Some Birth Control Pills Safer Than Others

European studies find types of hormones used raise or lower clot risk

By Ed Edelson
HealthDay Reporter

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THURSDAY, Aug. 13 (HealthDay News) -- The composition of a woman's contraceptive pill influences her risk of developing blood clots of the leg and lung, European researchers say.

Scientists have long known that oral contraceptives, which contain the female hormones estrogen and progestogen, increase the likelihood of deep vein thrombosis of the leg and pulmonary embolism, but new studies in Denmark and the Netherlands determined that some pills are safer than others. Neither study received funding from any companies that make oral contraceptives.

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Pills containing a second-generation progestogen -- levonorgestrel or norgestrel -- and a low dose of estrogen are safest, they concluded.

The overall risk of venous thromboembolism is low, perhaps three for 10,000 woman-years for women in general, said Dr. Ojvind Lidegaard, a professor of obstetrics and gynecology at the Rigshospitalet in Copenhagen, and lead author of one of two reports in the Aug. 14 online issue of BMJ. But older contraceptive pills double or triple that risk, and the newest generation of oral contraceptives increase the risk fourfold or fivefold, he said.

A first message from the studies is that "the risk when you are taking an oral contraceptive depends on both the estrogen dose and the progestogen dose, and the lower the dose, the less risk," Lidegaard said.

Both studies found that the risk decreases with the length of time a woman takes a combination pill, and that progestogen-only pills and the use of hormone-releasing intrauterine devices are not associated with an increased risk.

The Dutch study, which compared 1,524 women under the age of 50 who had deep venous thromboembolisms with 1,760 women with no such history, found a fivefold increased risk in oral contraceptive users. The risk of an event was highest in the first three months of use and lowest with pills containing levonorgestrel.

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