The U.S. Food and Drug Administration has added heart problems to the growing list of concerns about the smoking cessation pill Chantix.

The Pfizer drug may increase the risk of heart attacks among those who already have cardiovascular disease, according to a statement released by the agency.

FDA officials said they will change the label to reflect the cardiac risk following a trial involving 700 smokers with cardiovascular disease who took Chantix or a placebo. Although the drug was shown to be effective in helping patients quit smoking for as long as a year, those who took it were also slightly more likely to suffer heart problems.

The FDA said it was important to weigh the known benefits of Chantix against its potential risks given that “smoking is an independent and major risk factor for cardiovascular disease, and smoking cessation is of particular importance in this patient population.” The agency has asked Pfizer to conduct additional research on the drug’s cardiovascular effects.

The news announced on Thursday comes on the heels of another report by a watchdog group that found Pfizer failed to adequately alert federal authorities to more than half of reported suicides of patients who took Chantix. The Institute for Safe Medication Practices report found the company neglected to file 589 cases of serious psychiatric effects, including 150 suicides, through the electronic system intended to bring such incidents to the prompt attention of the FDA.

In December, FairWarning reported that Pfizer had failed to test the drug on mentally ill smokers, even though a disproportionate number of them suffer from some form of psychiatric illness, and they are a key part of the target market for smoking cessation treatments.

The drug, approved in May 2006, was later given a black box warning, the FDA’s strongest form of safety alert.

According to the Associated Press, sales of Chantix totaled $755 million last year. Most of the drug’s new sales are in markets outside the U.S., where warnings about the drug have been less prominent.

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