The Food and Drug Administration is adding a new warning to Pfizer's anti-smoking drug Chantix letting patients know the prescription can intensify the effects of drinking alcohol, sometimes leading to aggressive behavior or amnesia.

In a safety announcement published Monday, the FDA said, "Some patients experienced decreased tolerance to alcohol, including increased drunkenness, unusual or aggressive behavior, or they had no memory of things that happened."

The FDA also said it is keeping a strong "black box" warning on Chantix labels about suicidal behavior and other psychiatric side effects, after reviewing new data from the company. Last year Pfizer proposed that the FDA remove that warning based on the company's findings suggesting the drug does not increase those problems.

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The twice-a-day tablet has carried the FDA's strongest warning label since 2009, following reports of suicidal tendencies and violent or bizarre behavior among some patients.

The updated labeling also includes information from several studies and analyses conducted by Pfizer that found no difference in psychiatric problems between people taking Chantix and other stop-smoking treatments. But the FDA said Pfizer's research did not examine all types of psychiatric problems and had limitations preventing regulators "from drawing reliable conclusions."

The agency's update follows the recommendation of a panel of outside experts, who voted last October to keep the boxed warning on the drug until it can be reevaluated based on new information. New York-based Pfizer is expected to complete a larger study of Chantix's psychiatric side effects in late 2015.

The safety of Chantix (known generically as varenicline) has been debated in medical journals and courtrooms since 2007, when reports of suicide, agitation and other problems first began streaming into the FDA.

The drug's labeling currently tells patients to stop taking Chantix immediately if they experience agitation, depressed mood, suicidal thinking and other behavioral changes. Doctors are advised to weigh the drug's risks against its potential benefits of helping patients quit smoking.

Pfizer's drug works by binding to the same spots in the brain that are activated by nicotine when people smoke. The drug blocks nicotine from binding to those spots and prevents the release of "feel-good" brain chemicals that make smoking so addictive.

The Centers for Disease Control and Prevention reported recently that fewer American adults are smoking cigarettes than ever. The smoking rate has dropped from about 21 percent in 2005 to 18 percent in 2013. That adds up to about 42.1 million Americans who still smoke.