People using a widely prescribed class of antidepressants are twice as likely to attempt suicide, say the Canadian authors of the most comprehensive study of its kind.

The findings, published in the most recent issue of the British Medical Journal, have significant implications because of the widespread use of selective serotonin reuptake inhibitors (SSRIs).

Scientists at the Ottawa Health Research Institute analyzed more than 700 clinical trials involving almost 90,000 patients taking SSRIs, which include drugs sold under the name brands Prozac, Cipramil and Zoloft.

The Ottawa study compared the reported number of suicide attempts by patients on the drugs to the number of attempts by patients taking a placebo.

They found there were an average of 2.56 suicide attempts for every 1,000 patients taking SSRIs, compared with 1.15 suicide attempts for every 1,000 patients using placebos.

The risk of suicide is somewhat expected with depression, but the study found that the drugs contributed to an increased risk in patients suffering from other conditions. Doctors also prescribe SSRIs to treat anxiety, post-traumatic stress disorder and social phobia.

"The very interesting thing is it's occurring in non-depressed individuals," said Dr. Paul Hebert, co-author of the study. "We're seeing it in some diseases where suicide is unexpected, for example in panic disorder."

The study's authors say the risk could be higher than their results show because the published clinical trials they examined have limitations.

Most of the trials followed fewer than 100 people and many were short-term or had high drop-out rates. As well, some trials didn't report suicides or attempts if there were fewer than a certain percentage.

That points to the problem of under-reporting in clinical trials, said Dr. Dean Fergusson, who headed up the Ottawa study.

"It leaves all of us in the dark. When you have serious adverse effects, they need to be reported. We would have known about this sooner if there was more rigorous reporting," he said.

But the head of the University of Ottawa's psychiatry department told the Canadian Press that he is concerned about the way the findings were presented.

"I don't agree with the press release in the way it was worded. Because I think it's extremely alarmist and it could do harm," said Dr. Jacques Bradwejn, who is also psychiatrist-in-chief at the Royal Ottawa and Ottawa hospitals.

Bradwejn worries the public will miss the important caveat that two times is a very small number. "It might overamplify the danger and the frequency of the danger," he told CP.

In fact, the Ottawa researchers insist SSRIs are still safe for the majority of patients.

Both Health Canada and the U.S. Food and Drug Administration have warned of an increased risk of rare but serious side-effects such as suicidal thoughts and behaviour in children and teens who take certain antidepressants. Regulators have also boosted warnings on packaging for SSRIs.