A cohort study of oral contraceptive use and risk of benign breast disease

The purpose of the cohort study reported here was to investigate the associ ation between oral contraceptive use and risk of benign breast disease (BBD ), overall and by histological subtype, within the 56,537 women in the Cana dian National Breast Screening Study (NBSS) who completed self-administered lifestyle and dietary questionnaires. The NBSS is a randomized controlled trial of screening for breast cancer in women aged 40-59 at recruitment. Ca ses were the 2,116 women in the dietary cohort who were diagnosed with biop sy confirmed incident BED. For comparative purposes, a subcohort consisting of a random sample of 5,681 women (including 197 subjects with incident BE D) was selected from the full dietary cohort. After exclusions for various reasons, the analyses were based on 2,116 cases and 5,338 non-cases. There was an inverse association between use of oral contraceptives and risk of a ll types of BED combined. The reduction in risk was confined largely to pro liferative forms of BED (BPED), and in particular, to those forms of BPED w ithout histological atypia, in whom there was a progressive reduction in ri sk with increasing duration of use (the IRR (95% CI) for use of more than 7 years was 0.64 (0.47-0.87)); risk of BPED with atypia was increased somewh at in association with oral contraceptive use (the IRR (95% CI) for use of more than 7 years was 1.43 (0.68-3.01)), but not in a dose-dependent manner . The results were similar when examined separately in the screened and con trol arms of the NBSS and for screen-detected and interval-detected BPED. ( C) 1999 Wiley-Liss, Inc.