Etorphine is often used to immobilize elephants and other large mammals. Etorphine is available legally only for veterinary medicine|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.