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The new generation of birth control pills - which includes popular products like Yaz and Yasmin - increases the risk of gallbladder disease only marginally, a new Canadian study concludes.

The research, published in Monday's edition of the Canadian Medical Association Journal, challenges popular wisdom on the issue.

"The media has created some hysteria about this based on anecdotal reports, but our study shows the risk of gallbladder disease is pretty much the same for all oral contraceptives," Mahyar Etminan, of the Centre for Clinical Epidemiology and Evaluation at Vancouver Coastal Health Research Institute, said in an interview.

The new research involved 2.7 million women who took birth control pills for at least six months between 1997 and 2009. A total of 27,087 women in the study group underwent cholecystectomy (surgical gallbladder removal). Taking birth control pills is known to increase the risk of gallbladder disease, as does hormone replacement therapy.

The new study was designed to determine whether the risk varies substantially based on the chemical make-up of various drugs.

Birth control pills all contain a synthetic form of estrogen (usually ethinyl estradiol) and a synthetic form of progesterone called progestin; they come in various forms that distinguish products.

Researchers at the University of British Columbia and McGill University examined seven forms of birth control pills and found that their use increased the relative risk of women developing gallbladder disease between 5 and 20 per cent.

The highest increase in risk was seen in oral contraceptives that contained a form of progestin called drospirenone.

In Canada, these products are sold under the brand names Yaz and Yasmin, and marketed by Bayer Inc. of Toronto.

Bayer faces the threat of class-action lawsuits in both Canada and the U.S. from women who claim they were not adequately warned of higher risks of developing gallbladder disease in using these products. The link is also a popular subject of discussion on web forums.

The drug maker disputes the allegations. "Bayer's oral contraceptives have been and continue to be extensively studied worldwide and are safe and effective when used according to the product labelling," the firm said in a statement.

Dr. Etminan said the research is no way related to the legal dispute, but offered up his analysis of the data.

"Statistically, there is a slightly higher risk to women using an oral contraceptive containing drospirenone but, in the grand scheme of things, we don't think it's clinically significant," he said.

Dr. Etminan noted that it is exceedingly difficult to tease out information about specific oral contraceptives, because women routinely switch products, or discontinue and resume use. Only 11 per cent of women in the study used the same birth control continuously for more than two years.

The new research was funded by the Fonds de la recherche en santé du Québec, a government-financed research agency. "I want to be clear this wasn't funded by Bayer or lawyers or anything like that," Dr. Etminan said.

The gallbladder is a pear-shaped sac located under the liver that stores the bile secreted by the liver. Abnormal composition of bile leads to the formation of gallstones - small pebbles. Most people who develop gallstones don't have symptoms, but when the stones cause a blockage, a patient can develop severe abdominal pain or infections, and physicians usually recommend surgery.

Both estrogen and progesterone increase the risk of gallstones. Estrogen increases the production of cholesterol in the liver, which bolsters bile production and, in turn, leads to the formation of gallstones. Progesterone impedes bile flow and also leads to gallstone formation.

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