Fentanyl is a potent opioid analgesic that is a prominent member of the narcotic analgesic class of drugs designed to relieve pain. The drug acts primarily with the opioid mu-receptor binding sites that are discretely distributed in the human brain, spinal cord, and in other tissues. Due to its action on the central nervous system, fentanyl has therapeutic value in analgesia and sedation by increasing the patient's tolerance for pain and decreasing the perception of suffering. Like other narcotics, fentanyl may become habit-forming with continued use and doses should be monitored by a physician. Side effects include hallucinations, chest pain, irregular heartbeat, mood swings, and disorientation.