Case-control study of oral contraceptive use and risk of breast cancer.

The relation of oral contraceptive use to the risk of breast cancer in white women aged 25-59 years was assessed with data collected during 1977-1992 in a case-control surveillance system in hospitals in Boston, New York, and Philadelphia. A total of 3,540 cases with breast cancer were compared with 4,488 controls with nonmalignant nongynecologic conditions unrelated to oral contraceptive use. Relative risk estimates were obtained by unconditional logistic regression with control for major risk factors. For at least 1 year of use relative to less than 1 year, the multivariate relative risk estimate was 1.7 (95% confidence interval (CI) 1.3-2.3) in women aged 25-34 years, 0.9 (95% CI 0.7-1.0) in women aged 35-44 years, and 1.2 (95% CI 1.0-1.4) in women aged 45-59 years (p < 0.01 for the difference across age). Among women aged 25-34 years, the relative risk estimates were greatest for use of long duration, but the trend was not statistically significant (p = 0.17); in addition, the duration of use was correlated with the recency of use, and it was not possible to distinguish their effects. Among women aged 35-44 years, the relative risk estimate decreased with increasing duration of use (p = 0.01). Among women aged 45-59 years, some relative risk estimates were increased, but there was no consistent pattern. The results add to the evidence of an association between oral contraceptive use and an increased risk of breast cancer at young ages.