The New York drug maker updated the product label for Chantix to more prominently display post-marketing reports of adverse events that had been added in November. Pfizer said Chantix users should be observed for "serious neuropsychiatric symptoms, including changes in behavior, agitation, depressed mood, suicidal ideation and suicidal behavior."

In November, the FDA said it was probing reports of erratic behavior, suicidal thoughts and other potential side effects of Chantix. The agency advised doctors to monitor Chantix users for behavioral or mood changes.

Pfizer said Friday some reports of adverse events may have been complicated by the symptoms of nicotine withdrawal in patients who stopped smoking, but not all patients with these symptoms had quit smoking. Some patients with pre-existing psychiatric illness experienced a worsening of their conditions.