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

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.