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The study, of 2 million Danish women, found what numerous others have before: Women who used birth control pills or other estrogen-containing contraceptives had a heightened risk of developing a blood clot in the legs or lungs.
But researchers found an additional layer. The risk of a blood clot was further increased during weeks when those women also used a nonsteroidal anti-inflammatory drug, or NSAID -- including such common painkillers as ibuprofen and naproxen.
Experts stressed that for any one woman, the risk of a blood clot is very low.
For one, an NSAID may be the best option for pain relief, said Denny, who was not involved in the study.
It's also important, she said, to keep the blood clot risk associated with birth control in perspective: It's much lower, for example, than the odds of developing a clot during pregnancy or in the few months afterward.
Birth control pills, rings and patches, as well as some intrauterine devices (IUDs), use hormones to prevent pregnancy. Some hormonal contraceptives -- particularly those with higher amounts of estrogen -- have long been linked to a small increase in the risk of blood clots. Estrogen can increase blood levels of certain proteins that form clots.
Enter the new study, published Sept. 6 in the journal BMJ.
Researchers analyzed medical records from 2 million women ages 15 to 49 who were living in Denmark between 1996 and 2017. During that period, just over 8,700 developed a first-time venous thromboembolism -- a blood clot either in a leg vein or the lungs.
Overall, the study found, NSAID use alone was linked to an increased blood clot risk, compared to periods where women did not use the painkillers. Similarly, that risk was elevated among women who used estrogen-containing birth control alone.
But when the two were combined, that blood clot risk showed a notable bump.
"The interesting part is that the risk with simultaneous use was larger than the sum of the risks with individual use," said study leader Dr. Amani Meaidi, a researcher with the Danish Cancer Society in Copenhagen.
"The absolute risk of venous thromboembolism is still low in women using both drugs," she stressed.
Researchers estimated that:
- Around 4 of 100,000 women would develop a blood clot per one week of NSAID use.
- Roughly 2 in 100,000 would develop a clot per week of using "high-risk" contraceptives. Those included estrogen/progestin patches and rings, and pills with high estrogen doses (50 micrograms).
- When women used both high-risk contraceptives and an NSAID, an estimated 23 of 100,000 would develop a blood clot per week of use.
The painkillers had no apparent effect on blood clot risk among women on low-risk contraceptives like progestin-only pills and IUDs.
Although blood clots are uncommon complication of some contraceptives, they can also be very serious, Meaidi pointed out.
She said women who often need to take an NSAID for pain issues may want to opt for a low-risk form of birth control.
But any woman who has concerns about blood clots can consider low-risk contraceptives, like an IUD or progestin-only pills.
"Luckily," Denny said, "we have plenty of options."
Planned Parenthood has an overview of birth control options.
SOURCES: Amani Meaidi, MD, PhD, Danish Cancer Society Research Center, Copenhagen; Colleen Denny, MD, director, family planning, NYU Langone Hospital Brooklyn, associate professor, obstetrics and gynecology, NYU Grossman School of Medicine, New York City; BMJ, Sept. 6, 2023, online
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