Dr Graham Cope discusses cigarette smoking and its profound effects on the gastrointestinal system. Smoking cigarettes reduces the gastric mucosal barrier, increasing the risk of gastric ulceration and cancer, probably by reducing prostaglandin synthesis, which controls mucus and bicarbonate production. Smoking increases the risk of Crohn's disease in the small intestine and is detrimental to surgical and non-surgical treatment, possibly increasing mucosal permeability and reducing wound healing. In the colon, smoking is protective against ulcerative colitis, as nicotine suppresses the associated enhanced immune response. However, smoking is positively related to colon cancer and collagenous colitis, by the inhibition of other types of immune cells and the increase of free radical-mediated mutations and carcinogenic changes. Recent evidence shows that nicotine is a direct stimulus to the production of cancer cells, which may cast doubt on the use of nicotine-replacement therapy (NRT) as a long-term aid to smoking cessation