Many moons ago, when I was still working in private practice, a fellow entered the clinic and asked if we had any elephant juice. Elephant juice is one of the colloquial names for etorphine. Etorphine is a narcotic several thousand times stronger than morphine. It was developed by the South Africans to immobilise elephants, rhinos and other large difficult to handle animals. The drug is so potent that a scratch from a needle dipped in the stuff is potentially fatal.
Interestingly, while a standard dose results in immobilisation a reduced dose causes excitation. This manifests itself as compulsive walking or running. The affected animal appears oblivious to its surroundings and continues to run until it drops from exhaustion, meets an immovable object or receives veterinary intervention. It has been used in the horse racing industry when a bit of illegal zip is required. In Australia zoo vets are generally the only ones with legal access to it. As it was illegal to import etorphine into the US the Americans produced their own version, called carfentanil, which is even more potent.
Dr. F. Bunny