Women who smoke have a higher risk of developing coronary heart disease than men, a large international study finds.

The study published online in the Lancet Thursday, collected data on more than two million people and concluded that the increased risk of heart disease tied to smoking was 1.25 times higher for women.

The longer a women smoked, the greater her heart disease risk compared with a man who smoked for the same time period, researchers said.

The lead authors were Dr. Rachel Huxley, from the University of Minnesota and Dr. Mark Woodward, from Johns Hopkins University in Baltimore, Md. They received no funding for their research.

The finding that among smokers, the excess risk of heart disease in women compared with men increases by two per cent for every year lends support to the possibility that physiological differences between men and women make a difference, the researchers said.

For example, women might receive more carcinogens or toxins from smoking the same number of cigarettes than men, the investigators speculated.

"Whether mechanisms underlying the sex difference in risk of coronary heart disease are biological or related to differences in smoking behaviour between men and women is unclear," the study's authors concluded.

"Tobacco-control programs should consider women, particularly in those countries where smoking among young women is increasing in prevalence.

The study was large and examined a diverse range of populations worldwide, with consistent findings, Huxley and Woodward said.

They also acknowledged limitations of the research, such as an inability to take use of birth control pills into account in the analysis. Studies also defined non-smokers differently, with some including those who had never smoked and others defining them as not-current smokers.

Tobacco companies targeting women

In most societies, smoking rates are higher for men than for women, but more men than women are quitting, wrote Dr. Carolyn Dresler journal in a commentary that accompanies the research paper.

"What makes the realization that women are at increased risk worrisome is that the tobacco industry views women as its growth market," said Dresler of the Tobacco Prevention and Cessation Program at the Arkansas Department of Health, in Little Rock.

Tobacco companies are increasingly targeting women with slim brands and slick packaging, said Ellen Mason, senior cardiac nurse at the British Heart Foundation.

Mason suggested introducing plain packaging to increase the effectiveness of health warnings and to reduce the appeal of tobacco products.

Think of quitting

In 2010, about one in five women in Canada smoked, according to Health Canada. Among female smokers in 2002, less than half indicated that they were even considering quitting.

After quitting for one year, the risk of coronary heart disease drops by about half, and quitting has many other health benefits, said CBC medical specialist Dr. Karl Kabasele, a public health physician in Toronto.

Even if a woman is thinking of quitting, Kabasele suggested she talk to her doctor about the many tools and support groups that are now available. Many are covered by provincial health plans.