Drugs giant Pfizer is expected to see a sharp fall in the sales of one of its best-selling products following a warning by US regulators.
Chantix has proven to be a popular treatment
The Food and Drugs Administration (FDA) said it was "increasingly concerned" that Pfizer's anti-smoking drug Chantix could cause suicidal thoughts.
"There are a number of compelling cases that... look as if they are the result of exposure to the drug," it said.
Pfizer has already added warning notices to Chantix labels.
FDA officials said they have received almost 500 cases of suicidal behaviour and thinking, and 39 reports of completed suicides.
"Chantix may cause worsening of current psychiatric illness even if it is currently under control," said the FDA.
"It may also cause an old psychiatric illness to reoccur."
Chantix had sales of $280m (£142m) in the last three months of 2007.
The drug works by binding to nicotine receptors in the brain, thereby reducing withdrawal symptoms.