This article is from the Depression FAQ, by Cynthia Frazier with numerous contributions by others.
No matter what their exact mode of action may be, it is clear that antidepressants are not "happy pills." There is no street-market in antidepressants, for unlike "speed" which will improve the mood of almost everybody, antidepressants only improve the mood of depressed people. Also unlike the almost instant effects of speed, the mood-improving effects of antidepressants develop slowly over a number of weeks. "Speed" induces a highly artificial state, antidepressants cause the brain to slowly increase its production of naturally occurring neurotransmitters.