For people who want to prevent pregnancy, birth-control drugs come with an obvious reward. They also come with some small risks—including, for some forms, a slightly increased risk of breast cancer.
This has long been known about the most popular type: combined hormonal birth control, which is available in pill, patch, and ring form. These all contain both estrogen and progestin, a synthetic version of the naturally occurring reproductive hormone progesterone.
Less is known about the breast-cancer risks of progestin-only birth control, a type that’s growing in popularity because it only contains one hormone. By eliminating estrogen, these methods cut out some of the risks associated with the hormone, such as blood clotting.
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But new data from the U.K. have allowed researchers for the first time to compare the breast-cancer risks of different popular types of hormonal birth control—and the study found no difference in risk between women using progestin-only birth control and those using methods that combine progestin and estrogen.
The new research, published Mar. 21 in PLOS Medicine, addresses a longstanding knowledge gap and could help experts better understand the mechanisms behind the risks and benefits of birth control. “There’s a really limited understanding of the mechanisms for how oral contraceptives may be influencing breast-cancer risk,” says Shelly Tworoger, Associate Center Director for Population Science at the Moffitt Cancer Center, who was not involved in the study. Because the risk metrics look the same for formulations with and without estrogen, she says, “the findings could suggest that the progestins are the primary source of what’s going on to increase risk of breast cancer.”
Combined hormonal birth control has a well-documented yet complex relationship to various cancers and other minor risk factors because it’s been around since the 1960s. “There’s quite a lot of evidence already that women have an increase in breast-cancer risk while they’re taking combined oral contraceptives,” says Gillian Reeves, director of the cancer epidemiology unit at the University of Oxford and an author of the study. But “that increase is small, and it seems to decline once women stop taking it.” Many other options have since emerged that ditch the estrogen in favor of progestin-only formulas, including the progestin-only pill, also called POP or the “mini-pill.” While POPs are only used by about 4% of American women, their adoption worldwide is continuing to grow. Reeves says that in 2020, the U.K. saw nearly as many prescriptions for POPs as it did for combined hormonal pills. And of the 14% of women worldwide who use IUDs, the majority use a hormonal version, all of which only contain progestin.
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Reeves and her research colleagues were curious about the breast-cancer risk for people taking progestin-only birth control methods. For their new study, they sorted through prescription records from a group of almost 10,000 female, British breast-cancer patients under age 50, as well as a control group of similar-age women who did not have breast cancer. When comparing medical histories, the team found that current or recent use of a progestin-only contraceptive method was associated with an increase in breast-cancer risk of about 20% to 30%—the same increase associated with combined contraceptive methods.
While this might sound like a huge number, Reeves says that it’s important to consider that the number being increased—the average woman’s chances of developing breast cancer, if she has no other indicators or family history—is very small to begin with. “Twenty percent is not going to lead to many extra cases, because it’s so rare,” says Reeves. Though breast-cancer risk increases as women age, “for women who, say, take it for five years from 30 to 34, you’re talking about an increase in risk up to age 50 of something like 0.2%,” she says. “So it’s very small.”
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Traditional combined hormone oral birth control is protective against some other cancers, too, though experts aren’t entirely sure why. Although women who take them are at slightly higher risk of developing breast and cervical cancers, they have a lower likelihood of developing endometrial, ovarian, and colorectal cancers. The new research didn’t look at progestin’s-only birth control’s effects on risks of other cancers.
The findings could pave the way for new knowledge about how hormonal IUDs affect the body. Because IUDs are placed directly in the uterus, their hormonal effects only need to work locally. And for the most part, it seems like they do. But the new study found no difference in increased breast-cancer risk between different methods of progestin-only birth control—such as pills vs. IUDs—which means that the hormones in an IUD are either traveling more widely in the body than previously thought, or setting off some sort of hormonal chain reaction that begins in the uterus and ends up affecting breast tissue, Reeves says.
Reeves and Tworoger agree that the findings do not indicate the need for any changes in how physicians prescribe birth control for any of the myriad reasons that someone might want to take it. And nobody should be going to their doctor after reading the study asking to be taken off their medication, says Reeves. “There’s no reason to be any more concerned than you would be about using a combined oral contraceptives.” Picking the right birth control is all about balancing the small known risks with the proven rewards, says Reeves, “and that’s what women do every day.”