Feb. 27 (HealthDay News) -- While popular antidepressants such as Prozac are widely prescribed for people with varying degrees of depression, the drugs are only effective for those with the most severe depression, a new study suggests.

"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," lead researcher Irving Kirsch, a professor of psychology at the University of Hull in Great Britain, said in a prepared statement. "This means that depressed people can improve without chemical treatments," he added.

In the study, Kirsch and his colleagues collected data on 35 clinical trials of drugs called selective serotonin reuptake inhibitors, or SSRIs, whose results had been submitted to the U.S. Food and Drug Administration. The antidepressants included in the trials were fluoxetine (Prozac), venlafaxine (Effexor), nefazodone (Serzone), and paroxetine (Seroxat/Paxil).

An analysis of the data showed that patients taking antidepressants fared no better than patients receiving a placebo. This appeared to be the case whether the patients were mildly or moderately depressed.

The drugs only seemed to benefit a small group of patients -- those with the severest depression when the study began.

Based on these results, there appears to be little reason to prescribe these antidepressants to anyone but the most severely depressed patients, the study authors concluded.

The findings were published online Feb. 25 in the journal PLoS Medicine.

Dr. Nada Stotland, president-elect of the American Psychiatric Association, said she wasn't surprised that the study found that not every antidepressant works for every patient. Many people who are depressed don't respond to the first antidepressant they try. It can take up to an average of three different antidepressants until one works for a particular patient, she said.

"Medication helps some, but not all, people with depression," Stotland said in a prepared statement. "For people with mild to moderate depression, psychotherapy can work as well as medication. Studies have shown that between 70 and 80 percent of people can and do get better with a combination of treatment approaches, which will often include individual therapy, family therapy and/or medication.

"Therefore, testing any single antidepressant on a group of depressed individuals will show that many of them do not improve," Stotland added. "There is a small group of depressed individuals who do not respond to any antidepressant."

The new study highlights the fact that treatment for depression needs to be tailored to an individual, and that the most effective treatment will often include multiple approaches, Stotland said. The study results also suggest that more long-term follow-up trials are needed to determine which patients are most likely to benefit from specific therapies.

"Some of the most exciting research in progress at the present time concerns our attempt to match the antidepressant to the patient from the outset; we may be able to perform laboratory tests or identify clinical factors that let us know in advance which antidepressant will work for each person," Stotland said. "That will be an enormous advance for the millions of people suffering from this very painful and potentially disabling disease."

SOURCES: American Psychiatric Association, news release, Feb. 26, 2008; PLoS Medicine, Feb. 25, 2008, online