Fentanyl brand names include Actiq, Duragesic, Fentora, Onsolis, Sublimaze and Instanyl is a synthetic primary -opioid agonist commonly used to treat chronic breakthrough pain and is commonly used in pre-procedures. It is approximately 350 times more potent than morphine, with 100 micrograms of fentanyl approximately equivalent to 10 mg of morphine and 75 mg of pethidine (meperidine) in analgesic activity. It has an LD50 of 3.1 milligrams per kilogram in rats, 0.03 milligrams per kilogram in monkeys, and an undetermined LD50 in humans. mFentanyl was first synthesized by Dr. Paul Janssen in 1960 following the medical inception of meperidine several years earlier. Janssen developed Fentanyl by assaying analogues of the structurally-related drug meperidine for its opioid activity. The widespread use of fentanyl triggered the production of fentanyl citrate, which entered the clinical practice as a general anaesthetic under the trade name Sublimaze in the 1960s.