New evidence emerged on Friday that women taking Bayer's best-selling contraceptive Yasmin may run a higher risk of dangerous blood clots than those using older birth-control pills.

Two studies showed that users of pills containing drospirenone, one of the hormones used in Yasmin, had a two to threefold higher risk of venous thromboembolism than did users of those containing the older synthetic hormone levonorgestrel, according to British Medical Association's bmj.com site.

In the study cited by bmj.com, based on U.S. medical claims data, researchers found a twofold increased risk of non-fatal venous thromboembolism in women using drospirenone-containing contraceptives compared with women using levonorgestrel.

Thromboembolis is the medical term for a blood clot in the veins, often in the legs, that gets dislodged and can cause fatal clogging in the lungs' arteries.

The second study, based on data from the British General Practice Research Database, even found a threefold increased risk.

Still, the researchers described the overall risk as low, with rates for blood-clotting cases in U.S. study of 30.8 per 100,000 women years on drospirenone versus 12.5 in the control group.

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Bayer said in a statement that the methodology used showed "significant flaws."

"Given the already large and robust scientific body of evidence, in Bayer's opinion, these studies do not change the overall assessment about the safety of Bayer's oral contraceptives," the company said, citing 10 years of safety study results since the pill came to market.

Studies assessing the blood clotting risk of drospirenone had yielded inconsistent results in recent years with some studies showing elevated risks and others suggesting drospirenone was as safe as levonorgestrel.

In March 2010, Bayer put additional risk warnings on the European product label for the Yasmin pill but said at the time that the overall benefit-risk profile remained unchanged.

Bayer said in its 2010 annual report that there were about 6,850 lawsuits pending in the United States with plaintiffs claiming they had suffered injuries from Bayer's Yasmin and Yaz pills or generic copies sold by Teva's Barr Laboratories.