Antidepressants have made some sufferers of depression feel even more depressed, a fresh look at past studies has found.

Researchers from the Yale School of Medicine looked at results from seven different studies involving approximately 2500 patients, who were randomly assigned to different types of antidepressants. These included Cymbalta and placebo drugs.

The results found that at least one-fifth of those who had taken Cymbalta had worsened symptoms of depression after a two-month period.

The patients were categorized into two groups. The responders, who showed signs of improvement after the treatment and the non-responders, who didn't improve, but reported side effects such as poor sleep and stomach problems, which brought down their overall depression score.

It's, however, difficult to say a priori who will be in which group, Ralitza Gueorguieva, the study's lead author from the Yale University School of Medicine in New Haven, Conn. told Reuters Health.

According to the researchers, the study highlights just how important it is to identify, which patients will respond to the drug.

You know within the first couple weeks of starting a treatment who's the most likely to benefit because they're already starting to show improvement, Dr. Michael Thase, a psychiatrist from the University of Pennsylvania Perelman School of Medicine who wasn't involved in the new study, told Reuters.

Those who were given the Cymbalta drug in the experiment reported either a steep or steady improvement while 16 percent said they didn't get better and in some cases felt worse. Those who took the placebo drug also reported a steady, but more slight, improvement.

According to Thase, the findings hold true to many other antidepressants that are widely used, but highlighted that patients respond to some antidepressants a lot better than others.

If you can identify people who would be potential responders to a particular medication...it would be a great, huge advantage for the field, C. Hendricks Brown, who has studied depression treatments at the University of Miami Miller School of Medicine, told Reuters.

The new study appears in the journal Archives of General Psychiatry.