Blood clots known as deep vein thromboses have been linked to newer types of oral contraceptives--the so-called third-generation birth control pills.

These pills differ from older versions in the type of progestin they contain. Progestin is a female hormone, and oral-contraceptive combinations contain some form of it plus a form of estrogen.

Cyproterone, which acts like progestin, has also been linked to a higher blood clot risk in small studies. Because cyproterone acts against male hormones called androgens, oral contraceptives containing the drug have been given to women with serious acne or abnormal hair growth--both androgen-related. Cyproterone is not available in the US.

To confirm a link between the drug and blood clots, US researchers looked at data on nearly 100,000 women taking oral contraceptives. They found that while blood clots were rare, cyproterone-containing pills carried a four-times higher risk compared with pills containing the progestin levonorgestrel. Levonorgestrel is found in older contraceptives known as second-generation birth control pills.

Catherine Vasilakis-Scaramozza and Dr. Hershel Jick, of Boston University School of Medicine in Lexington, Massachusetts, report their findings in the October 27th issue of The Lancet.

The researchers looked at data on more than 24,000 women with prescriptions for low-dose estrogen contraceptives containing cyproterone and 75,000 women who had received levonorgestrel-containing pills between 1992 and 1999.

From 1992 on, 26 women suffered confirmed or ``probable'' deep vein thrombosis. When the investigators considered other risk factors such as body mass and smoking, they found that cyproterone was linked to a fourfold increase in the risk of blood clots.

Deep vein thrombosis, also known as venous thromboembolism, occurs when blood clots form in the veins. In some cases, these clots may travel through the bloodstream and lodge in the lungs, where they can cause a potentially deadly condition called pulmonary embolism. Ten women in this study had a pulmonary embolism.

``Our study results,'' the authors conclude, ``accord with those of previous small studies, which have shown an association between use of oral contraceptives containing cyproterone and increased risk of venous thromboembolism.''

SOURCE: The Lancet 2001;358:1427-1429.

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