The Food and Drug Administration announced today that it is requiring the smoking-cessation drugs Chantix and Zyban to carry the strongest type of safety warning possible to alert patients that the medications can cause serious mental health problems, including depression and suicide.

The agency said it took the action requiring "boxed warnings" after finding a surprisingly high number of reports of problems involving changes in behavior among people taking the medications, including depression, hostility, suicidal thoughts and attempted and successful suicides. The agency had received 98 reports of suicide among patients taking Chantix and 188 reports of attempted suicide, and 14 suicides and 17 suicide attempts among patients taking Zyban, officials said.

Agency officials said they were unsure why the drugs would cause these problems and noted that many of the symptoms can occur from nicotine withdrawal as well. The agency is requiring the companies that make the drugs to conduct follow-up studies to try to explore how commonly mental health problems occur.

The agency previously had issued warnings about mental health problems among patients taking Chantix, and required similar warnings on Zyban when it is sold as Wellbutrin and in generic forms for use as an anti-depressant.

The move is the latest in a serious of actions the FDA has taken after becoming concerned about psychiatric side effects of medications, including anti-depressants. The agency recently issued similar warnings about drugs used to treat epilepsy.

The FDA stressed that it was not advising people against using the medications because smoking causes serious health problems and quitting smoking is important. But patients taking the drugs and their doctors need to be aware of the potential problems and monitored closely, officials said.