FDA to Pfizer: Toughen Warning on Chantix

U.S. regulators on Friday said Pfizer'santi-smoking drug Chantix appears increasingly likely to be linked to serious psychiatric behavior, and called for stronger label warnings.

The Food and Drug Administration said after an analysis of reports of suicidal thoughts and behavior potentially linked to the medication, it was calling for Pfizer to strengthen the label warnings on the drug.

FDA said the drug is effective but that patients should talk to their doctors about this new safety information.

The drug has been prescribed to about 4 million people in the United States since approval in 2006, according to Pfizer. (See Mike Huckman's on-air report at left.)

Shares of Pfizer were up marginally Friday after the Chantix news broke, but were off their highs for the day.