The Legislative Analysis and Public Policy Association is monitoring the emergence of novel psychoactive substances (NPS) appearing on the illicit drug market in the United States. The term “novel” does not denote a new, never-before-seen substance but rather a substance that is newly available in the drug market. This fact sheet examines medetomidine, a powerful veterinary sedative that has been appearing as an adulterant in fentanyl mixtures in the U.S. Medetomidine is a synthetic alpha-2 agonist that is used as a sedative in veterinary medicine and sold under the brand name Domitor®. Medetomidine belongs to the same drug class as xylazine, but its potency is 200 times greater, and has a longer duration of action, than xylazine. The U.S. Food and Drug Administration has only approved medetomidine for use in veterinary medicine, and information about the use of the substance in humans is limited. The effects of medetomidine on humans are not well understood but include prolonged sedation, slow heart rate, slowed breathing, and low blood pressure. Similar to xylazine, the combination of medetomidine with other substances, like fentanyl, can enhance or prolong the effects of the substance and place the individual at a higher risk of overdose than from...