Approximately 140 million women around the world rely on hormonal contraception. According to new research published in the New England Journal of Medicine, the use of such contraceptives can increase a woman’s risk of breast cancer.

The study’s authors analyzed data from the Danish Sex Hormone Register Study, which tracks the health of women between the ages of 15 and 79. Of more than 1.7 million women, there were more than 11,00 incidents of breast cancer. Compared with the women who had never used hormonal contraceptives, the women who were “current or recent users” had a relative risk of breast cancer of 1.20. In addition, the risk increased along with the duration of use.

“As compared with women who had never used hormonal contraception, an increased risk of breast cancer was observed among woman who had previously used hormonal contraception for long periods of time,” wrote lead author Lina S. Mørch, PhD, University of Copenhagen in Denmark, and colleagues. “Among these women, the risk appeared to remain increased for at least 5 years after discontinuation. No increased risk was found among women who had previously used hormonal contraception for less than five years.”

Mørch et al. noted that the increase in risk was quite small. The “overall absolute increase” in breast cancers diagnosed among women who were current or recent users of hormonal contraceptive was approximately one extra breast cancer per 7,690 women using such contraception for one year.

“The estimated number of additional breast cancers among premenopausal women that were attributable to hormonal contraception is likely to be low,” the authors 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.”

The authors added that their study did have limitations. Such factors as alcohol consumption and physical activity were not taken into account, for instance, and body-mass index information was not available for all women.