updated 2/1/2008 4:33:01 PM ET 2008-02-01T21:33:01

Government regulators said Friday the connection between Pfizer's anti-smoking drug Chantix and serious psychiatric problems is "increasingly likely."

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The Food and Drug Administration began in November investigating reports of depression, agitation and suicidal behavior in patients taking the popular twice-daily pill.

The agency's announcement comes two weeks after Pfizer added stronger warnings to the drug. In doing so, the company stressed that a direct link between Chantix and the reported psychiatric problems has not been established, but could not be ruled out.

In a public advisory released Friday, FDA said patients taking Chantix should tell their doctor about any history of mental illness.

"Chantix may cause worsening of current psychiatric illness even if it is currently under control," reads the statement. "It may also cause an old psychiatric illness to reoccur."

FDA noted that patients with psychiatric problems were not included in the original studies used prove Chantix's safety.

Approved in May 2006, Chantix, already prescribed 4 million times in the U.S., has been one of the few bright spots on Pfizer's balance sheet. For full-year 2007, the drug had sales of $883 million, helping offset lower sales of older drugs, such as the antidepressant Zoloft, which face generic competition.

The tablets work by binding to nicotine receptors in the brain, reducing the symptoms of withdrawal.

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