Meta-analyses of all available data have shown that smoking is associated with a 50% increased risk of MS.1
A key aspect in establishing causation of any environmental factor is a dose effect. Many studies do demonstrate increasing risk with increasing numbers of cigarettes smoked.2 Further, temporality is important to prove causation.
Longitudinal studies have shown that individuals in whom MS develops are more likely to smoke before disease onset.2 Longitudinal analyses have also shown that disability accumulates more rapidly in MS patients who smoke than in never-smokers.3
The mechanism by which smoking affects susceptibility to and progression of MS is unclear. Studies have shown that tobacco snuff use does not increase the risk of MS and thus cigarette smoking does not appear to influence MS risk solely by nicotine(Drug information on nicotine).4
1. Handel AE, Williamson AJ, Disanto G, et al. Smoking and multiple sclerosis: an updated meta-analysis. http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0016149
2. Hernán MA, Olek MJ, Ascherio A. Cigarette smoking and incidence of multiple sclerosis. Am J Epidemiol. 2001;154:69-74. http://www.ncbi.nlm.nih.gov/pubmed/11427406
3. Healy BC, Ali EN, Guttmann CR, et al. Smoking and disease progression in multiple sclerosis. Arch Neurol. 2009;66:858-864. http://www.ncbi.nlm.nih.gov/pubmed/19597087
4. Hedström AK, Bäärnhielm M, Olsson T, Alfredsson L.Tobacco smoking, but not Swedish snuff use, increases the risk of multiple sclerosis. Neurology. 2009;73:696-701. http://www.ncbi.nlm.nih.gov/pubmed/19720976