Smoking as a risk factor for multiple sclerosis

Background: Smoking has been associated with an increased risk for multiple sclerosis, but no studies have measured levels of the nicotine metabolite cotinine in prospectively collected samples to assess exposure. Objective: To investigate the effects of laboratory defined tobacco use on the risk for multiple sclerosis using prospectively collected biobank blood samples. Methods: Levels of cotinine were measured in n=192 cases, and n=384 matched controls, using an immunoassay. The risk for multiple sclerosis was estimated using matched logistic regression. Results: Elevated cotinine levels (≥10 ng/ml) were associated with a significantly increased risk for multiple sclerosis, (odds ratio, OR 1.5, 95% confidence interval, CI 1.0–2.1). This association was only present in young individuals (below median age at blood sampling, <26.4 years), (OR 2.2, 95% CI 1.3–3.8). Conclusions: This study confirms that smoking is a risk factor for multiple sclerosis. It has the advantage of using analyses of cotinine levels in samples that were collected several years before disease onset, thus excluding any risk for recall bias and minimising the risk for reversed causation. Our results also suggest that the smoking related immunological events that contribute to the development of multiple sclerosis occur early in life.