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A health warning meant to alert doctors about the potential risks of prescribing antidepressants to youth may have actually triggered a significant rise in suicides among Canadians under age 18, a new study has found.

The findings, published today in the Canadian Medical Association Journal, are adding new fuel to an already heated controversy about whether certain antidepressants may pose a risk to Canadians, particularly teens and children, or if the drugs help reduce suicide rates.

Researchers examined antidepressant prescription information for young Canadians in Manitoba before and after Health Canada issued a safety warning in 2004, which was intended to warn doctors and caregivers that young people taking selective serotonin reuptake inhibitors such as Prozac, Zoloft and Paxil, and other newer antidepressants, may be at an increased risk for mood and behavioural changes.

But researchers found the warning resulted in several unintended consequences that may have contributed to a 25-per-cent increase in youth suicides in the two years afterward.

After the warning was issued, there was a 14-per-cent drop in the number of Canadians under 18 being prescribed certain antidepressants, while prescription rates for newer antidepressants, excluding fluoxetine (also known by the brand name Prozac), fell as much as 40 per cent.

In addition, there was a significant decrease in the number of Canadian children and adolescents who saw their doctor for treatment of depression.

Even though the warning did not tell doctors to stop prescribing antidepressants, it seems to have had a major impact on the level of care young Canadians received for depression, said Laurence Katz, lead author of the study and a psychiatrist at the University of Manitoba.

"Untreated depression is a major concern," Dr. Katz said. "That's the major issue here."

Similar changes were not found among young Canadians from ages 19 to 24, a group that was not targeted by the warning.

For the study, researchers examined antidepressant prescriptions and health information for youth in Manitoba in the nine years before and two years after the health warning was issued for antidepressants. During that period, 4,521 children ages 5 to 11 and 16,791 adolescents ages 12 to 17 were prescribed antidepressants. From 1995 to 2005, 99 children and adolescents committed suicide in the province.

The study is one of only a handful that examine the impact of drug safety warnings on the public and could highlight the need for better ways to deliver warnings and monitor their effect on the general population.

"Unfortunately, it appears that somehow we went from a warning that is reasonable to an effect that would appear unintended," Dr. Katz said.

While the study focuses narrowly on antidepressants and youth, the findings that safety warnings had an impact on prescription rates is probably not an isolated incident, said Dr. Katz, who specializes in child and adolescent psychiatry.

He said one possible problem is that the antidepressants warning was fairly vague. It didn't tell doctors whether to stop prescribing the drugs to children and adolescents, or what other action they should take.

"There's something in the communication on the warning," Dr. Katz said. "The warning itself didn't say what doctors should do."

Ginny Dennehy, who lives in Whistler, B.C., knows better than most people the difficulty of treating youth depression and the debate over the safety of drugs. Her son, Kelty, committed suicide in 2001, when he was 17. He had just begun taking antidepressants to help his bouts of severe illness, Ms. Dennehy said. When safety warnings about antidepressants and youth came out a few years later, the family wondered whether the medication contributed to Kelty's decision to kill himself.

But more than anything, his death highlights an overall lack of understanding and support services for young people suffering from depression, Ms. Dennehy said.

"That's the problem with mental illnesses. There's a big stigma," she said. "Some parents are ashamed their kid is like this."

Ms. Dennehy and her husband, Kerry, started the Kelty Patrick Dennehy Foundation in order to raise money and promote the need for improved treatment for youth depression.

"One of the things our foundation is focusing on is to help remove the shame," Ms. Dennehy said. "It's slow, but I think we're moving in the right direction."

The study was funded by a grant from the Health Sciences Centre Foundation, a Canada Research Chair Award and a Canadian Institutes of Health Research New Investigator Award. One study author, Jitender Sareen, is a member of the speakers' bureau for Wyeth, AstraZeneca and Biovail.