Etorphine (M99) is a semi-synthetic opioid possessing an analgesic potency approximately 1,000–3,000 times that of morphine.[1] It was first prepared in 1960 from oripavine, which does not generally occur in opium poppy extract but rather the related plants Papaver orientale and Papaver bracteatum.[2] It was reproduced in 1963 by a research group at MacFarlan Smith in Gorgie, Edinburgh, led by Kenneth Bentley.[3] It can be produced from thebaine.[4]

Etorphine is available legally only for veterinary use and is strictly governed by law. It is often used to immobilize elephants and other large mammals. Diprenorphine (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 along with the etorphine.

The human antidote is generally naloxone, not diprenorphine, and is always prepared before the preparation of etorphine to be immediately administered following accidental human exposure to etorphine. The LD50 in humans is 3 μg which led to the requirement that the medicine include an equal dose of an antidote, diprenorphine or naloxone.