A few years go, a friend of mine was rushed to the hospital with a searing pain in her chest that was so intense, she thought she was having a heart attack. It turns out she had a blood clot, a side effect from the birth-control pill that’s more common in women over 35 who smoke (which she was, and she did) — one that can be fatal if it reaches your lungs.
Two recent case-controlled studies published in the British Medical Journal found that the type of pill you’re taking could also increase your risk of clots. The studies examined the records of thousands of women in the United States and UK who filed an insurance claim for oral contraceptives between 2002 and 2009. American researchers looked at women aged 15 and 44, and found the risk of non-lethal thromboembolism is about twice as high in women who take pills with the synthetic progesterone drospirenone, like Yasmin and Yaz, than it is for those whose pills contain levonorgestrel. And another study out of the UK that looked at women the same age found that the risk was three times as high.
That said, the risk of blood clots is still low for individual women on the pill — in the American study, the risk of non-fatal blood clots was 30.8 per 100,000 in the women taking the newer drospirenone pills and 12.5 per 100,000 in those taking levonorgestrel pills. The UK study found 23 per 100,000 for the newer pills, compared to 9.1 per 100,000 for the older pills. Women who are obese, smokers or older than 35 are generally at an overall higher risk for blood clots.
Bayer responded in a press release that “that the manner in which the authors applied the study methodology reported in these two publications and the databases used provide less reliable conclusions than are available from existing scientific evidence.” Bayer is facing lawsuits from women in the U.S. and Canada who say they were not properly warned about the risks of clots from birth-control pills; the lawsuits have not yet been argued in a court.