Etorphine

Etorphine (Immobilon or M99) is a semi-synthetic opioid possessing an analgesic potency approximately 1,000-3,000 times that of morphine. It was first prepared in 1960 from oripavine, which does not generally occur in opium poppy extract but rather in "poppy straw" and in the related plants Papaver orientale and Papaver bracteatum. It was later reproduced in 1963 by a research group at MacFarlan Smith in Gorgie, Edinburgh, led by Professor Kenneth Bentley. It can also be produced from thebaine.

Etorphine is often used to immobilize elephants and other large mammals. Etorphine is available legally only for veterinary use and is strictly governed by law. Diprenorphine (M5050), also known as Revivon, 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. For this reason the package as supplied to vets always includes the human antidote as well as Etorphine.

One of its main advantages in general veterinary work is its speed of operation and, even more important, the speed with which Revivon reverses the effects. For example, operations on valuable animals such as racehorses, using alternative anesthetics risks the animal injuring itself as the anesthetic wears off. The rapid action of Immobilon and Revivon means the animal can be back on its feet within a relatively short time and aware of its surroundings more quickly, thus reducing any tendency to panic and move around rapidly while still partially under the influence of the anesthetic. For this reason, its use is popular with many vets.

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.