The U.S. Food and Drug Administration is investigating whether a Pfizer Inc. drug has any connection with suicidal thoughts and violent behavior among its users.

Chantix, a drug used to help smokers quit cigarettes, gained its approval last year and generated about $600 million in sales so far, according to the MSNBC website.

FDA revealed on Tuesday that it had received reports of mood disorder and erratic behavior among patients using Chantix. They have already advised doctors to closely supervise their patients and report whether they have any behavior changes. The federal agency also said that the changes have often been reported within days or weeks of people first taking the drug.

Pfizer said in response that it added information about the reports to the product’s label. However, it stressed that there is no scientific evidence that could correlate Chantix and these reports and that there were no cases of suicide among 5,000 patients who had participated in a study of the drug.

FDA announced that it is investigating at least one incident of a patient who died while being under Chantix treatment. Musician Carter Albrecht died shot in the head in September by his girlfriend’s neighbor while he tried to kick down the man’s door. His family said he had been under Chantix treatment for several years and the drug might have caused his rage. His autopsy report revealed that Albrecht’s blood alcohol was three times the legal limit, instead.

FDA asked Pfizer to report any information on similar cases. It also added that there is no clear connection between Chantix and behavioral changes, since lack of nicotine seems to often aggravate existing mental problems in people who quit smoking.

New York-based Pfizer also submitted reports to the FDA about any symptoms of drowsiness among people using Chantix, especially whether the drug might influence their ability to drive. However, Pfizer said that there is no significant difference in drowsiness between patients using Chantix and those taking a dummy pill, according to clinical studies of the drug.