The truth about cigarette smoking and the risk of inflammatory bowel disease.

The etiology of inflammatory bowel disease (IBD) is generally believed to be multifactorial in nature involving both genetic and environmental factors. Cigarette smoking has been shown through previous retrospective observational studies to be an environmental factor with both positive and negative influences in IBD. Smoking increases the risk of developing Crohn's disease (CD) but not the risk of ulcerative colitis (UC). Meanwhile smoking cessation increases the risk of a UC flare while CD patients are more likely to show a decrease in disease severity. Unfortunately, these observational studies cannot control for bias the way a randomized controlled trial can, however, they still reveal meaningful truths about smoking and IBD. The study by Higuchi et al. (1) adds to our understanding of the impact of smoking on IBD in several ways. They showed that increasing exposure to smoking is associated with an increased risk of developing CD. They also showed that the risk of UC is highest in the first 2-5 years after smoking cessation but remains elevated for > 20 years. This research also raises several new issues regarding the association between smoking and IBD, which hopefully will be answered through future well-designed observational studies.