The FDA started an investigation on the smoking cessation aid, Chantix, in November, after it had received reports of 37 suicides and more than 400 cases of suicidal behavior among patients taking the popular pill. Unlike other antismoking drugs such as nicotine gums, patches, and lozenges, which are only nicotine substitute when the patients quits, Chantix acts directly on brain centers affected by nicotine. 

"As the agency's review of the adverse event reports proceeds, it appears increasingly likely that there may be an association between Chantix and serious neuropsychiatric symptoms," the FDA says in a press release.

FDA released a “Public Health Advisory” Friday afternoon, telling that patients taking Chantix should inform their doctors about any mental diseases they may have had in the past, and advised family members of those under Chantix treatment to watch for any changes in mood and behavior. "Chantix may cause worsening of current psychiatric illness even if it is currently under control.” "It may also cause an old psychiatric illness to reoccur,” FDA reports.

The FDA noted that symptoms such as anxiety, nervousness, tension, depressed mood, unusual behaviors, and thinking about or attempting suicide, could develop both during Chantix treatment, but sometimes following withdrawal of Chantix.

Patients also may experience strange dreams, inability to drive or handle heavy machinery.

FDA added that patients with psychiatric problems were not included in the initial studies used to test Chatix’ safety.

Pfizer declared that the neuropsychiatric symptoms manifested in some patients under Chantix treatment, may also be the result of nicotine withdrawal.

Chantix was approved in 2006, and since then it has been prescribed to approximately 4 million patients in the United States to help them quit smoking.