Nonspecific inflammatory bowel disease and smoking.

The authors assessed the relation between cigarette smoking and nonspecific inflammatory bowel disease in a case-control study of 124 cases of ulcerative colitis, 109 cases of Crohn's disease, and 250 age- and sex-matched control subjects in hospital for acute nongastric or intestinal conditions unrelated to smoking. For ulcerative colitis, the risk for current smoking compared with never smoking was 0.5, with a 95% confidence interval (Cl) of 0.3-1.0. They observed decreasing risk with increasing number of cigarettes smoked. The risk for ex-smokers, however, was greater than that for never smokers (relative risk = 2.7; 95% Cl = 1.5-4.9). The elevated risk of ulcerative colitis in ex-smoking in the presence of an overall lack of association with ever-smoking may plausibly be attributed to either 1) brief induction time of a protective effect of smoking on ulcerative colitis or 2) selective cessation of smoking due perhaps to very early symptoms of the disease. If time at first onset of bowel symptoms, instead of clinical diagnosis, is considered as the index date, the negative association between ulcerative colitis and current smoking would have weakened in men and disappeared in the overall series. There was clear evidence of a positive association between cigarette smoking and Crohn's disease (relative risk for ever smokers vs. never smokers = 4.0; 95% Cl = 2.2-7.3). The risk estimates increased with the number of cigarettes smoked per day and duration of habit. The association between current smoking and Crohn's disease was even stronger when age at first onset of bowel symptoms was considered as the index date, but the risk for ex-smokers fell below unity.