Smoking may be considered an established risk factor for sporadic ALS.

OBJECTIVES: A 2003 evidence-based review of exogenous risk factors for sporadic amyotrophic lateral sclerosis (ALS) identified smoking as the only risk factor that attained "probable" (more likely than not) status, based on 2 class II studies. The purpose of the current, evidence-based, update was to see if the conclusion of the previous review needed to be modified, based on studies published since.
METHODS: A Medline literature search was conducted for the period between 2003 and April 2009 using the search terms smoking and (ALS or "amyotrophic lateral sclerosis" or MND or "motor neuron disease"). The references of primary articles and reviews were checked to assure completeness of the search. Primary articles published since the previous review were classified as before.
RESULTS: Twenty-eight titles were identified, but only 7 articles met inclusion criteria. Of these, 1 provided class II evidence, and 1 class III evidence: both showed increased risk of ALS with smoking. The class II study showed a dose-response effect, and risk decreasing with number of years since quitting smoking. Five articles provided class IV or V evidence, which may not be relied upon to draw conclusions.
CONCLUSIONS: Smoking may be considered an established risk factor for sporadic amyotrophic lateral sclerosis (ALS) (level A rating; 3 class II studies, 1 class III study). Evidence-based analysis of epidemiologic data shows concordance among results of better-designed studies linking smoking to ALS, and lets those results drive the conclusions.