Carfentanil or Carfentanyl (R33799) is an analogue of the popular synthetic opioid analgesic fentanyl, and is one of the most potent opioids known (also the most potent opioid used commercially). Carfentanil was discovered by Janssen Pharmaceutica. It has a quantitative potency approximately 10,000 times that of morphine and 100 times that of fentanyl, activity in humans starting at about 1 μg. It is marketed under the trade name Wildnil as a tranquilizer for large animals. Carfentanil is intended for animal use only as its extreme potency makes it inappropriate for use in humans.
A good practical example of the drug in use in veterinary practice was shown in an episode of the Discovery Channel series, Animal Cops: Houston, where carfentanil was administered orally (dissolved in honey, specifically) to a full-grown brown bear to tranquilize it so that it could be safely relocated to the Houston Zoo from a south Texas animal abuser's property. A few tablespoons' worth of the anesthetic in a diluted solution was sufficient to put the grizzly bear, weighing more than 1000 pounds, to sleep.
It is thought that in the 2002 Moscow theater hostage crisis, the Russian military made use of an aerosol form of either carfentanil or another similar drug such as 3-methylfentanyl to subdue Chechen hostage takers. Its short action, easy reversibility and therapeutic index (10600 vs. 300 for fentanyl) would make it a near-perfect agent for this purpose. Wax et al. surmise from the available evidence that the Moscow emergency services had not been informed of the use of the agent, but were instructed to bring opioid antagonists. Because of the lack of information provided, the emergency workers did not bring adequate supplies of naloxone or naltrexone (opioid antagonists) to prevent complications in many of the victims. Assuming that carfentanil was the only active constituent (which has not been verified by the Russian military), the primary acute toxic effect to the theatre victims would have been opioid-induced apnea; in this case mechanical ventilation and/or treatment with opioid antagonists would have been life-saving for many or all victims.