The problem with studies of antidepressants is that they're almost all based on short-term, fixed-length treatment courses. After all, you can't do a double blind trial that lasts years on end; it would be unethical to give someone a placebo for that long, when antidepressants are believed to work better. Yet (AFAIK) people who go on antidepressants typically stay on them for years, so the studies are reviewing a fairly artificial use case. It's like the handful of controlled trials that attempt to compare programming languages, which by necessity ask participants to solve short, fixed programming exercises, even though that bears little resemblance to the process of real software development.<p>In the case of antidepressants - on one hand, there are some reasons to expect short-term interventions to be the <i>best</i>-case scenario in terms of evaluating benefit, such as the greater risk of side effects with long-term use, and drug tolerance effects. On the other hand, I suspect (but don't have data to prove) that <i>placebos</i> are much less effective in the long term. People think the placebo effect is in part a reaction to the social experience of interacting with a doctor, getting personal attention and concern for whatever condition is supposedly being treated. To the extent this is true, the novelty of the experience is probably a large factor, and over the long term you'd expect a reversion to the mean. So even if antidepressants are less effective in the long term than the short term, they might be <i>more</i> compelling as a treatment option, because the alternative (placebo) loses even more of its effectiveness.<p>Edit: Another factor is that the effects of reduced depression may take a long time to be fully apparent. Depression tends to work in feedback loops: as an oversimplified example, you feel bad about yourself, so you lose motivation to take care of your life, so you start neglecting essential tasks, the consequences of which make you feel even worse about yourself. And lifting yourself out of depression is the same thing in reverse. So if an antidepressant has the effect of reducing your <i>susceptibility</i> to depression - i.e. under the same life circumstances, you wouldn't lose quite as much motivation, or see things quite as darkly - then even a small change might tip the balance and let you stay at equilibrium in a more functional state of mind. But before you can reach that equilibrium, you have to go through a long process of getting your life back in order and regaining self-confidence.