The purpose of this study is to provide new information and to determine which kinds of brief, tolerable, experimental pain are affected by the drugs being studied in patients with irritable bowel syndrome, a disease that causes chronic abdominal pain. The effects of dextromethorphan, naloxone, and fentanyl on experimental pain stimuli will be tested in these studies to better understand what causes irritable bowel syndrome.

• Patients with irritable bowel syndrome and healthy pain-free individuals (as controls) will be studied.
• No medical condition that would contraindicate use of dextromethorphan, fentanyl or naloxone.