An increase in the number of suicides among people attempting to stop smoking using the smoking-cessation medicines Chantix and Zyban has prompted US regulators to issue a warning and require stronger worded messages on packaging.

Federal drug regulators have instructed Pfizer, the manufacturer of Chantix and GlaxoSmithKline’s Zyban products be labelled with so-called black box warnings — the agency’s most serious caution — on the prescribing information for both drugs.

Both companies will also be required to conduct clinical trials to assess the mental health risks associated with the drugs’ uses.

European officials first alerted the US Food and Drug Administration (FDA) in 2007 to problems associated with Chantix after Carter Albrecht, a keyboard player from the pop-music group Edie Brickell and the New Bohemians, was killed by a neighbor who complained that Mr. Albrecht was banging on his door, ranting.

Mr. Albrecht’s girlfriend blamed Chantix, which she said had made him hostile.

The widely publicized event led to a cascade of similar reports and close scrutiny by FDA safety officials, who have now received 98 reports of completed suicides and 188 reports of attempted suicides among those taking Chantix.

Officials examining the drugs found that Zyban has similar associated risks. The agency received 14 reports of suicides and 17 reports of attempted suicides among those taking Zyban.

No one knows why the drugs are associated with mental problems. In some cases, patients could be experiencing nicotine withdrawal, but some of the reports involved patients who had yet to stop smoking.

Chantix and Zyban join a growing list of medicines that may be associated with suicidal thoughts and behaviors, including anti-depressant, anti-seizure and some antibiotic medicines.

Zyban and its generic equivalents, known as bupropion, already had a suicide warning on their prescribing labels because of the drug’s use as an antidepressant known as Wellbutrin.

Dr. Curt Rosebraugh, director of a drug evaluation office at the FDA, said, “stopping smoking is a goal we should all be working towards”.

“We don’t want to scare people off from trying a medication that could help them achieve this goal. You should just be careful,” he added.