Health and government officials are warning of a potential deadly substance in the illegal drug supply.
The Centers for Disease Control and Prevention (CDC) and the White House Office of National Drug Control Policy (ONDCP) issued a health advisory on Thursday about reports of medetomidine being detected in fentanyl.
Also known as "rhino tranq," "mede" or "dex," medetomidine is a veterinary sedative that causes severe, prolonged sedation. Classified as an alpha-2 agonist, it acts on the nervous system similar to other veterinary sedatives, like xylazine, and can cause life-threatening withdrawal symptoms.
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The advisory is based on surveillance data, including forensic drug testing, wastewater analysis, clinical case reports and overdose investigations.
In 2023, there were 247 incidences of medetomidine detected in drug samples, which rose to 2,616 in 2024 and 8,233 in 2025 – a more than 3,000% increase. Forensic drug reports showed that about 98% of medetomidine-positive samples also contained fentanyl.
The drug has been detected in at least 18 states and Washington, D.C., with a concentration in the Northeast and Midwest regions of the U.S., per the CDC alert.
Dr. Adam Scioli, chief medical officer of Caron Treatment Centers in Pennsylvania, said the alert highlights a "concerning and rapidly evolving development" in the illicit opioid supply.
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"Its co‑occurrence with fentanyl significantly complicates overdose presentation and withdrawal management, further increasing both clinical acuity and unpredictability," he told Fox News Digital.
"Medetomidine is not routinely detected on standard toxicology screens, increasing the risk of under‑recognition without a high index of clinical suspicion."
Naloxone (Narcan), known for its overdose reversal effects, does not counteract medetomidine, the advisory warned.
"While naloxone remains essential for reversing opioid‑induced respiratory depression, it does not address the sedative effects of medetomidine," Scioli confirmed.
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The initial effects of the drug include deep sedation or decreased consciousness, bradycardia (slow heart rate), hypotension (low blood pressure) and respiratory depression, especially when combined with fentanyl and other opioids.
Withdrawal symptoms typically begin about a few hours after taking medetomidine. They can be severe and rapid, peaking at about 18-36 hours, according to experts.
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Initial withdrawal symptoms include rising blood pressure, tachycardia, agitation and nausea/vomiting, gradually leading to severe hypertension, altered mental status, and possible cardiac or neurologic complications, per the alert.
Severe cases require intensive care in a hospital. "Management may require ICU‑level monitoring and care," said Scioli.
In a May 2024 example cited by the CDC, medetomidine in the illegal opioid supply was linked to a cluster of overdoses in Chicago, potentially exceeding 175. At least 16 people were hospitalized and one died.
There were some limitations to the surveillance, the agency acknowledged. Surveillance systems rely on small samples that may not be representative of all areas.
There is also the chance that the numbers could be overestimated due to contamination or repeated use of drug paraphernalia. Conversely, numbers could actually be higher than recorded, as medetomidine is rapidly metabolized in the body and is not typically tested in clinical settings.
The surveillance was conducted by the CDC with support from federal public health programs and collaboration with other agencies.
Scioli noted that the alert highlights the need for "careful assessment beyond standard opioid toxicity models," as well as close coordination with toxicology, emergency medicine and public health partners.
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"The drug supply is evolving in ways that strain traditional opioid‑focused frameworks and demand greater clinical vigilance," he said.
"From a treatment perspective, this is another clear example of why addiction care must be adaptive, medically sophisticated, and grounded in whole‑person, long‑term recovery — not solely acute stabilization," he added.
