Effect of smoking on inflammatory bowel disease: Is it disease or organ specific?

Smoking is an important environmental factor in inflammatory bowel disease (IBD) with differing effects in ulcerative colitis (UC) and Crohn's disease (CD). Never smoking and formerly smoking increase the risk of UC, whereas smoking exacerbates the course of CD. The potential mechanisms involved in this dual relationship are yet unknown. A reasonable assumption is that smoking has different effects on the small and large intestine. This assumption is based on animal and human studies that show that the effects of smoking/nicotine on CD and UC depend on the site of inflammation and not on the type of disease.