Patients with inflammatory bowel disease (IBD) often require a combination of drugs, some of which are taken for many years, to control their disease. Some of these drugs have potentially serious side effects, which may be initiated or exacerbated by interaction with other agents used to treat IBD. Furthermore, patients with IBD may take treatment for other, unrelated conditions. It is important for doctors who manage patients with IBD to be aware of, and thereby minimize, the dangers presented by such drug interactions. In this review, we summarize the common and important interactions of drugs used in patients with IBD, including some that may be of therapeutic benefit. Particular attention is paid to interactions that occur where both drugs are used to treat IBD.