Animal tranquilizers found in illegal opioids may suppress the lifesaving medication naloxone − and cause more overdose deaths

In March 2023, the U.S. Food and Drug Administration approved naloxone as a nonprescription nasal spray to reverse the effects of an opioid overdose. Michael Siluk/UCG/Universal Images Group via Getty Images

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

These are the key findings of my recent study, published in March 2025 in the Journal of Pharmacy Technology.