Carfentanil or carfentanyl (Wildnil) 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 first synthesized in 1974 by a team of chemists at Janssen Pharmaceutica which included Paul Janssen. It has a quantitative potency approximately 10,000 times that of morphine and 100 times that of fentanyl, with activity in humans starting at about 1 microgram. It is marketed under the trade name Wildnil as a general anaesthetic agent for large animals. Carfentanil is intended for large-animal use only as its extreme potency makes it inappropriate for use in humans. Currently sufentanil, approximately 10–20 times less potent (500 to 1000 times the efficacy of morphine per weight) than carfentanil, is the maximum strength fentanyl analog for use in humans.
Effects and side effects of carfentanil in humans are similar to those of other and include euphoria, relaxation, pain relief, pupil constriction, drowsiness, sedation, slowed heart rate, low blood pressure, lowered body temperature, loss of consciousness, and suppression of breathing. The effects of carfentanil, including overdose, can be reversed by the opioid antagonists naloxone and naltrexone, though higher or multiple doses than usual may be necessary compared to other opioids