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Etorphine (Immobilon or M99) is a semi-synthetic opioid possessing an analgesic potency approximately 1,000-3,000 times that of morphine depending on the situation. It 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 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.. For this reason the package as supplied to vets always includes the human antidote as well as Revivon.

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 risk 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.


Etorphine is an agonist at μ, δ, and κ opioid receptors. It also has a weak affinity for the ORL1 nociceptin/orphanin FQ receptor.

Legal status

In Hong Kongmarker, Etorphine is regulated under Schedule 1 of Hong Kong'smarker Chapter 134 Dangerous Drugs Ordinance. It can only be used legally by health professionals and for university research purposes. The substance can be given by pharmacists under a prescription. Anyone who supplies the substance without prescription can be fined $10000(HKD). The penalty for trafficking or manufacturing the substance is a $5,000,000 (HKD) fine and life imprisonment. Possession of the substance for consumption without license from the Department of Health is illegal with a $1,000,000 (HKD) fine and/or 7 years of jail time.

In the Netherlandsmarker, Etorphine is a list I drug of the Opium Law. It is only used for veterinary purposes in zoos to immobilize large animals.

In the US, Etorphine is listed as a Schedule I drug, although Etorphine hydrochloride is classified as Schedule II.

In The Media

Dexter - M99 is the drug Dexter uses to sedate his victims.


  1. Bentley KW, Hardy DG. "Novel analgesics and molecular rearrangements in the morphine-thebaine group. 3. Alcohols of the 6,14-endo-ethenotetrahydrooripavine series and derived analogs of N-allylnormorphine and -norcodeine." Journal of the American Chemical Society. 1967 Jun 21;89(13):3281-92. PMID 6042764
  2. Opium: the king of narcotics
  3. Bentley KW, Hardy DG. "New potent analgesics in the morphine series." Proceedings of the Chemical Society. 1963;220.
  4. Zoo Vet At Large - Sky Travel (8 July 2007, Part 10 of 26)
  5. Hawkinson JE, Acosta-Burruel M, Espitia SA. "Opioid activity profiles indicate similarities between the nociceptin/orphanin FQ and opioid receptors." European Journal of Pharmacology. 2000 Feb 18;389(2-3):107-14. PMID 10688973

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