More people without any documented psychiatric condition are taking antidepressants, according to a new study, and some of them are likely receiving little benefit.

Nearly three-quarters of antidepressants in the U.S. were prescribed by non-psychiatrists in 2007, up from 60 percent a decade earlier, according to the analysis of a national sample of 233,144 doctor office visits, the latest data available.

The percentage of these patients prescribed antidepressants without being diagnosed with a mental illness more than doubled in that period to 6.4 percent in 2007 from 2.5 percent in 1996

The findings spark concern about whether these medicines are effective for the patients receiving them, since previous research has shown that antidepressants are most effective for people with severe symptoms, said Mark Olfson, a professor of clinical psychiatry at Columbia University and an author of the study published in the journal Health Affairs.

"We're left wondering whether the level of illness severity is rather low in these patients," said Olfson. In those cases, antidepressants "may not offer therapeutic benefits ... or the medication may not work any better than a placebo."

Antidepressants were the second most widely prescribed class of medicine in the US in 2010, after cholesterol-lowering statins, according to IMS Health. Their use continues to grow: Some 253 million prescriptions for antidepressants were written in 2010, compared with 231 million in 2006.

The generic version of Zoloft, known as sertraline, was the top antidepressant dispensed in 2010 with nearly 36 million prescriptions, and Celexa, or citalopram, was the second-most popular with 32 million prescriptions, according to IMS Health.

G. Caleb Alexander, a professor of medicine at the University of Chicago who was not involved in the study, said the reasons behind the prescribing pattern were not clear from the data, but that Olfson's interpretation was plausible.

"I think many physicians regard these drugs as relatively safe and are willing to try them in settings where there is limited effectiveness," said Alexander, who recently published a paper on trends in the use of antipsychotic drugs.

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