The US Food and Drug Administration (FDA) said it has issued an early warning about Pfizer's smoking-cessation drug, Chantix, amid reports of erratic behaviour and suicidal thoughts.

FDA is currently evaluating post marketing adverse event reports for Chantix (varenicline), a prescription medicine to help adults stop smoking.

Based on FDA's request for information from the manufacturer, Pfizer, Inc., the company recently submitted reports to the agency describing suicidal ideation (thoughts). In the wake of a case report citing erratic behaviour in an individual who had used Chantix, FDA has also asked the company for any information on additional cases that may be similar in patients who have taken the drug.

The FDA said it's Center for Drug Evaluation and Research is working to complete an analysis of the available information and data. When this analysis is completed, FDA will communicate the conclusions and recommendations to the public.

A preliminary assessment reveals that many of the cases reflect new-onset of depressed mood, suicidal ideation, and changes in emotion and behaviour within days to weeks of initiating Chantix treatment. The role of Chantix in these cases is not clear because smoking cessation, with or without treatment, is associated with nicotine withdrawal symptoms and has also been associated with the exacerbation of underlying psychiatric illness. However, not all patients described in these cases had pre-existing psychiatric illness and not all had discontinued smoking.

FDA is aware of a highly-publicized case of erratic behaviour leading to the death of a patient using Chantix to attempt to quit smoking. Although other factors, including alcohol consumption, appear to have played a part in this specific case, FDA asked Pfizer for additional cases that might be similar. We are currently evaluating the material Pfizer submitted in response to our request, said the FDA report.

In the meantime, FDA recommends that health care providers monitor patients taking Chantix for behaviour and mood changes. Patients taking Chantix should contact their doctors if they experience behaviour or mood changes.

FDA also advises that, due to reports of drowsiness, patients should use caution when driving or operating machinery until they know how using Chantix may affect them.