Carfentanil or carfentanyl, sold under the brand name Wildnil, is an extremely potent opioid analgesic used in veterinary medicine to anesthetize large animals such as elephants and rhinoceroses.

[2] Chosen for its high therapeutic index, carfentanil was first sold in 1986 under the brand name "Wildnil" for use in combination with an α2-receptor agonist as a tranquilizing agent[3]: 9  for large mammals like hippos, rhinos, and elephants.

Over three hundred cases of overdose related to fentanyl and Carfentanil analogues were reported between August and November 2016 in several of the United States, including Ohio, West Virginia, Indiana, Kentucky and Florida.

[17][18] Around 2016, the United States and Canada reported a dramatic increase in shipment of carfentanil and other strong opioid drugs to customers in North America from Chinese chemical supply firms.

In June 2016, the Royal Canadian Mounted Police seized one kilogram of carfentanil shipped from China in a box labeled "printer accessories".

[17] Carfentanil was not a controlled substance in China until 1 March 2017,[19] and until then was manufactured legally and sold openly over the Internet, being actively marketed by several Chinese chemical companies.

[20] Researchers had previously surmised from the available evidence that the Moscow emergency services had not been informed of the use of the agent, despite being instructed to bring opioid antagonists to the scene.

Unaware that hundreds of patients had been exposed to high doses of strong opioids, the emergency workers failed to bring sufficient quantities of naloxone and naltrexone to counteract the effects of carfentanil and remifentanil.

[21] The toxicity of carfentanil in humans and its ready commercial availability has raised concerns over its potential use as a chemical weapon of mass destruction by rogue nations and terrorist groups.

Carfentanil is classified as Schedule II under the Controlled Substances Act in the United States with a DEA ACSCN of 9743 and a 2016 annual aggregate manufacturing quota of 19 grams (less than 0.7 oz.).