The Food and Drug Administration (FDA) said Pfizer's anti-smoking drug called Chantix needs to change its labeling in order to warn patients that using the medicine can slightly increase the risk of heart problems for those who already suffer from cardiovascular disease.

The study looked into 700 smokers with heart related diseases who were being treated with Chantix. The study has discovered the drug, also called varenicline, was linked to higher risk of heart attack, clogged arteries and angina in number of patients compared to patients who took a placebo. Placebo is a substance that has no therapeutic effect, used as a control in testing new drugs.

However, the agency said that the drug did help patients quit smoking and that this benefit should be weighed against its potential risks when deciding to use the drug in smokers with cardiovascular disease.

Further details of the new results will be shared with doctors and will be shown in the drug's medication guide to patients, the agency said. FDA also said they will continue to evaluate the pros and cons of using the medicine.

Pfizer mentioned that patients should consult with their doctors to decide what medications are right for them.

The drug was first approved in May 2006 and it has been used by millions of smokers in the U.S. However the sales have decreased since 2008 when the drug was first associated to psychological side effects such as depression and suicidal thinking.

Pfizer Inc. reported sales of $755 million for Chantix in 2010.