Etorphine

Etorphine, also known by its development code name M99, is a highly potent semi-synthetic opioid analgesic developed from thebaine in the 1950s by Janssen. It is used primarily to immobilize large mammals in veterinary medicine and wildlife management, including elephants, rhinoceroses, and other large ungulates. It is not approved for human use due to extreme potency and risk of fatal respiratory depression.

Etorphine acts as a mu-opioid receptor agonist and is estimated to be about 1,000 times more potent than morphine, substantially more potent than fentanyl. It is administered by injection in very small doses and has a rapid onset. Side effects include respiratory depression, hypotension, bradycardia, and hypersalivation, and adverse effects can be fatal if not managed with supportive care. Because of its potency, accidental exposure to humans can be deadly, so handling requires strict safety protocols.

In veterinary practice, etorphine is typically used with sedatives or tranquilizers for immobilization, and its effects are reversed with diprenorphine. In some cases, naloxone or naltrexone may be used as auxiliary reversers, but diprenorphine is the primary antagonist.

Etorphine is tightly regulated worldwide and is available only to licensed veterinarians or research institutions. Possession, distribution, or use outside approved veterinary contexts is illegal in many jurisdictions in addition to being hazardous to humans.

Etorphine was developed in the 1950s by Janssen Pharmaceutica and first marketed as M99. It has since become an essential tool in wildlife immobilization but remains among the most restricted opioid analgesics.