High-dose estrogen birth control may increase breast cancer risk

The recent use of oral contraceptives, particularly high-dose estrogen formulations, is associated with an increased risk for breast cancer, according to recent findings.

“Our results suggest that use of contemporary oral contraceptives in the past year is associated with an increased breast cancer risk relative to never or former oral contraceptive use, and that this risk may vary by oral contraceptive formulation,” researcher Elisabeth F. Beaber, PhD, MPH, of the public health sciences division of the Fred Hutchinson Cancer Research Center in Seattle, said in a press release.

In the nested case-control study, researchers evaluated 1,102 women with invasive breast cancer who were enrolled at Group Health Cooperative, a health care system in the Seattle-Puget Sound area, between 1990 and 2009. The researchers randomly sampled 21,755 controls from the enrollment records of Group Health Cooperative and matched them to breast cancer cases based on age, year, length of enrollment and availability of medical charts. All participants were aged 20 to 49 years.

The researchers reviewed medical and pharmacy records for all oral contraceptive exposures in the 12 months before diagnosis (reference date) and categorized them by formulation. The formulations were grouped based on estrogen and progestin levels, doses and dosing schedules (monophasic or triphasic schedule). The contraceptives were classified as low- (20 mcg ethinyl estradiol), moderate- (30 mcg to 35 mcg ethinyl estradiol or 50 mcg mestranol) or high- (50 mcg ethinyl estradiol or 80 mcg mestranol) dose estrogen.

Recent users were defined as those who filled at least one oral contraceptive prescription within the past year, and recent users were compared with those who did not fill a prescription for oral contraceptives within the year before the reference date.

Conditional logistic regression, ORs and 95% CIs were used to analyze these data.

The researchers found that oral contraceptive use within the past year was correlated with increased risk for breast cancer (OR=1.5; 95% CI, 1.3-1.9) vs. never-use or past oral contraceptive use. Although this correlation was stronger for ER-positive (OR=1.7; 95% CI, 1.3-2.1) vs. ER-negative breast cancer (OR=1.2; 95% CI, 0.8-1.8), it was not statistically significantly different (P=.15).

An especially high risk was observed with the recent use of oral contraceptive formulations involving high-dose estrogen (OR=2.7; 95% CI, 1.1-6.2), ethynodiol diacetate (OR=2.6; 95% CI, 1.4-4.7) or triphasic dosing with an average of 0.75 mg norethindrone (OR=3.1; 95% CI, 1.9-5.1). Low-dose estrogen oral contraceptives were not associated with elevated risk (OR=1; 95% CI, 0.6-1.7).

“Our results require confirmation and should be interpreted cautiously,” Beaber said in the press release. “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.”

Disclosure: The researchers report no relevant financial disclosures.