It's been known that compared with other women, those on birth control pills have a higher risk of gallstones and other gallbladder disorders, such as gallbladder inflammation.
So far, there have been about 3,000 lawsuits in the U.S. against the products' manufacturer, Bayer AG.
But until now, there had been no studies on whether gallbladder risks vary with different pill formulations, according to lead researcher Mahyar Etminan, an assistant professor of medicine at the University of British Columbia and a scientist at Vancouver Coastal Health Research Institute in Canada.
Despite that lack of hard evidence, he said, the lawsuits have generated "fear" among women about the risks of the newer birth control pills.
"I think at least on the issue of gallbladder disease, our study is reassuring," Etminan said.
The study, reported in the Canadian Medical Association Journal, looked at rates of gallbladder removal among more than 2.7 million U.S. women who used birth control pills between 1997 and 2009.
Overall, the risk of needing gallbladder surgery was not significantly higher among women on pills with newer versions of the hormone progestin -- including drospirenone, the progestin used in Yasmin and Yaz -- versus women on pills with an older progestin called levonorgestrel.
Of more than 495,700 levonorgestrel users, 1 percent had gallbladder surgery over an average follow-up period of about one year. The rate was the same among the more than 448,000 women on pills containing drospirenone.
When Etminan's team weighed other factors -- like obesity and smoking habits -- there was a small statistical increase in the risk of gallbladder surgery related to drospirenone and two other newer progestins, versus levonorgestrel.
But those small differences are unlikely to be "clinically significant," the researchers say.
"Strictly on the side effect of gallbladder disease with these drugs, it appears from our study that the risks are pretty much the same amongst different agents," Etminan told Reuters Health.
Other risks common to all birth control pills include blood clots, heart attack and stroke. Those side effects are rare, but relatively higher among smokers and women older than 35.
And some studies have found that the risk of blood clots appears higher with newer pill formulations, including Yasmin and Yaz, versus pills with levonorgestrel.
Obesity is one of the general risk factors for gallbladder disease, so one way to limit the risk with pill use is to maintain a healthy weight, according to Etminan.
He also suggested that women on the pill be aware of the potential signs of gallstones or other gallbladder problems, including severe abdominal pain and nausea.
Most cases of gallstones, however, do not cause symptoms and need no special treatment.
The study was funded by Canadian health agencies. Etminan and his co-authors report no ties to Bayer or any other conflicts of interest.
SOURCE: bit.ly/efCI9C CMAJ, online April 18, 2011.