Objective: To assess the risk of colorectal cancer among women who have used oral contraception or hormone replacement therapy (HRT), especially those exposed to both classes of exogenous hormones.
Design: Nested case-control study using prospectively collected data from the Royal College of General Practitioners' Oral Contraception Study (OCS). The 146 cases were women with a diagnosis of colorectal cancer recorded on the OCS database by March 2003. Each case was matched with three controls who were free of the disease at the time of the case's diagnosis, of similar age and with similar length of follow-up in the OCS.
Results: Compared with never use, current use of oral contraception was associated with a reduced risk of colorectal cancer: adjusted odds ratio (OR) 0.38 [95% confidence interval (CI) 0.11-1.32]; former use 0.89 (95% CI 0.59-1.33); ever use 0.84 (95% CI 0.56-1.24). Similar comparisons for HRT were: current use 0.34 (95% CI 0.13-0.91); former use 0.59 (95% CI 0.30-1.14); ever user 0.49 (95% CI 0.27-0.87). The OR among women who had used both classes of hormones was 0.34 (95% CI 0.15-0.79) compared with those who have used neither.
Conclusions: Ever users of oral contraceptives do not benefit from a long-term reduction in colorectal cancer, although current and recent use may confer some protection. Women who have ever used HRT appear to have important reductions in their risk of colorectal cancer, especially while using these hormones. Further study is needed in order to determine how long any benefits last and whether these are stronger in women exposed to both classes of exogenous hormones.