Many drugs that are assumed to be effective are probably little better than placebos, but there is no way to know because negative results are hidden. One clue was provided six years ago by four researchers who, using the Freedom of Information Act, obtained FDA reviews of every placebo-controlled clinical trial submitted for initial approval of the six most widely used antidepressant drugs approved between 1987 and 1999—Prozac, Paxil, Zoloft, Celexa, Serzone, and Effexor. They found that on average, placebos were 80 percent as effective as the drugs. The difference between drug and placebo was so small that it was unlikely to be of any clinical significance. The results were much the same for all six drugs: all were equally ineffective. But because favorable results were published and unfavorable results buried (in this case, within the FDA), the public and the medical profession believed these drugs were potent antidepressants.
