Healthy, middle-aged smokers who take Pfizer's Chantix or Champix, one of the most popular quit-smoking drugs on the market, have a higher risk of suffering heart attacks or other serious heart problems, a study found on Monday.

British and American scientists analyzed 14 clinical trials of Champix, sold as Chantix in the United States and known generically as varenicline, and found the likelihood of developing serious heart problems resulting in hospitalization, disability or death was almost 72 percent higher in patients taking the drug compared with those taking a placebo.

The researchers urged U.S. drug regulators, who have already issued warnings about Chantix's safety in certain patient groups, to take note of these new findings.

"I think our new research shifts the risk-benefit profile of varenicline," said Sonal Singh of Johns Hopkins University School of Medicine, who led the research and published it in the Canadian Medical Association Journal.