WASHINGTON (Reuters) - Use of antidepressant drugs in the United States doubled between 1996 and 2005, probably because of a mix of factors, researchers reported on Monday.

A woman holds the antidepressant drug Prozac, also known as fluoxetine, in Leicester, central England February 26, 2008 in this posed photograph. REUTERS/Darren Staples

About 6 percent of people were prescribed an antidepressant in 1996 -- 13 million people. This rose to more than 10 percent or 27 million people by 2005, the researchers found.

“Significant increases in antidepressant use were evident across all sociodemographic groups examined, except African Americans,” Dr. Mark Olfson of Columbia University in New York and Steven Marcus of the University of Pennsylvania in Philadelphia wrote in the Archives of General Psychiatry.

“Not only are more U.S. residents being treated with antidepressants, but also those who are being treated are receiving more antidepressant prescriptions,” they added.

More than 164 million prescriptions were written in 2008 for antidepressants, totaling $9.6 billion in U.S. sales, according to IMS Health.

Drugs that affect the brain chemical serotonin like GlaxoSmithKline’s Paxil, known generically as paroxetine, and Eli Lilly and Co’s Prozac, known generically as fluoxetine, are the most commonly prescribed class of antidepressant. But the study found the effect in all classes of the drugs.

Olfson and Marcus looked at the Medical Expenditure Panel Surveys done by the U.S. Agency for Healthcare Research and Quality, involving more than 50,000 people in 1996 and 2005.

“During this period, individuals treated with antidepressants became more likely to also receive treatment with antipsychotic medications and less likely to undergo psychotherapy,” they wrote.

SOCIAL ACCEPTANCE

The survey did not look at why, but the researchers made some educated guesses. It may be more socially acceptable to be diagnosed with and treated for depression, they said. The availability of new drugs may also have been a factor.

“Although there was little change in total promotional spending for antidepressants between 1999 ($0.98 billion) and 2005 ($1.02 billion), there was a marked increase in the percentage of this spending that was devoted to direct-to consumer advertising, from 3.3 percent ($32 million) to 12 percent ($122.00 million),” they added.

Dr. Eric Caine of the University of Rochester in New York said he was concerned by the findings. “Antidepressants are only moderately effective on population level,” he said in a telephone interview.

Caine, who was not involved in the research, noted that several studies show therapy is as effective as, if not more effective than, drug use alone.

“There are no data to say that the population is healthier. Indeed, the suicide rate in the middle years of life has been climbing,” he said.

Olfson and Marcus said out-of-pocket costs for psychotherapy and lower insurance coverage for such visits may have driven patients away from seeing therapists in favor of an easy-to-prescribe pill.

The rise in antidepressant prescriptions also is seen despite a series of public health warnings on use of antidepressant drugs beginning in 2003 after clinical trials showed they increased the risk of suicidal thoughts and behaviors in children and teens.

In February 2005, the U.S. Food and Drug Administration added its strongest warning, a so-called black box, on the use of all antidepressants in children and teens.