The protective effect of the oral contraceptive pill on rheumatoid arthritis: an overview of the analytic epidemiological studies using meta-analysis.

The oral contraceptive pill (OCP) has been implicated as having a protective effect on the development of rheumatoid arthritis (RA). The results of 12 studies have now been reported and produced differing results and conclusions. Because of the discrepancy in results and the importance of the issue we undertook a review of the studies and performed a meta-analysis. In all, 9 independent studies satisfied the criteria for selection, 6 case-control design and 3 longitudinal. Using standard meta-analysis techniques, the overall pooled odds ratio for all the studies was 0.68 for the crude results (95% CI 0.58-0.78) and 0.73 for the adjusted results (95% CI 0.61-0.85). The graphical odd-man-out method produced a 94% interval of 0.70-0.72. The pooled odds ratio of the case-control studies was lower than for the longitudinal studies. However, subdividing studies by the type of case source produced a pooled odds ratio for studies using hospital-based cases of 0.49 (95% CI 0.39-0.63) which was considerably less than that of studies using population-based cases: 0.95 (0.78-1.16). This difference was unlikely to have explained by bias due to selection of controls. We suggest that OCP use may not have a "protective effect" on the development of RA but may prevent the progression to severe disease by modifying the disease process.