Smoking and ulcerative colitis.

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Abstract
In a case-control study of smoking and ulcerative colitis patients with the disease were much less likely to smoke than community controls matched for age and sex. The difference was substantial, with an estimated relative risk of 3.8 for non-smoking on current habits, was even larger (6.2) when habits at onset of the disease were examined, and was mainly accounted for by 42 of 55 patients who had given up smoking a mean of eight years before onset. The association could not be explained by confounding by social class. These findings suggest that smoking directly or indirectly confers protection against ulcerative colitis.