Numerous epidemiological studies have been performed to determine factors that might contribute to the development of inflammatory bowel disease. Although the role of oral contraceptive agents in Crohn's disease (CD) and ulcerative colitis (UC) have been assessed, most studies were of small sample size and characterised by low statistical precision. A meta-analysis was performed to increase the statistical power and to investigate the association between the use of oral contraceptives and the development of CD and UC. The study was based on a search of a Medline database from 1975 to October 1993 and a review of reference lists from published articles, reviews, symposia proceedings, and abstracts from major gastrointestinal meetings. All studies specifically designed to evaluate this association were selected. The combined results of nine studies--two cohort studies (30,379 unexposed and 30,673 exposed patients) and seven case-control studies (482 CD, 237 UC, and 3198 controls)--which satisfied our selection criteria were evaluated. The pooled relative risk (adjusted for smoking) associated with oral contraceptive use was 1.44 (1.12, 1.86) for CD and 1.29 (0.94, 1.77) for UC. These results suggest modest associations between the use of oral contraceptives and the development of CD and UC. As these associations are weak, non-causal explanations for the findings cannot be eliminated.
PIP: The GI Research Group of the University of Calgary in Alberta, Canada, used data from seven case control studies (719 cases of inflammatory bowel disease and 3198 controls) and two cohort studies (32,673 users of oral contraceptives [OCs] and 30,379 nonusers) to conduct a meta-analysis to increase the statistical power to examine the association between OC use and the development of Crohn's disease (CD) and ulcerative colitis (UC). The studies tended to have a small sample size and a low statistical power. The estimated relative risks (RRs) in the studies of CD varied from 0.7 to 2.5. The pooled RR for both cohort and case control studies of CD stood at 1.44 when controlled for smoking and 1.68 when not controlled for smoking. The confidence intervals in seven of the eight studies of CD and for all four studies of UC included unity. The RRs in the studies of UC ranged from 0.7 to 2.4. The pooled RR for the studies of UC were 1.29 when adjusted for smoking and 1.68 when not adjusted for smoking. The researchers controlled for smoking because smoking protects against UC and is a risk factor for CD. These findings suggest a positive, albeit modest, association between OC use and both CD and UC, even when smoking is controlled. Since the associations are modest, the GI Research Group could not eliminate non-causal explanations for the findings.