The exact nature of the association between hormones and the development of breast cancer remains uncertain. Studies of endogenous hormone levels and breast cancer suggest a possible role of estrogens and androgens in the cause of breast cancer. Numerous studies have been conducted to assess the risk of breast cancer among women exposed to exogenous hormones, Several studies of women who have used oral contraceptives have shown a twofold increase in risk for the onset of breast cancer at an early age associated with 10 years of oral contraceptive use. One of the largest studies, the Cancer and Steroid Hormone Study, found no association between breast cancer and oral contraceptive use for women up to the age of 54. A meta-analysis combining the results of 31 published studies of the association between hormone replacement therapy and breast cancer revealed no increased risk of breast cancer associated with ever-use (risk ratio [RR], 1.02; 95% confidence interval [CI], 0.93-1.12). Use of oral contraceptives for more than 10 years was associated with a small increased risk (RR, 1.23; 95% GI, 1.08-1.40), but few studies have examined very long term use. A recent study of pregnancy and breast cancer outcome found no adverse influence of pregnancy shortly before or after diagnosis of breast cancer on prognosis. The results of a wide range of studies should be considered when weighing the benefits and risks of hormone use among women at increased risk of breast cancer or with a history of breast cancer.
