Oral contraceptive use and colorectal cancer in the Nurses' Health Study I and II.

BACKGROUND: It remains unclear if oral contraceptive (OC) use is associated with the incidence of colorectal cancer. Few studies have examined this association by duration of OC use, time since last OC use, and different cancer subsites.
METHODS: Among 88,691 participants of the Nurses' Health Study I (NHSI) and 93,080 participants of the Nurses' Health Study II (NHSII), we assessed OC use every 2 years between 1976 and 2010 and categorized it as ever use, duration of use, and time since last use. We included incident colorectal cancer cases through 2010 (NHSI: age at diagnosis = 36-88, N = 1,764; NHSII: age at diagnosis = 33-64, N = 206). Multivariable hazard ratios and 95% confidence intervals were estimated using Cox proportional hazards regression models.
RESULTS: Ever OC use was not associated with colorectal cancer in NHSI [1.01 (0.91, 1.12)] nor NHSII [1.03 (0.69, 1.53)]. In NHSII, when compared with never-users, longer durations (5+ years) of OC use were inversely associated with the risk of colon cancers (Ptrend = 0.02) but the number of endpoints was limited. No other colorectal cancer subsites were associated with OC durations or times since last OC use in either cohort.
CONCLUSIONS: In two large prospective cohorts, we found little evidence that OC use may be protective for colorectal cancer, except potentially with longer durations of use among younger women. IMPACT: Our results do not support the previous initial studies that reported an inverse association of recent OC use with colorectal cancer but instead support newer, larger studies demonstrating no such association. ©2015 American Association for Cancer Research.