Using hormonal birth control methods — including newer types of birth control pills, as well as intrauterine devices (IUDs) and implants — may slightly increase women's risk of breast cancer, according to a new study from Denmark.
The study builds on earlier findings linking hormonal birth control and breast cancer, but the new study focused on newer forms of birth control.
The study, which included about 1.8 million women in Denmark, found that those who used hormonal birth control methods were 20 percent more likely to develop breast cancer over an 11-year period, compared with those who never used hormonal birth control.
Still, a woman's overall chance of developing breast cancer linked to hormonal birth control use was quite small: The researchers estimate that there would be 1 extra case of breast cancer for every 7,690 women who took hormonal contraception (or 13 extra cases of breast cancer for every 100,000 women who used hormonal contraception). [10 Do's and Don’ts to Reduce Your Risk of Cancer]
When the researchers examined a number of different hormonal formulations used in birth control, they found that all of the formulations raised the risk of breast cancer by about the same amount. (Hormonal birth control methods typically use either a combination of the hormones estrogen and progestin, or progestin by itself.)
The study is published today (Dec. 6) in The New England Journal of Medicine.
Not a "new" link
The findings of alink between hormonal contraception and breast cancer is not new; studies going back decades have suggested that the hormones in birth control could raise the risk of breast cancer. But these earlier studies looked mainly at older types of birth control pills, which had a higher dose of estrogen than today's pills. Therefore, it wasn't clear if this risk applied to newer formulations of birth control pills or to other birth control methods, including intrauterine devices (IUDs) and implants that contain only the hormone progestin.
The new study "confirms that the increased breast cancer risk ... that was initially reported with the use of older, often higher-dose formulations also applies to contemporary formulations" of birth control, David Hunter, a professor of epidemiology and medicine at Oxford University's Nuffield Department of Population Health in the United Kingdom, wrote in an editorial that accompanied the study. "These results do not suggest that any particular preparation is free of risk," Hunter added.
But this risk should be weighed against the important benefits of hormonal contraception, which is an effective method of birth control, the researchers, from the University of Copenhagen, wrote in their study. What's more, other studies have found that taking hormonal birth control may actually reduce the risk of other cancers, including ovarian cancer, endometrial cancer and colorectal cancer, they said.
Risk with longer use
The new study involved women in Denmark ages 15 to 49 who had not previously been diagnosed with cancer. The researchers used nationwide registries to collect information about prescriptions that were filled for hormonal contraception, as well as diagnoses of breast cancer.
The longer women used hormonal contraception, the greater their risk of breast cancer, the researchers found. Using hormonal contraception for less than one year did not increase women's risk of breast cancer. However, using hormonal contraception for 10 years was linked with a 40 percent increase in the risk of breast cancer, compared with those who had never used hormonal contraception.
Once women stopped using these forms of birth control, the increased risk of breast cancer disappeared if the women had used hormonal contraception for less than five years. But if they had taken hormonal contraception for more than five years, the higher risk of breast cancer persisted for at least five years after their discontinuation of hormonal birth control, the study found. [Beyond Birth Control: 5 Conditions 'The Pill' Can Help Treat]
The findings held even after the researchers took into account some factors that can affect the risk of breast cancer, such as becoming pregnant or having a family history of the disease.
But the study did not account for some other things that affect breast cancer risk, including physical activity levels and alcohol consumption.
Still, the researchers noted that any unaccounted factors would need to have a large effect on the risk of breast cancer and be very common in the population to explain the results.
The study was funded by the Novo Nordisk Foundation, a commercial foundation in Denmark that funds research to support its business interests, which include the pharmaceutical company Novo Nordisk. It had no role in the design, analysis or interpretation of the study, or in writing the paper.
Original article on Live Science.
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Rachael is a Live Science contributor, and was a former channel editor and senior writer for Live Science between 2010 and 2022. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.