Drugs used for treating inflammatory bowel disease are known to have a number of gastrointestinal and liver adverse effects. 5-ASA products are relatively safe and have few adverse events. In contrast sulfasalazine has side effects in 11-40% of treated patients including fatigue, nausea, abdominal pain and diarrhoea. Glucocorticoids can induce or propagate peptic ulcers and upper GI bleeding especially in combination with NSAIDs. Thioguanins may have severe gastrointestinal side effects including gastrointestinal complaints (in up to 12%), hepatotoxicity (up to 4%) and pancreatitis (1%). Nodular regenerative hyperplasia (NRH) is an important potential side effect of thiopurine therapy especially in men with Crohn's disease after ileocecal resection. NRH may ultimately lead to portal hypertension. A major concern of methotrexate therapy in IBD besides myelosuppression and pulmonary fibrosis is hepatotoxicity. 5mg of folic acid substitution per week potentially decreases gastrointestinal side effects by 80% without interfering with the efficacy of methotrexate. Besides renal dysfunction, tremor, hirsutism, hypertension and gum hyperplasia cyclosporine is known to have a number of gastrointestinal side effects that occur with less frequency such as diarrhoea (up to 8%) nausea and vomiting (up to 10%) and hepatotoxicity in 1-4%. Rare gastrointestinal adverse events are gastritis and peptic ulcers. Paying attention to these potential deleterious side effects is mandatory for physicians treating IBD patients.