A common anti-smoking drug significantly increases the risk of suicide and should only be used as a last resort, doctors warned at a meeting on Wednesday evening.
Doctors say patients who take Champix were eight times more likely to be linked to depression and reported cases of suicidal behaviour than those who use other nicotine products, such as the nicotine patch or gum.
Prescribed more than one million times in Britain last year, Champix has previously been linked to heart attacks, strokes, unprovoked violence and blackouts.
Dr Curt Furberg, from the Wake Forest School of Medicine in North Carolina, and the professor of public health sciences behind the latest study, said "the risks simply outweigh the benefits."
Dr Furberg would like the drug to be banned but accepts this will be highly unlikely. A year after people start taking the drug, only 10 per cent of people are off cigarettes, he said.
"Champix should be the last resort. You should give counselling to help people quit and if you need medication use nicotine replacement or Zyban," advised Dr Furberg.
"If you give Champix, keep track of the person's mental status."
Since its introduction in Britain in 2006, Champix has been linked to 80 deaths, including 39 suicides. Additionally, hundreds of patients have reported complaints of suicidal thoughts while taking the drug.
Dr Furberg and his colleagues studied the number of serious side effects of anti-smoking treatments reported to the U.S. drug watchdog between 1998 and 2010.
Champix, also known as varenicline and sold in the U.S. as Chantix, topped the list, even though it was only approved for four of the years included in the nearly 13 year study.
Ninety per cent, or 2,925 cases, of reported depression and suicidal behaviour linked to the drug. By comparison, Zyban, accounted for seven per cent while patches, gums and other nicotine replacement products accounted for just three per cent.
"We found that Champix is associated with more suicidal behaviour reports than any other smoking-cessation drug on the U.S. market," said Dr Furberg.
According to a report in the Daily Mail, Britain's drug watchdog, the Medicines and Healthcare products Regulatory Agency (MHRA) said all medicines have side effects and linking reaction to a drug does not prove it was caused by the drug.
A spokesperson for MHRA said they would "carefully consider" the results of this study to see whether further advice to health professionals and patients is required.
Although Champix carries strong warnings about side effects on its label, Dr Furberg and colleagues want the FDA to further restrict its use by making it a second choice drug when other treatments have failed.
The anti-smoking tablets work by binding to nicotine receptors in the brain, reducing the symptoms of withdrawal.