Cigarette smoking and incidence of multiple sclerosis

Experimental data suggest that cigarette smoking may play a role in the dev elopment of multiple sclerosis (MS), but epidemiologic studies have been sm all and inconclusive. The authors assessed the association between MS incid ence and smoking in two cohort studies of US women, the Nurses' Health Stud y (121,700 women aged 30-55 years at baseline in 1976) and the Nurses' Heal th Study II (116,671 women aged 25-42 years at baseline in 1989). Smoking h istory was assessed at baseline and updated on biennial questionnaires. A t otal of 315 definite or probable cases of MS were documented. Compared with that for women who never smoked, the relative incidence rate was 1.6 (95% confidence interval: 1.2, 2.1) among current smokers and 1.2 (95% confidenc e interval: 0.9, 1.6) among past smokers after adjustment for age, latitude , and ancestry. The relative rate increased significantly with cumulative e xposure to smoking (p for trend < 0.05), from 1.1 (95% confidence interval: 0.8, 1.6) for 1-9 pack-years to 1.5 (95% confidence interval: 1.2, 2.1) fo r 10-24 pack-years and 1.7 (95% confidence interval: 1.2, 2.4) for 25 or mo re pack-years. Similar results were obtained after adjustment for other pot ential confounding factors. Although these prospective results do not prove a cause-and-effect relation, they suggest that smoking is associated with an increased risk of MS.