Ohmefentanyl (β-hydroxy-3-methylfentanyl) is an extremely potent analgesic drug which selectively binds to the µ-opioid receptor.

Ohmefentanyl is one of the most potent μ-receptor agonists known, comparable to super-potent opioids such as carfentanil and etorphine which are used for tranquilizing large animals such as elephants in veterinary medicine. In mouse studies, the most active isomer 3R,4S,βS-ohmefentanyl was 28 times more powerful as a painkiller than fentanyl, the chemical from which it is derived, and 6300 times more effective than morphine.^[1]. Ohmefentanyl has three stereogenic centers and so has eight stereoisomers, which are named F9201-F9208. Researchers are studying the different pharmaceutical properties of these isomers.^[2].

The 4″-fluoro analogue of the 3R,4S,βS isomer of ohmefentanyl is the most powerful opioid yet discovered, possessing an analgesic potency approximately 18,000-fold greater than morphine.^[3]