Etorphine (Immobilon or M99) is a semi-synthetic opioid possessing an analgesic potency approximately 10,000 times that of morphine and was first prepared in 1960 from oripavine, which does not generally occur in opium poppy extract but rather in "poppy straw" and in related plants, Papaver orientale and Papaver bracteatum. It was later reproduced in 1963 by a research group at Macfarlan-Smith and Co. in Edinburgh, led by Professor Kenneth Bentley. It can also be produced from thebaine.
Etorphine 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 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 an opioid painkiller for human usage in China. It is claimed to be less addictive than traditional opioids but this has yet to be confirmed.
In the US, Etorphine is listed as a Schedule I drug, although Etorphine hydrochloride is classified as Schedule II.