Antidepressants may not work as well as in people who are mildly depressed as they do in patients with major depression, according to a new study published in the journal PLoS Medicine.

Researchers from various U.S., U.K. and Canadian universities found that some patients taking antidepressants believe the drugs are working for them, but many times it is only a placebo effect.

The research team based its findings on 47 clinical trials, using drugs such as fluoxetine (Prozac) and paroxetine (Paxil), which are selective serotonin re-uptake inhibitors (SSRIs), and the venlafaxine (Effexor), a serotonin-norepinephrine re-uptake inhibitor.

The study found that the benefits of the drugs were only significant in a small group of the most severely depressed patients, Sky News reported.

"Most patients can benefit as much from other treatment, such as physical exercise and psychotherapy, as they can from medication," University of Hull, U.K., professor of psychology Irving Kirsch told Sky News. "Antidepressants should be reserved for patients at the very extreme levels of depression."

Makers of antidepressants, such as Eli Lilly and GlaxoSmithKline, disagree with the study.

Doctors advise patients who are on antidepressants not to discontinue their medication without consulting a physician first, as there can be serious side effects.

Click here to see the study.