Antidepressant/SSRIs are a much bigger one. The actual experimental evidence for these drugs, contrasted against placebo, is shockingly weak. Moreso, SSRIs come with tremendous side effects. At the very least we should be making an effort to gradually phase these out of society. In reality, we've gone the other direction and floored it - with SSRI prescriptions growing exponentially.
I have seen the argument that the weak evidence for their effectiveness is entirely due to them being better placebos. What makes them a better placebo? Ironically, it is (according to this theory) that their side-effects are obvious to the patient, so those who are actually on them are more sure than the controls are that they are taking the drug under test, as opposed to the placebo being used in the trial.
SSRIs do have a studied and recorded physical effect of inhibiting serotonin reuptake, but what that actually does to someone is still not entirely understood, past "it can help with depression and/or maybe ADHD".
It's unfortunate that SSRIs have harmful long-term effects—I know some people who have been dependent on SSRIs for around a decade, and probably will be forever because of the neurotoxicity.
The main point is it's not even clear it can help with depression, at least not anymore than a sugar pill. I'm going to avoid referencing any single study since you can find a study to support practically any reasonable view. Instead I'd just encourage you to look up: 'ssri vs placebo' anywhere. The overwhelming body of evidence concludes that if there's any effect, it's absolutely negligible. And, as a previous poster mentioned, it may well simply be due to the fact that doing a double blind with SSRIs is not really possible. They have such a tremendous physical effect than any subject will absolutely know if they're getting the "real" thing.
And to be clear a placebo doesn't mean the effect of such isn't real. Mental state provably affects the body in all sorts of ways. Interestingly enough the placebo effect is often substantially stronger in Americans than in other groups as well [1]. The two main hypothesis there are that more poorly performing drugs are making it past regulators, the other is that the overwhelming amount of direct-to-consumer medical advertising creates a greater faith in the power of pills than in other countries - US/New Zealand are the only developed countries that allow direct to consumer medical ads. Or perhaps a mixture of both, like in many things.
