Two studies published in the British Medical Journal confirm that birth control pills increase the risk of blood clots.
The two trials, one of which involved a U.S. population of women and the other a British cohort, found that women taking newer forms of oral contraceptives, which include drosperinone, a synthetic version of the hormone progesterone, increased their risk of developing a blood clot by two or three times over those taking older birth control pills that rely on a different form of the hormone known as levonorgestrel.
The blood clots were not fatal, and the absolute number of cases was small, but the results highlight what doctors have known ever since birth control pills were approved — that the mix of hormones needed to prevent pregnancy also promote clotting factors.
Newer oral contraceptives include Yaz, Yasmin and Ocella, and their makers all include warnings about increased risk of blood clots on the drugs’ labels. Bayer, which manufactures Yaz and Yasmin, said in a statement that the study had “significant flaws” and defended the safety of its product:
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.
Dr. Grace Lau, an obstetrician and gynecologist at New York University Langone Medical Center, said that while the absolute risk of developing a blood clot on the newer contraceptives was small — 30.8 per 100,000 women years compared with 12.5 events in the control group in the U.S. study — there is still an increased risk. “If a woman has been on Yaz and has had no problems with it, then I don’t have a problem continuing to write her prescriptions for it,” she says. “But for someone who hasn’t been on a contraceptive, as a provider I think about what would decrease their risk and I want the best for my patients. So this may not be the first thing I give them, since it may not be the best option they could possibly get.”
However, Lau stresses that the absolute risk for individual women is still low, so each woman should decide for herself which option is right for her. Birth control pills can cause other side effects, from spotting to cramps, and some women may respond better to the newer pills. Taking birth control, however, is not indicated for women who have a history of blood clots or are over 35 and smoke, since smoking increases the risk of clots.
