Birth control may be increasing women’s chance of developing breast cancer by as much as 38 percent. The link with cancer risk exists not only for older generations of hormonal contraceptives but also for the products that many women use today, according to a paper published Wednesday in the New England Journal of Medicine. The study used an average of 10 years of data from more than 1.8 million Danish women.
These results sound scary at first. After all, it means that nearly a quarter of American women are doing something that might increase their risk of developing breast cancer by a third—in theory. However, in practice the picture is far more complex.
Overall, the study found that women who used birth control had a 20 percent increase in their relative risk for developing breast cancer. However, that number varied depending on how long women had used their particular method. Women who had used hormonal birth control for less than a year had only a 9 percent increase in their relative risk, while women who had used birth control for more than 10 years had a 38 percent increase.
Compared to what the group of researchers found in one of their other papers—that using hormonal contraception was associated with a 300 percent increase in suicide risk—"it is a modest increase,” said Dr. Øjvind Lidegaard, one of the authors of the paper and a gynecologist at the University of Copenhagen. (“Some people ask me, 'we have the impression that you hate hormonal contraception,'” he said, given his history of publishing papers highlighting associated risks. He assured Newsweek that was not the case.)
In fact, birth control increases breast cancer risk about as much as drinking alcohol does, said Dr. Mary Beth Terry, an epidemiologist at the Columbia University Mailman School of Public Health. Relative to the increased risk posed by other environmental factors, like smoking for lung cancer—that's about a 10 times greater risk—and having a human papillomavirus infection for cervical cancer—that may increase risk about 50 or 60 times—38 percent really isn't that much. "The range of risks we're talking about here is much much smaller," she said.
Lindegaard speculated that the hormones in birth control may trigger certain cells that are ready to turn into cancer, he said, given that the risk seems to increase after only a few months of use. Dr. Charles A. Leath, a gynecologic oncologist at the University of Alabama at Birmingham, said that while this pathway was plausible, it was far from certain.
What those numbers mean in terms of actual women getting breast cancer who otherwise may not have is a bit less striking: there was about one extra breast cancer case diagnosed for every 7690 women who used hormonal contraception for a year.
For some perspective, about 252,710 American women were diagnosed with breast cancer in 2017, according to estimates from the National Institutes of Health; 12.4 percent of women will hear the diagnosis at some point in their lives. About 40,000 women died of breast cancer in 2017.
The paper did not make any note of whether birth control impacted mortality from breast cancer, Leath noted.
However, Lidegaard noted, pretty much everything in life carries risks and women know that. “[Contraceptives] also brings benefits, and we should not forget them. But we should make an individual assessment—doctor and a woman, together—to see what is the most appropriate thing for her to use.”
Of course, finding a safe and effective form of birth control is more than just a personal concern. Unintended pregnancies cost the U.S. government $21 billion in 2010, according to a report from the Guttmacher Institute.
“For many women, hormonal contraception—the pill, the patch, the ring, IUDs, and the implant—is among the most safe, effective and accessible options available,” said the American College of Obstetricians and Gynecologists’s vice president of practice, Dr. Chris Zahn. “As with any medical intervention, hormonal contraception is associated with specific health risks.” (Pregnancy, Zahn noted, carries its own set of risks.)
And not only are there a wide variety of other factors that can influence an individual’s risk of breast cancer—including certain genetic mutations and their family history—but using hormonal birth control may also be associated with a decreased risk of other kinds of cancers. According to an editorial that accompanied the study in NEJM, birth control may actually be protective against cancer on the whole despite this increased risk for one type.
One thing reiterated by every doctor Newsweek spoke to: Women who are worried about how their contraception might increase their risk of breast cancer should speak with their health care provider.
More from Newsweek