Carfentanil is an ultra-potent opioid of public health and chemical weapons defense concern. Exposure to carfentanil has been seen in both illicit drug use and in the resolution of a hostage situation in Moscow, Russia in 2002. While opioid pharmacology and toxicology are well researched topics, carfentanil remains unstudied relative to its clinically used counterparts like fentanyl. Similarly to fentanyl, carfentanil elicits is toxicity from central nervous system depression, largely in regions of the brain involved in spontaneous respiratory rhythmogenesis. By depressing the respiratory centers of the brain, respiratory failure can occur, and can lead to death. Carfentanil is of concern because it has demonstrated 100 times higher potency than its prototype, fentanyl, in analgesic assays in rodents. Carfentanil has very little human relevant data to indicate its toxicity in man for use in public health or chemical defense risk assessments.