Irving Kirsch of the University of Hull and his colleagues did a meta-analysis on the clinical data from all the trials submitted to the U.S. Food and Drug Administration for four drugs: fluoxetine, or Prozac; venlafaxine, or Effexor; nefazodone, or Serzone; and paroxetine or Seroxat/Paxil.

The researchers included unpublished, as well as published, trials.

The meta-analysis, published in PLoS Medicine, found the improvement in depression among patients receiving the trial drugs, as compared to those receiving placebos, was not clinically significant in mildly depressed patients or even in most patients who suffer from very severe depression.

"Although patients get better when they take antidepressants, they also get better when they take a placebo, and the difference in improvement is not very great," Kirsch said in a statement. "This means that depressed people can improve without chemical treatments."

The researchers conclude there is little reason to prescribe new-generation antidepressant medications to any but the most severely depressed patients unless alternative treatments have been ineffective.