Fentanyl is an opioid analgesic, first synthesized by Janssen Pharmaceutica (Belgium) in the late 1950s, with an analgesic potency of about 80 times that of morphine. Fentanyl was introduced into medical practice in the 1960s as an intravenous anesthetic under the trade name of Sublimaze. Fentanyl has an LD50 of 3.1 milligrams per kilogram in rats, and, 0.03 milligrams per kilogram in monkeys.