U.S. Food and Drug Administration said that Pfizer Inc's anti-smoking drug Chantix seems to be linked to serious psychiatric symptoms, according to a FDA's public health advisory issued Friday.

FDA said after an analysis of cases of depression, suicidal thoughts and other unusual behavior in patients on the medication, evidence shows a strong association between these symptoms and Chantix.

Doctors have written 5 million Chantix prescriptions in U.S. since the drug to help smokers quit came on the market in 2006. Reports of psychiatric side effects have grown in recent months.

A total of 39 patients have committed suicide while taking the medicine. In addition to the suicides, the FDA said there have been 491 incidents of suicidal thoughts and behavior.

The agency said it has also received reports of agitation, depression and other mood changes.

"We've become increasingly concerned as we've seen there are a number of compelling cases that truly look as if they are the result of exposure to the drug," Bob Rappaport, a director in FDA's unit that oversees Chantix, said.

The FDA said it is working with Pfizer to develop a so-called medication guide highlighting the warnings for patients. A drug's label is typically directed at doctors.

Approved by FDA in May 2006 as a smoking cessation drug, Chantix acts at sites in the brain affected by nicotine and may help those who wish to stop smoking by providing some nicotine effects to ease the withdrawal symptoms and by blocking the effects of nicotine from cigarettes if users resume smoking.

(Agencies via Xinhua February 2, 2008)