LONDON (Reuters) - 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.

"People should be concerned. They don't need Chantix to quit and this is another reason to consider avoiding Chantix altogether," he said in a statement. "People want to quit smoking to reduce the risk of cardiovascular disease, but in this case they're taking a drug that increases the risk for the very problems they're trying to avoid."

Investors had high hopes for Chantix when Pfizer first launched it in 2006, but reports of suicidal thoughts and other mental health problems in users led Food and Drug Administration (FDA) officials to order a "black box" warning on the drug's label in 2009.

Chantix, which reduces both the craving for and pleasurable effects of cigarettes, is used by heavy smokers who find it difficult to quit. It is one of the biggest-selling stop-smoking drugs in the United States, and has more than 70,000 prescriptions every month in Britain.

Annual sales of around $800 million make Chantix a moderate-sized product for Pfizer, the world's biggest drug maker.


In the study, Singh's team reviewed and analyzed 14 trials involving more than 8,200 healthy people who were given either Champix or a placebo, or dummy pill.

Whereas the number of people who died in each group was the same, at seven, the increased risk of a major harmful cardiovascular event requiring hospitalization was 72 percent among those taking Champix/Chantix. None of the studies followed people for longer than a year. The average age of study participants was less than 45 years and the majority were men.

Yoon Loke of Britain's University of East Anglia, who co-led the research, said that while the number of serious heart problems was low, at around 1 percent, it was worth noting that most of the studies were carried out in healthy people.

"These are life-threatening diseases and so any increased risk should be carefully avoided -- particularly as heavy smokers are already susceptible to cardiovascular disease," Loke said in a statement.

Loke said the results suggested that in smokers who do have a history of heart disease, an estimated 1 in 28 would experience extra heart problems if they used Champix for a year.

Smoking kills up to half of those who do it and is predicted to claim up to 8 million lives a year by 2030 if current trends persist.

It causes lung cancer, which is often fatal, and other chronic respiratory diseases, and is also a major risk factor for cardiovascular diseases -- the world's number one killers.

The FDA said last month it was changing the Chantix label to make clear that it carried an increased heart risk for people who already have cardiovascular disease.

In May, France said it was removing Champix from a register of reimbursable treatments on state social security funds after questions were raised about the drug.

(Reporting by Kate Kelland; editing by Sophie Walker)