The association between birth control pills and breast cancer was identified years ago. However, many women and their doctors have assumed that newer, lower-dose pills — and other hormonal delivery methods beyond the pill — were much safer. Is there still a risk?
The researchers analyzed data on 1,797,932 women younger than 50 who did not have cancer and had not undergone fertility treatment. About 60 percent of the women used some type of hormonal contraception, and about 40 percent did not. In about an 11-year span, 11,517 women got a breast cancer diagnosis. The study found that those using hormonal contraception of any sort were 20 percent more likely to have developed breast cancer than were women who did not use it. The longer women had used hormonal contraception, the greater their risk — increasing from a 9 percent heightened risk with less than one year of use to a 38 percent greater risk after more than 10 years of use.
Risk fell rapidly once women stopped using hormonal contraception, except among women who stopped after having used it for five years or longer. Their likelihood of developing breast cancer remained elevated for at least five more years. Risk varied some among the different types of contraception, but major differences were not found. An editorial that accompanied publication of the study noted that "these results do not suggest that any particular preparation is free of risk."
Women who use hormonal contraception, which includes patches, implants, injections, vaginal rings and intrauterine devices as well as birth control pills. It also includes various hormones and combinations, though generally at lower dosages than decades ago. In the United States, about 10 million women use oral contraceptives, and more than 4 million others use IUDs or implants.
Despite the finding of increased risk, breast cancer is fairly rare among younger women. The study estimated that for every 100,000 women, hormone contraceptive use causes an additional 13 breast cancer cases a year overall but just 2 for every 100,000 women younger than 35. That means that while the percentage increases look substantial, in fact they reflected a fairly small increase in actual cases.
The risk analysis did not take into account some factors — such as physical activity, alcohol consumption and breast-feeding history — that can affect the risk for breast cancer. The study also did not include women 50 or older, who are the most likely to develop breast cancer. The study was funded by the Novo Nordisk Foundation.
Dec. 7 issue of the New England Journal of Medicine (nejm.org).
The research described in Quick Study comes from credible, peer-reviewed journals.