Abstract
During the pa t decade, smoking habit has been identified as a major exogenous factor in inflammatory bowel disease (IBD). It is associated not only with the development of the disease but al o with the clinical course in established disease. IBD combines absolute opposites as smoking is associated with Crohn’s disease and nonsmoking or former smoking with ulcerative colitis. The first reports of a negative association between smoking and ulcerative colitis were based on independent, clinical observations; from those studies a positive association was found between smoking and Crohn’s disease. Epidemiological studies that followed consistently showed that smokers have a reduced risk of ulcerative colitis and an increased risk of Crohn’s disease and that exsmokers have an increased risk of ulcerative colitis. In ulcerative colitis, but not in Crohn’s disease, a dose-response pattern has been demonstrated. Changes in clinical course, in disease severity and extension, and in recurrence rate indicate substantial clinical effects of smoking with a protective effect of smoking in ulcerative colitis and an aggravating effect in Crohn’s disease. There are also indications of smoking’s effects on changes in IBD epidemiology and sex distribution. The biological explanation to the finding is unknown. Smoking may aggravate Crohn’s disease by vascular effects. Theories on the protective effect in ulcerative colitis include effects on immune and inflammatory response, on mucus and on intestinal permeability. Possibly, beneficial effects in ulcerative colitis are exerted by nicotine but further studies are needed. Due to overall negative effects of smoking, IBD patients should not smoke. It seems, however, reasonable to give individual advice in patients with ulcerative colitis who have experienced a beneficial effect of ·making considering both current health status and life situation.