An examination is needed of the potential adverse effects of the agents most commonly used to treat inflammatory bowel disease. Most of these therapies can be used safely to induce or maintain remissions, although some aspects of monitoring for toxicity are necessary. Aminosalicylates, including sulfasalazine and mesalamine delivery systems, are most commonly associated with sulfa-related effects (sulfasalazine) or intolerance, with rare instances of nephritis, pulmonitis, hepatitis, or worsening colitis. The immunomodulators are most commonly associated with bone marrow suppression, hepatitis, and the risk of opportunistic infections. Methotrexate is contraindicated in pregnancy. Antibiotics used for inflammatory bowel disease are generally safe and well tolerated, although metronidazole carries a long-term risk of peripheral neuropathy. The well-recognized multitude of adverse effects from corticosteroids are eliminated or minimized when rapidly metabolized steroids, such as budesonide, are used.