Oral contraception, smoking and inflammatory bowel disease--findings in the Royal College of General Practitioners Oral Contraception Study.

Data from the Royal College of General Practitioners Oral Contraception Study have been examined to determine whether oral contraceptive use was associated with the development of Crohn's disease or ulcerative colitis. Over a 17 year period Crohn's disease developed in 42 women and ulcerative colitis in 78. The incidence of both diseases was greater in oral contraceptive users compared to non-users with the rate ratio for Crohn's disease being 1.7 (95% confidence limits, 0.88, 3.2) and for ulcerative colitis being 1.3 (95% confidence limits 0.82, 2.0). For both diseases incidence was unrelated to parity or social class, but women smoking at recruitment had a greater incidence of Crohn's disease (rate ratio 1.8, 95% confidence limits 0.93, 3.3) and a reduced incidence of ulcerative colitis (rate ratio 0.68, 95% confidence limits 0.41, 1.1). Although these results are based on small numbers and could be chance findings, they are consistent with other studies showing associations between oral contraceptive use, smoking and the development of Crohn's disease and ulcerative colitis.