Etorphine (Immobilon or M99) is a synthetic cousin of morphine and 1500-3000 times more powerful. It was invented in 1963 by a research group at McFarlan-Smith and Co. in Edinburgh, led by Professor Kenneth Bentley. It can be produced from thebaine. It is most often used to immobilize elephants and other large mammals. Etorphine is only available legally for veterinary use and is strictly governed by law. Diprenorphine (M5050) is an opioid receptor antagonist that can be administered in proportion to the amount of etorphine used (1.3 times) to reverse its effects. Veterinary-strength etorphine is absolutely fatal to humans; one drop on the skin can cause death within a few minutes.

Large Animal Immobilon is a combination of etorphine plus acepromazine maleate. An etorphine antidote Large Animal Revivon contains mainly diprenorphine for animals and a human-specific naloxone-based antidote, which should be prepared prior to the etorphine.

A close relative, dihydroetorphine has been used as opiate painkiller for human USAge in China. It is claimed to be less addictive than traditional opioids but this has yet to be confirmed.