Adverse drug effects on the gastrointestinal (GI) tract can occur: (1) as a predictable result of a drug’s mode of action; (2) by direct injury; (3) through compromising GI mucosal integrity; or (4) as a consequence of changes in colonic microbiota. Non-steroidal anti-inflammatory drugs are the most common cause of gastroduodenal injury, owing to inhibition of prostaglandin synthesis, and increase the risk of serious GI complications. Low doses of aspirin are associated with an increased risk of upper GI haemorrhage.