The FDA-sponsored studies find that Chantix does not increase a person's risk of psychiatric hospitalization. But the studies "do not rule out an increased risk of other neuropsychiatric events with Chantix," the FDA says in a statement.
This means that warnings on the current Chantix label are strong enough for the FDA.
"The Agency continues to believe that the drug's benefits outweigh the risks and the current warnings in the Chantix label are appropriate," the FDA says.
In 2009, the FDA required Chantix -- and Zyban, a different kind of stop-smoking drug -- to warn users of the risk of "serious psychiatric symptoms." These include behavior changes, hostility, agitation, depressed mood, suicidal thoughts, and attempted suicide.
More recently, the FDA warned that Chantix may add to the risk of heart attack in people with underlying heart disease.
The FDA cited two studies. In the first, the Veterans Administration compared over 14,000 Chantix users to people trying to quit smoking by using a nicotine patch or other nicotine replacement therapies (NRTs). In the second, the Department of Defense compared nearly 20,000 Chantix users to some 16,000 NRT users.
Overall, the studies found no increased risk of psychiatric hospitalization in Chantix users. However, the Department of Defense study followed patients for only 30 days. And neither study looked at psychiatric symptoms that were not serious enough to require hospitalization.
Pfizer, which makes Chantix, is conducting a large safety study to look more closely at the drug's psychiatric risks. Results of that study should be announced in 2017.
Chantix works by partially blocking nicotine receptors in the brain. This takes away much of the rewarding effect of smoking. Chantix also mimics the effects of nicotine on the brain, although to a lesser extent than nicotine itself.