Chronic inflammatory bowel disease, cigarette smoking, and use of oral contraceptives: findings in a large cohort study of women of childbearing age.

Since the start in 1968 of the Oxford Family Planning Association contraceptive study 31 women have developed ulcerative colitis and 18 have developed Crohn's disease, giving incidences of 0.15 and 0.09/1000 woman years respectively. The incidence of ulcerative colitis in women who were non-smokers on entry to the study was 0.17/1000 woman years and the incidence in smokers was 0.11/1000 woman years. The findings for Crohn's disease were entirely different, the corresponding incidences being 0.05 and 0.17/1000 woman years respectively. Both ulcerative colitis and Crohn's disease were more common among women currently using oral contraceptives than among those not doing so. Incidences per 1000 woman years for ulcerative colitis were 0.26 in users and 0.11 in non-users; for Crohn's disease the incidences were 0.13 and 0.07 respectively. Though the association between the use of oral contraceptives and chronic inflammatory bowel disease cannot be regarded as established, the effects of smoking have been shown consistently in many studies. This observation provides an important clue to the aetiology of chronic inflammatory bowel disease.