Some Birth Control Pills May Increase Risk of Breast Cancer

Women who have recently used high-estrogen birth control pills may be at a higher risk for developing breast cancer, according to a new study.

Women who have recently used high-estrogen birth control pills may be at a higher risk for developing breast cancer, according to a new study. The study, published in Cancer Research, shows that women who used birth control pills within the last year may have as much as a 50% relative increased risk of breast cancer compared with women who never or formerly took birth control pills. Those taking a pill with the highest levels of estrogen had the highest risk; pills that contained low estrogen levels did not increase breast cancer risk.

Recent use of a birth control pill with a high estrogen dose, ethynodiol diacetate, or triphasic dosing with an average of 0.75 mg of norethindrone was associated with a particularly higher risk-between a 2.6- and 3.1-fold higher risk-compared with using other types of birth control pills. Those who took a pill with a moderate estrogen dose had a 1.6-fold higher risk of breast cancer.

“Our results require confirmation and should be interpreted cautiously,” said Elisabeth F. Beaber, PhD, MPH, a staff scientist in the public health sciences division of Fred Hutchinson Cancer Research Center in Seattle, in a statement. “Breast cancer is rare among young women, and there are numerous established health benefits associated with oral contraceptive use that must be considered. In addition, prior studies suggest that the increased risk associated with recent oral contraceptive use declines after stopping oral contraceptives.” The risk of a woman being diagnosed with breast cancer at age 40 is about 1.5%.

The case-control study analyzed 1,102 women diagnosed with breast cancer and compared them to 21,952 controls. All of the study participants were patients in the Group Health Cooperative in the Seattle area. More than 80% of the women diagnosed with breast cancer were between the ages of 40 and 49; the rest were between the ages of 20 and 39. Beaber and colleagues examined electronic pharmacy records for information on the women’s oral contraceptive type and duration of use.

The researchers divided birth control pills into three categories-low-level synthetic estrogen (about 20 mcg of ethinyl estradiol), moderate-level synthetic estrogen (30 to 35 mcg of ethinyl estradiol or 50 mcg of mestranol), and high-level synthetic estrogen (50 mcg of ethinyl estradiol or 80 mcg of mestranol).

The link of high estrogen level birth control use was stronger for estrogen receptor (ER)-positive breast cancer compared with ER-negative breast cancer, although the difference was not statistically significant (odds ratio of 1.7 vs 1.2; P = .15).

The authors also highlighted some of the established health benefits of taking a birth control pill-reproductive planning, menses regulation, decreased dysmenorrhea, and decreased risk of benign breast conditions, “which must also be considered when making individual choices,” said the authors.

This study was funded by the National Cancer Institute.