The animal tranquilizers xylazine[1] and medetomidine[2] are in approximately one-third of the illegal opioids available in the U.S., including fentanyl[3], heroin[4] and oxycodone[5]. Animal tranquilizers enhance the user’s euphoric high from opioids, particularly in those who have developed a tolerance to the opioid[6]. But adding the tranquilizers to these already illicit drugs could keep naloxone[7], a medication known to prevent...