Smoking: effects on multiple sclerosis susceptibility and disease progression.

Multiple sclerosis (MS) is associated with both genetic and environmental factors that influence disease susceptibility. Exposure to cigarette smoke is emerging as a viable environmental risk factor for MS that contributes to both increased disease susceptibility and more rapid disease advancement. The relative risk for MS development is approximately 1.5 for smokers compared with nonsmokers. Furthermore, there may be important interactions between smoking, an individual's genetic background, and other environmental risk exposures. This review summarizes the current evidence supporting the association of smoking with MS risk and disease course, with additional comments on causation.