(USA Today) -- The smoking cessation drugs Chantix and Zyban must now carry a boxed warning -- the strongest type possible -- about the risk of serious mental health problems, including depression, behavior changes and suicidal thoughts, the Food and Drug Administration announced Wednesday.

These have occurred in people with and without underlying psychiatric illnesses as well as those still smoking, Curt Rosebraugh, head of the FDA office that approved the stop-smoking drugs, said at a news conference. The symptoms linked to the drugs resemble those of nicotine withdrawal.

Rosebraugh emphasized that such events appear to be rare and that for many people, the drugs' benefits outweigh their risks. "The health benefits of quitting smoking ... are immediate and substantial," he said.

Besides the immediate labeling change, the FDA is requiring the drugs' makers to conduct a clinical trial to determine how often serious psychiatric symptoms occur in people using them. Earlier clinical trials didn't uncover a potential psychiatric risk, Rosebraugh said, but "they weren't specifically looking for it."

The FDA decided to require the boxed warning after reviewing voluntary adverse-event reports for stop-smoking products, Rosebraugh said.

Chantix was approved in May 2006. In September 2007, a "bizarre" Texas case spurred the FDA to look at adverse-event reports, Rosebraugh said, referring to musician Carter Albrecht, 34, whose girlfriend blamed Chantix for the abnormal violent behavior that led to his shooting death.

In May 2008, a watchdog group posted a study online that linked Chantix to loss of consciousness, lapses in alertness, dizziness and muscle spasms. That led the Federal Aviation Administration to remove it from the list of drugs considered safe for pilots and air traffic controllers.

For Chantix, the FDA has received 98 reports of suicides; for Zyban and bupropion, the drug's generic name, 14. There also were 188 reports of attempted suicides by Chantix users, 17 by smokers trying to quit with bupropion. Nicotine-replacement products haven't generated similar reports, Rosebraugh said.

He cautioned "these are crude counts" and attributed the higher number for Chantix to publicity about the drug and its larger market share.

Chantix maker Pfizer and Zyban maker GlaxoSmithKline say they don't believe the suicide reports establish their drugs as the cause.

"If some of this is nicotine withdrawal, it really doesn't matter," the FDA's Robert Temple said. Smokers trying to quit and their doctors need to pay attention to behavior changes, no matter their cause, Temple said.

Bupropion is also sold as Wellbutrin, an antidepressant. Generic bupropion and Wellbutrin already carry a boxed warning about suicidal behavior in patients who receive the drug for psychiatric disorders.

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