Federal regulators, worried about potential links to suicides and hostile behavior, ordered tough new warning labels for Chantix and Zyban, the two most popular prescription smoking-cessation pills.

The move by the Food and Drug Administration could discourage smokers who want to quit the habit since the two drugs are widely considered among the most effective antismoking medications.

Chantix, made by Pfizer Inc., has been dogged by concerns that the once widely popular medicine is associated with suicidal behavior, mood swings, seizures, accidents, and most recently to an unusual skin allergy.

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