WASHINGTON  Federal drug regulators warned Wednesday that patients taking two popular drugs to stop smoking should be watched closely for signs of serious mental illness, as reports mount of suicides among the drugs’ users.

But officials emphasized that fear should not stop patients from taking the smoking-cessation medicines, Chantix, made by Pfizer, and Zyban, made by GlaxoSmithKline, which also sells it under the brand name Wellbutrin, for depression.

“Stopping smoking is a goal we should all be working towards,” said Dr. Curtis J. Rosebraugh, director of a drug evaluation office at the Food and Drug Administration. “We don’t want to scare people off from trying a medication that could help them achieve this goal. You should just be careful.”

Pfizer will add a so-called black box warning  the F.D.A.’s most serious caution  to the packaging information for Chantix.