Studies Suggest Higher Risk Of Blood Clots With Some Birth Control Pills

A couple of papers just published by the British Medical Journal add to the evidence that the latest type of birth control pills may confer a higher risk of blood clots than older kinds of oral contraceptives.

The newer pills — including Yaz and Yasmin — contain a synthetic hormone called drospirenone. Many older contraceptives rely on levonorgestrel, another type of the progestin hormone.

As NPR's Richard Knox reported last year, Yaz quickly became the best-selling birth control pill in the U.S. after its introduction in 2006. More than

Here's a recap of the latest data.

In an analysis of a sample of U.S. insurance claims, the researchers found that women taking contraceptives with drospirenone had more than twice the risk of developing blood clots than those taking contraceptives that used levonorgestrel. The rates of blood clots (as calculated using the number per years of 100,000 women taking the medicines) was 30.8 for drospirenone and 12.5 for levonorgestrel.

A second study looked at cases of blood clots in British women who started taking an oral contraceptive containing either the new kind of progestin or the older one. The researchers found women taking the drosperinone contraceptives had a 2.7 times greater risk of developing a blood clot than those taking the older type. The rates for clots were 23 vs. 9.1 (again calculated using the clots per years of 100,000 women taking the medicines).

So what does it all mean? The increased risk suggested by these studies is about two to three times those from older pills, but the chances that any particular woman taking the newer medicines will get a blood clot is still pretty low.

Still, the studies aren't definitive, as editorial in BMJ notes. Women concerned about the issue could consider other, older options, if there's no medical reason to rule them out.

In a statement, Bayer, maker of Yaz and Yasmin, criticized the studies as flawed and affirmed the safety of its pills. "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.

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