Exposure to passive smoking during adolescence is associated with an increased risk of developing multiple sclerosis

Background: Environmental factors are associated with acquiring multiple sclerosis (MS) particularly in adolescence. Objective: To test for association between MS and exposure to passive smoking at the age of 10–19. Methods: A total of 919 patients from the Danish MS Registry and Biobank and 3419 healthy blood donors who had not smoked before the age of 19 were targeted. We analyzed separately for each sex and for those never-smokers (cohort 1) and active smokers above the age of 19 (cohort 2). All participants completed standardized questionnaires about smoking and lifestyle. We matched cases and controls in the ratio of 1:2 by propensity scores discarding unmatchable individuals and used logistic regression adjusted for all covariates and interactions. Results: After matching, we included 110/213 male cases/controls and 232/377 female case/controls in cohort 1. In cohort 2, the numbers were 160/320 and 417/760, respectively. Among women in cohort 1, the odds ratio (OR) for MS by passive smoking at the age of 10–19 was 1.432 ( p = 0.037) but in men it was 1.232 ( p = 0.39). Among men in cohort 2, OR was 1.593 ( p = 0.022) but among women it was only 1.102 ( p = 0.44). Conclusion: Among never smokers, female MS cases were more often than female controls reported with passive smoking between the age of 10 and 19, and among smokers above the age of 19, male MS patients were more often than male controls reported with passive smoking.