"What we found is that patients who are on the lower end of the severity, even on the sort of middle range of severity, the medications weren't doing much more than the placebo was. For patients at the higher end of severity the medications had a very potent effect," said Jay C. Fournier, University of Pennsylvania.
Scientists say for people who are severely depressed, antidepressants work well. But for those with mild to moderate depression - antidepressants are about as effective as a placebo.
In the study provided by the Journal of the American Medical Association, doctors looked at data from six different randomized studies completed over the last 20 years.
"It both highlights the importance of the medicines for those patients who are at the more severe ranges of depression, but it also questions the importance of those same medicines for patients whose severity levels are moderate or less," said Robert DeRubeis, a psychologist at the University of Pennsylvania, Philadelphia, who with colleagues performed the meta-analysis.
Although, doctors say, antidepressants are generally effective they do have their limits.
"You had to be pretty severe in this study to show a difference, an actual benefit of the medication versus placebo," said Fournier.
How medications affect different degrees of depression could change the way doctors treat the condition.
Again, patients with mild to moderate depression did not do any better on antidepressants than those on placebo. But researchers say the positive news is that some of the patients taking a placebo did see improvement in their symptoms.