fentanyl (sometimes spelled fentanil) is a synthetic opioid that is used as a painkiller. it also acts as a sedative. it has a short duration of action and a rapid onset. the two most common forms are patches that are put on the skin, and an intravenous solution. paul janssen developed fentanyl in 1960. fentanyl is about 120 times as potent as morphine; this means, much less fentanyl is needed to get the same effect.