Breast cancer risk among women under 55 years of age by joint effects of usage of oral contraceptives and hormone replacement therapy

Objective: To assess effects on breast cancer risk of exposure to both oral contraceptives and menopausal hormones, an increasingly common exposure. Design: A case-control study of breast cancer among women under the age of 55 years in Atlanta, GA involving 1,031 cases and 919 population controls was conducted. Results: Ever use of oral contraceptives was associated with a relative risk of 1.1 (95% 0.9-1.4), whereas the relative risk for hormone replacement therapy was 0.9 (95% CI 0.7-1.2). Seventeen percent of the cases versus 19% of the population controls reported exposure to both agents, resulting in a relative risk of 1.0 (95% CI 0.7-1.4) relative to those unexposed to either preparation. Although there was little variation in risk associated with joint effects by either age or race, there were statistically nonsignificant elevations in risk for this exposure among women who had experienced a natural menopause (relative risk = 2.0, 95% CI 0.7-5.6), were relatively thin (relative risk = 1.5, 0.8-3.0), or who had a first degree relative with breast cancer (relative risk = 2.0, 0.6-7.0). When joint effects of longer term use of both agents were considered, subjects who reported use of oral contraceptives for 10 or more years and hormone replacement for 3 or more years had a relative risk of 3.2 (95% CI 1.4-7.4) compared with nonusers of either preparation. Conclusions: Although our results must be cautiously interpreted given small numbers within subgroups, they raise concern and emphasize the need for further evaluation on breast cancer risk of the increasingly common exposure to both oral contraceptives and hormone replacement therapy.