The Food and Drug Administration announced Wednesday that Chantix and Zyban are two anti-smoking drugs that will now carry warnings about increased risks of depression and suicidal thoughts.

The FDA said patients who are using these smoking cessation drugs should not stop taking them due to the fears but should be closely monitored.

Dr. Curtis J. Rosebraugh, director of a drug evaluation office at the Food and Drug Administration 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.”

Apart from reports of suicide, the drugs have also been reported to cause a change in behavior and increased hostility and agitation in users irrespective of whether the users had a history of psychiatric illness or not.

In many cases the side effects were reported to begin just after beginning the drugs and ended once the medication was discontinued.

"The risk of serious adverse events while taking these products must be weighed against the significant health benefits of quitting smoking," said Dr. Janet Woodcock, director of the FDA's Center for Drug Evaluation and Research. "Smoking is the leading cause of preventable disease, disability and death in the United States, and we know these products are effective aids in helping people quit."

Chantix, made by Pfizer, and Zyban, made by GlaxoSmithKline, which also sells it under the brand name Wellbutrin, for depression have to undertake clinical trials to assess the mental health risks associated with the products use. Pfizer is already enrolling schizophrenic patients in a trial.

The FDA said Chantrix was approved in 2006 and since then there have been 98 suicides and 188 attempted suicides reported while Zyban has had reports of 14 suicides and 17 attempts.

Both the drugs are required to carry black box warnings and Pfizer said it updated its label in 2007 to include information of neuropsychiatric symptoms and has revised the label again to reflect Wednesday's warning.

Dr. Briggs W. Morrison, senior vice president of the primary care development group at Pfizer said, "Quitting smoking is one of the best things people can do for their health, but the quitting process is both difficult and complex. The labeling update underscores the important role of health care providers in treating smokers attempting to quit and provides specific information about Chantix and instructions that physicians and patients should follow closely."

Glaxo will expand its existing black box warning on Wellbutrin, citing suicidal thoughts by patients who use it for depression, to include Zyban.

No one knows why the drugs are associated with mental problems. The FDA said neither Chantix nor Zyban contains nicotine, and some symptoms could be the result of nicotine withdrawal.

“If this is nicotine withdrawal, it really doesn’t matter,” said Dr. Robert Temple, an F.D.A. official. “You need to pay attention to them.”