Do oral contraceptives prevent rheumatoid arthritis?

Two studies have suggested that the risk of rheumatoid arthritis in women using oral contraceptives is less than half that of nonusers. When a third study from the Mayo Clinic failed to confirm these findings, it was criticized for inclusion of ineligible subjects, misclassification of oral contraceptive use, and inadequate statistical power. Recent expansion of the Mayo Clinic's data resources provided a unique opportunity to resolve the controversy, and a new population-based case-control study was undertaken. In comparison with the previous study, the new investigation had 2.2 times as many eligible cases and more complete ascertainment of oral contraceptive use via access to the records of Planned Parenthood of Minnesota. Comparing any prior use of oral contraceptives with never having used them, the relative risk of rheumatoid arthritis estimated from 182 cases and their 182 matched controls was 1.1 (95% confidence interval 0.7 to 1.7). The relative risk for current use was 1.3 (95% confidence interval, 0.7 to 2.4). The lack of a protective effect was independent of age, disease severity, and disease end point (date of confirmed diagnosis or symptom onset).