel abnormal states that I sometimes needed pharaceuticals to get myself out of. For example, I once took too much vitamin D, and (since about a year's supply of D was being stored in my body) I had to go around wearing particularly dark sunglasses and avoiding the sun for about 12 months and also take an ACE inhibitor called Benicar to mitigate the adverse effects of the D. Point is I have taken many kinds of pharmaceuticals for many different purposes.
Although most of my evidence comes from personal experience (and the experience of my friends, most of whom also have serious chronic illness), I have seen significant evidence in the form of arguments that cite studies that the most commonly used family of antidepressants, the SSRIs, have no positive effect on most of the depressed people who take them. I do not pay much attention to the medical literature, though, relatively to how much I pay attention to the stream of my treatment attempts, my symptoms, how they might be related and (vitally) how things (both natural human cognitive process as are discussed here and the disease process itself) might be biasing my perceptions of those things.
Also there is a basic argument from information theory to the effect that health is maintained through an elaborate network of negative-feedback loops (maintaining what the medicos call "homeostatis") plus the observation that the most common way health is lost is when some of the feedback loops get stuck as a single setting plus the observation that it is vastly easier for a pharmaceutical or prospective pharmaceutical to freeze a feedback loop (in the "always-on" or "always-off" "position" usually) than to unfreeze it.
We have a lot of knowledge of the mechanism by which various pharmaceuticals exert their effect, and the vast majority of them do it by inhibiting or interrupting a "pathway" that is one side of a feedback loop. In other words, a feedback loop consists of a "pathway" pushing in one direction and another "pathway" pushing in the other direction, and the pharmaceutical inhibits or blocks one of the pathway with the result that the feedback loop no longer works but rather is stuck in one "position".
This is all very vague and hand-wavy, and I have probably driven any medicos reading this crazy by now, and there is a good chance that I should not have published it, but you asked and I answered with the limited time and energy I had available.
Also there is the hypothesis that most people desperately want the health care system to be effective because it would be ever so convienient if it were so. Since I assign a significant probability to that hypothesis, I aggressively discount the common wisdom about effectiveness of medical interventions.
I am not thrilled with the fact that someone armed with just my name and a little patience with a search engine can find this comment, and ask repliers not to quote me or imply with their replies that I have serious chronic ilnesses so that I can in the future delete this comment.
Ok, thanks for answer. It is certainly true that people have a desire to believe that medicine works independent of its actual effectiveness; you only have to look back a few generations to when people were very serious about prescribing and using remedies that came down to various combinations of opiates, laxatives, and alcohol.
Today I received some shoes in the post, which included a couple of packets of silica gel. I don't think I have ever seen a packet of silica gel that didn't have "DO NOT EAT" printed on it, and it's always bemused me. It doesn't look edible, or smell appetising, and isn't even especially harmful to ingest in most circumstances. Chances are that if I ever did want to eat silica gel, I'd probably have a damn good reason, and a lifetime of being told to not eat it is an obstacle to that.
This has started me thinking about all the other things we internalise as serious hazards contrary to reality. As a child, I was told that picking my nose and eating it would have some sort of cumulative toxic effect. This was obviously a lie manufactured by my parents (or maybe their parents) to get me to stop doing it, but a couple of decades later I felt positively scandalised when I read about an Austrian pathologist who claimed the practise was beneficial to the immune system. (Although this is mentioned in the delightful Wikipedia page on nose-picking, the reference links are dead, so I'd actually treat this assertion with caution, but feel free to munch away on your own dried nasal mucus anyway).
Although nose-picking and eating the packaging that shoes come in are pretty trivial examples, I do wonder how many of these prohibitive false dire consequences I'm still labouring under, invisibly making my life more difficult. I also wonder how many full-grown adults still don't accept sweets from strangers.
Do you have any examples of an authority figure, or a prevailing piece of cultural conditioning, giving warnings of dire outcomes you later discovered to be false, misleading or based on an agenda you were naive to at the time?