4-fluoroisobutyrylfentanyl (also known as 4-fibf and p-fibf) is an opioid analgesic that is an analog of butyrfentanyl and structural isomer of 4-fluorobutyrfentanyl and has been sold online as a designer drug. it is closely related to 4-fluorofentanyl, which has an ec value of 4.2 nm for the human μ-opioid receptor. 4-fluoroisobutyrylfentanyl is a highly selective μ-opioid receptor agonist whose analgesic potency is almost ten times of that reported for morphine.