Breast cancer in relation to early use of oral contraceptives. No evidence of a latent effect.

A long-term effect of oral contraceptives (OCs) on breast cancer risk has been suggested as an explanation for some studies' failure to detect an association between OCs and breast cancer. To address this latency hypothesis, we analyzed data on 4714 case subjects and 4540 control subjects from the population-based Cancer and Steroid Hormone Study. No support was evident for a latent effect of OCs on breast cancer risk through age 54 years: among parous women who had cumulated more than six years of OC use before their first term pregnancy, the risk of breast cancer, relative to nonusers before first term pregnancy, was 0.6 at zero to four years after first term pregnancy (95% confidence interval [Cl], 0.2 to 1.8), 0.7 at five to nine years (95% Cl, 0.3 to 1.7), and 1.1 at ten to 14 years (95% Cl, 0.3 to 3.9). Among nulliparous women with more than six years of OC use in total, the relative risk of breast cancer, by interval from last use of OCs, was 1.3 at zero to four years (95% Cl, 0.8 to 2.0), 1.1 at five to nine years (95% Cl, 0.5 to 2.0), and 0.6 at ten to 14 years (95% Cl, 0.1 to 3.7).