Regulators in the USA have told Pfizer and GlaxoSmithKline to include a boxed warning on their smoking cessation products, Chantix and Zyban, about the risk of "serious mental health events”.

The US Food and Drug Administration said that the warnings on Chantix (varenicline) and Zyban (bupropion) will highlight the risk of changes in behaviour, “depressed mood hostility, and suicidal thoughts when taking these drugs”.

Janet Woodcock, director of the FDA's Center for Drug Evaluation and Research, was quick to note that “the risk of serious adverse events while taking these products must be weighed against the significant health benefits of quitting smoking”. She added that “smoking is the leading cause of preventable disease, disability and death in the USA and we know these products are effective aids in helping people quit.”

The FDA’s request for the additional warnings is based on a review of reports submitted to the agency’s adverse event reporting system and on an analysis of information from clinical trials and scientific literature. These show that some people who have taken Chantix and Zyban have reported experiencing unusual changes in behaviour, become depressed, or had their depression worsen, and had thoughts of suicide or dying.

In many cases, “the problems began shortly after starting the medication and ended when the medication was stopped”, the agency stated, though some people continued to have symptoms after ending treatment. The FDA went on to say that neither Chantix nor Zyban contain nicotine and some of these symptoms may be a response to nicotine withdrawal.

The agency added that GSK and Pfizer willalso be required to conduct a clinical trial to determine how often serious neuropsychiatric symptoms occur in patients using their smoking cessation therapies, including patients who currently have psychiatric disorders. The FDA concluded by saying that its review of adverse events for patients using nicotine patches “did not identify a clear link between those medications and suicidal events”.

The companies do not believe there is enough evidence to prove a link between these serious mental problems and their drugs but have agreed to add the warnings to their labels.