Smoking is a risk factor for multiple sclerosis: a metanalysis.

Several case control studies have probed a link between cigarette smoking and subsequent multiple sclerosis (MS). Data collection and statistical methods have varied, and frequently, case numbers have been small. Publications relating to MS and smoking are reviewed and combined where comparable methods have been used. Metanalysis of six informative studies show significantly elevated odds or rate ratios, ranging from 1.22 to 1.51, depending on the method of analysis, confirming that the risk of MS is increased for those who smoke prior to disease onset, as measured by commencement of symptoms. A variety of direct causative mechanisms are discussed, but an indirect association through health adverse conduct is favoured.