Carfentanil is an analogue of the synthetic opioid analgesic fentanyl, and is one of the most potent opioids known (also the most potent opioid used commercially). Carfentanil 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. Carfentanil is intended for large-animal use only as its extreme potency makes it inappropriate for use in humans.