The epidemiology of oral contraceptives and breast cancer.

Concern about a possible link with risk of breast cancer and use of oral contraceptives was raised soon after oral contraceptives were approved for general use. Over 35 epidemiologic studies have addressed this issue, but the controversy persists stimulated in particular by several recent studies. Overall, most studies have observed no consistent association between oral contraceptive use and risk of breast cancer. Nevertheless, the possibility remains that the risk may be elevated in certain subgroups, especially users of relatively long duration prior to their first full-term pregnancy. Most of the data regarding this subgroup derives from case-control studies, in which selection and recall bias may occur. To address this issue prospectively, our research group has established a new cohort of female nurses in the United States, aged 25-42. Enrollment is now complete, over 116,000 participants have returned the baseline questionnaire. A large proportion of these women are current or past oral contraceptive users. Within the next several years, substantial data will accrue to assess the relation between use of oral contraceptives at younger ages and risk of breast cancer.