A study which followed more than 1.8 million Danish women for over 11 years worryingly reports that women who used birth control had a 20 percent increase in their relative risk for developing breast cancer, and the longer they used it , the higher their risk.

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Over the course of the study, 11,517 cases of breast cancer occurred; that’s a low incidence number overall, but within this small incidence, the increase in risk was significant. Overall, the study reports that women who used birth control had a 20 percent increase in their relative risk for developing breast cancer.

However, the risk varied significantly based on how long the women took the contraceptives. Women who took them less than one year only had a 9% increase in relative risk, whereas women who took them for more than 10 years had a 38% increase. This translates into about one extra breast cancer case for every 7,690 women who used hormonal contraception for a year.

The risk was associated with all types of hormonal contraception including the pill, injections or IUDs.

“These results do not suggest that any particular preparation is free of risk,” wrote David Hunter, professor of epidemiology and medicine at the Nuffield Department of Population Health in the UK, in an accompanying editorial. He said that the link between breast cancer and contraceptives is well established, but this study is valuable because it provides quantified information and offers information about new preparations of contraception. “The number of cases increases with age because the risk of breast cancer increased with age,” said Hunter.

If all this scares you, it’s important to note that oral contraceptives decrease the risk of endometrial cancer by 50% and ovarian cancer by up to 30%. Contraceptives are a mixed bag, coming with both positives and negatives and right now, there’s no need to panic. Corresponding author Øjvind Lidegaard comments:

“[Contraceptives] also bring 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.”

The study concludes:

“The risk of breast cancer was higher among women who currently or recently used contemporary hormonal contraceptives than among women who had never used hormonal contraceptives, and this risk increased with longer durations of use; however, absolute increases in risk were small.”

The study was published in The New England Journal of Medicine.

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