Cigarette smoking increases the risk of rheumatoid arthritis: Results from a nationwide study of disease‐discordant twins

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Abstract
Objective. To test the hypothesis that cigarette smoking is associated with susceptibility to rheumatoid arthritis (RA) by comparing smoking history between twins with RA and their unaffected co‐twins. Methods. Interview questionnaires on smoking history were administered to 79 identical (monozygotic [MZ]) and 71 same‐sex nonidentical (dizygotic, [DZ]) twin pairs who were discordant for RA, recruited from the Arthritis and Rheumatism Council Twin Study. Results were expressed as odds ratios (OR) with 95% confidence intervals (95% CI). Results. Although most twin pairs were concordant for smoking history, there was a strong association between ever smoking and RA in the MZ pairs (OR 12.0, 95% CI 1.78–513), with a similar trend observed in the DZ pairs (OR 2.5, 95% CI 0.92–7.87). Conclusion. The discordance in cigarette smoking history for individuals who are at presumed identical genetic risk for RA supports other data suggesting the role of smoking in disease susceptibility.