Regulators in the USA have said that Pfizer’s smoking cessation drug Chantix could be linked to serious psychiatric problems, just a fortnight after the firm updated its label on the product.

In November, the US Food and Drug Administration issued an early communication that it was evaluating post-marketing adverse event reports on Chantix (varenicline) related to “changes in behaviour, agitation, depressed mood, suicidal ideation, and actual suicidal behaviour”. The agency now says that as its review proceeds, “it appears increasingly likely that there may be an association” between the drug and “serious neuropsychiatric symptoms”.

The FDA said they have received 491 reports of suicidal behaviour and thinking (420 in the USA) and 39 actual suicides, 34 of which took place on home soil. Around four million people in the USA have used Chantix since it was approved in May 2006 and the drug had global sales of $280 million in the fourth quarter.

Bob Rappaport, director of the FDA's Division of Anaesthesia, Analgesia and Rheumatology Products, said "Chantix has proven to be effective in smokers motivated to quit” but added that “it is important to consider these safety concerns and alert the public about these risks”. He added that patients should talk with their doctors about “whether Chantix is the right drug for them” and health care professionals should closely monitor patients for behaviour and mood changes.

This latest missive from the FDA has probably taken Pfizer aback, seeing as the label update it announced two weeks ago covers agitation, depression and suicidal ideation and behaviour. At the time the New York-based major said there is no evidence of a causal link, and limited itself to noting that in some cases an association could not be excluded. Pfizer added that the adverse event reports may have been complicated by symptoms of nicotine withdrawal itself.