Pfizer's stop-smoking drug Chantix can lead to a small increase in cardiovascular problems such as heart attacks for patients who already have cardiovascular disease, U.S. drug regulators said on Thursday.

The Food and Drug Administration is changing the label for Chantix after reviewing the results of a clinical trial.

An independent randomized trial of 700 smokers with cardiovascular disease who were treated with Chantix or a placebo showed that Chantix was effective in helping patients quit smoking for as long as one year.

However, patients who took the pill were also slightly more likely to have a heart attack or other adverse cardiovascular event versus patients on a placebo.

Many smokers who try to quit do so to prevent the risk of heart attacks, which may now be associated with Chantix.

"The known benefits of Chantix should be weighed against its potential risks when deciding to use the drug in smokers with cardiovascular disease," the FDA said in a statement.

The FDA said it is also requiring Pfizer to evaluate the cardiovascular safety of Chantix by conducting a large, combined analysis of randomized, placebo-controlled trials.
Pfizer's non-nicotine pill has already come under fire for psychiatric side effects, which have crimped global sales and prompted the FDA to issue a restrictive "black box" warning label for the drug.

Investors had high hopes for the drug -- called Champix in Europe -- when Pfizer first launched its smoking-cessation aid in 2006, but reports of serious side effects have prevented strong sales growth.

Annual sales are now about $800 million, making the pill a moderate-sized product for the world's biggest drug maker.

Chantix has been associated with agitation, depression and suicidal thoughts, and, in clinical trials, linked with nightmares. Psychiatric symptoms have occurred in people without a history of mental illness and have worsened in people who already had mental illness.