A new study finds antidepressants may be less effective than doctors have been led to believe. That's because it appears that studies that have found the drugs do little to help with depression are not being published.

And if studies that have found less-than-positive results have been published, they have been written as if they were positive.

The finding comes from a new analysis in the New England Journal of Medicine.

Dr. Erick Turner, an assistant professor of psychiatry at Oregon Health and Science University led a team who reviewed studies that the U.S. Food and Drug Administration has done on 12 widely prescribed antidepressant drugs.

His team looked to see whether results of these studies had been published in medical journals. They then compared the published version of the results with the FDA version of the results.

Turner's team found that nearly all the studies (94 per cent) that were published in journals suggested positive treatment results. But FDA data showed that in fact only 51 per cent of the studies were positive.

Of the 74 studies conducted on the 12 antidepressants, 38 produced positive results for the drug. All but one of those studies were published. But only three of the 36 studies with negative or questionable results were published. Another 11 were written as if the drug were effective.

Turner says this is just another example of "selective reporting," in which good news about a drug is made public but the bad news isn't.

"Selective publication can lead doctors and patients to believe drugs are more effective than they really are, which can influence prescribing decisions," said Turner.

Turner doesn't believe that the antidepressants studied are ineffective. In fact, his team found through an analysis of all the studies done on each drug that the medications were superior to treatment with placebos (sugar pills).

But after reviewing the FDA data on the drugs, they found that the published literature often exaggerated the drugs' effectiveness.

Turner said that he didn't know whether study sponsors chose not to submit manuscripts for studies that had less-than-positive results, or whether the journal editors and reviewers rejected the studies.

"Regardless, doctors and patients must have access to evidence that is complete and unbiased when they are weighing the risks and benefits of treatment," he emphasized.

David Shern, Ph.D., president of Mental Health America cautions that this study should not discourage people with depression from seeking care and treatments.

In a news release, Shern said the idea that research papers that don't produce positive results are not the ones that get published is not a new one.

"Historically, papers that get published have been those with positive results. This study highlights what is known as the "waste basket" problem in which studies with positive outcomes are more likely to be published than those with neutral or negative outcomes," said Shern.

The fact remains, antidepressants do work and have helped many depressed people to recover, he added.