Danish study supports link between hormonal contraception and breast cancer

The risk of breast cancer is higher among women who have recently used hormonal contraception than in those who have never used it, according to a large prospective cohort study published in the New England Journal of Medicine (2017;377:2228–39).

The study investigators assessed the association between the use of hormonal contraception (including combined hormonal and progestogen-only contraception) and the risk of invasive breast cancer in 1.8 million Danish women aged between 15 and 49 years with no history of cancer or venous thromboembolism, and who had not received infertility treatment.

After a mean follow-up of 10.9 years, a total of 11,517 cases of breast cancer occurred, according to data from the Danish National Patient Registry. The study investigators found that the relative risk (RR) of breast cancer among all current and recent users of hormonal contraception was 1.20 (95% CI 1.14–1.26) compared with those who had never used hormonal contraception.

The breast cancer risk increased according to the duration of use of hormonal contraception (RR 1.09 with less than one year of use vs 1.38 with over 10 years of use; p=0.002). This increased risk persisted after discontinuation, with the risk of breast cancer still higher among women who had used hormonal contraceptives for five years or more than in those who had never used it.

However, the study authors point out that the absolute increase in breast cancer risk is small. Overall, the absolute increase in breast cancer cases diagnosed among current and recent users of hormonal contraception was 13 per 100,000 person-years, which equates to one extra breast cancer for every 7690 women using hormonal contraception for one year.

One drawback of the study was that it did not measure cancer mortality after breast cancer diagnosis, and it is possible that the increased number of breast cancer diagnoses among hormonal contraception users could be partially attributed to the increased contact with healthcare services compared with never-users. However, the study authors argue that this cannot fully explain the association.

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