The risk of developing breast cancer is about 20% higher among women who are current or recent users of hormonal contraceptives, compared with women who never used them, according to a prospective cohort study that included current contraception formulations.

Lina S. Mørch, PhD, of the University of Copenhagen, and colleagues conducted a nationwide, prospective cohort study of 1,797,932 women in Denmark who were aged 15-49 years between 1995 and 2012. The risk of developing breast cancer among all women currently using or who had recently used hormonal contraception correlated to a relative risk (RR) of 1.20 (95% confidence interval [CI], 1.14-1.26). The report was published in the New England Journal of Medicine.

The risk of breast cancer was heavily associated with the length of time during which the women had taken hormonal contraceptives. Women with less than a year of exposure had the lowest risk elevation (RR 1.09, 95% CI, 0.96-1.23), while women with more than 10 years of exposure faced the greatest increase in risk (RR 1.38, 95% CI, 1.26-1.51, P = .002).

The increased risk persisted even after women had stopped taking contraceptives. Researchers found that contraception use of less than a year (RR 1.01, 95% CI, 0.88-1.15), between 1 year and 5 years (RR 1.07, 95% CI, 0.94-1.20), and between 5 years and 10 years (RR 1.30, 95% CI, 1.06-1.58) correlated to increased risks of breast cancer 5-10 years after cessation of use. The number of events in women who had used hormonal contraceptive for more than 10 years was too small to accurately determine risks and was not reported in the study.

The combinations and formulations of oral contraceptives had little effect on breast cancer risk elevation, the researchers found. Triphasic and monophasic formulations containing levonorgestrel had similar relative risk levels of 1.21 (95% CI, 1.04-1.41) and 1.45 (95% CI, 1.26-1.67), respectively (P = .07). The relative risk was similar for intrauterine devices containing levonorgestrel (RR 1.21, 95% CI, 1.11-1.33).

The difference in risk between current and recent users of hormonal contraception and women who never it was 13 (CI, 10-16) per 100,000 person-years, which translates into an one extra breast cancer diagnosis for every 7,690 women using hormonal contraception for 1 year.

“The estimated number of additional breast cancers that were associated with hormonal contraception did not include extra cases diagnosed after the discontinuation of long-term use. Even so, the estimated number of additional breast cancers among premenopausal women that were attributable to hormonal contraception is likely to be low,” the researchers wrote. “This risk should be weighed against important benefits of hormonal contraceptives such as good contraceptive efficacy and reduced risks of ovarian, endometrial, and perhaps colorectal cancer (at least for combined oral contraceptives that were commonly used in the 1970s and 1980s).”Dr. Mørch and a coauthor received grant support from the Novo Nordisk Foundation and became employed by Novo Nordisk Foundation after the study was accepted for publication. No other relevant disclosures were reported.

Source: Mørch L. et al. N Engl J Med 2017;377(23):2228-39 .