Experts have known that women who take birth-control pills have a lower risk of endometrial cancer, and a new study offers insight into how long the protective effect lasts and how many cancer cases have likely been prevented.
The analysis, published Tuesday in the journal The Lancet Oncology, analyzed data from 36 studies on women with endometrial cancer, which starts in the lining of the uterus known as the endometrium; it’s the most common type of uterine cancer. Researchers compared information about more than 27,000 women who developed endometrial cancer with 115,000 who didn’t and found that every five years of birth-control-pill use was associated with a 24 percent lower cancer risk. After ten years of use, the odds of developing the cancer before age 75 fell by more than 50 percent, from 2.3 cases per 100 women to 1.3. The effect lasted for at least 30 years after women stopped taking the pill — a significant benefit since the American Cancer Society says most cases are diagnosed in women over 55. The authors estimate that, in developed countries, the reduced risk translates to about 400,000 fewer cases in the last 50 years, with about 200,000 of those prevented between 2005 and 2014 alone.
This finding isn’t surprising but it’s “quite reassuring,” says Mary Jane Minkin, M.D., F.A.C.O.G., and clinical professor of obstetrics, gynecology, and reproductive sciences at the Yale School of Medicine, because the risk reduction appears to be even higher than previously thought. Dr. Minkin, who was not involved in the study, says the progestin (synthetic progesterone) in combination birth-control pills is probably the mechanism because it helps limit the growth of the uterine lining, which can be more of a problem in developed countries with high obesity rates. “Fat tissue manufactures estrogen, so heavy women are more at risk for overgrowth of the lining of the uterus,” she says. “If women take birth-control pills that contain a good dose of progestins, it limits the buildup of the lining, so fewer funky cells can grow in the lining, too. And heavy women incidentally make less of their own progesterone so it can spiral even more out of control.”
The National Institutes of Health says women can lower their endometrial cancer risk by maintaining a healthy weight and taking a combination pill for at least a year, and this study and others suggest that the longer you take it, the greater the benefits. The same can be said for other cancers: Being on the pill for five years or more is also associated with a 50 percent lower risk of ovarian cancer. While experts still don’t know for sure if oral contraceptives can increase breast-cancer risk, it seems that they’re not only safe to use but can also be good for your health in the long run.