It was previously believed that the combination of prescription anticoagulants (blood thinners) and hormone replacement therapy increased women’s risk of blood clots. However, a new study published in the journal Blood suggests the combination is safe.

"While it has been common practice among health care providers to avoid prescribing hormone therapy and anticoagulants at the same time, there has been no evidence to support this decision," said senior author Dr. Ida Martinelli, of the A. Bianchi Bonomi Hemophilia and Thrombosis Center in Milan, in a press release. So her research team set out to address these concerns, which have been "felt by both the physician and patient when making the decision to stop or continue hormone therapy in this setting."

For the study, the researchers looked at 1,888 women aged 60 and younger between 2007 and 2011. Of these women, 475 were using hormone therapy medications, like estrogen-only pills, combined estrogen-progesterone contraceptives, or progestin-only contraceptives. The rest were only taking blood thinners. The women were asked about uterine bleeding and blood clots, and subsequently followed up three, six, and 12 months later.

The results revealed women who took blood thinners — like Coumadin, Xarelto, or Jantoven — and hormone therapy together did not experience an overall increased risk of clotting or uterine bleeding. In fact, these women experienced 1 percent fewer blood clots than those who were not on the concurrent therapy. Meanwhile, 22.5 percent of women taking both therapies experienced uterine bleeding, compared to 21.4 percent of women who only took blood thinners.

Blood clots tend to form inside small veins near the surface of the skin as well as veins deeper within the body, which cause a condition called deep vein thrombosis. Not only can they be painful, but they also occasionally break away from their original location and travel to the lungs or the heart. When this happens, the blood clot can become life-threatening. Anticoagulants are commonly prescribed in these instances, however, doctors have been cautious to prescribe them when the patient is also on hormone therapy. These actions had become common practice despite there being no clinical evidence, Martinelli said.

This new information is important for women because it allows them to safely take the concurrent therapy without risk, thus “providing women the freedom to choose the method of birth control and other hormone-containing medications they prefer,” Martinelli said, adding that research will continue. In the meantime, she hopes her research will help “clinicians to confidently treat their patients who take blood thinners and hormones concurrently.”

Source: Martinelli I, Lensing A, Middeldorp S. Recurrent venous thromboembolism and abnormal uterine bleeding with anticoagulant and hormone therapy use. Blood. 2015.