Crohn disease is a chronic inflammatory bowel disease. It involves swelling and irritation of the lining of the digestive tract. Symptoms include diarrhea, fever, abdominal pain, weight loss, and fatigue. Patients with Crohn disease usually have signs and symptoms of illness that recur at intervals throughout their lives. Doctors use several drugs to treat recurring bouts of the disease. If symptoms are severe or if patients do not respond to other therapies, doctors may prescribe powerful new drugs called anti–tumor necrosis factor (anti-TNF) agents. Some patients who initially respond to an anti-TNF agent may develop tolerance to that agent, and it will no longer work for them. Other patients may have severe adverse effects that make them stop taking the anti-TNF agent. Few studies tell us whether it is worthwhile to try a second anti-TNF agent in patients who have already tried an anti-TNF agent.