Last spring, patients addicted to opioids started showing up at Philadelphia’s emergency rooms with unusually severe symptoms when they stopped using drugs being sold on the street.

Their hearts would race, their blood pressure would skyrocket, and they would shake uncontrollably in bed. They would vomit over and over again.

Medications to treat opioid withdrawal didn’t work. Neither did medications for withdrawal from xylazine, the animal tranquilizer added in recent years to illicit opioids sold in Philadelphia.

At Penn Presbyterian Medical Center’s ER, physician Jeanmarie Perrone wasn’t sure what she was seeing.

There were concerning reports from the streets, too.

Paramedics were treating opioid overdose victims who began breathing again with the overdose-reversing drug naloxone but would not wake up, suggesting that the opioids they were using had been laced with another powerful sedative.

Emergency physicians across the city began meeting regularly with the city health department to figure out what was causing the overdoses and the withdrawal symptoms.

They went through a list of substances being added to illicit drugs and detected through the city’s drug-checking program everything from local anesthetics to an industrial chemical.

“We knew what was in the drug supply, and we knew there was an abnormal syndrome in people, but it was hard to tie those together,” Perrone said. “Ultimately, it was a process of elimination.”

By January, they zeroed in on another veterinary tranquilizer that was quickly replacing xylazine in the city’s drug supply, called medetomidine.

Medetomidine includes the drug dexmedetomidine, typically used in intensive care settings. In approved medical applications, it is generally regarded as a safer sedative option for some patients, because it does not cause slow or labored breathing, Perrone said.

But it also can cause severe withdrawal symptoms, similar to what Perrone and her colleagues were seeing in their ERs.

The rapid rise of medetomidine

At three Philadelphia-area health systems, Perrone and other emergency room doctors collected data on patients who had experienced severe withdrawal symptoms that did not respond to standard treatment options for opioid and xylazine withdrawal.

Their findings were featured in the Centers for Disease Control and Prevention’s Morbidity and Mortality Weekly Report on Thursday, along with reports on the drug from Chicago and Pittsburgh.

In Philadelphia, the study authors noted, 72% of illicit opioids tested contained medetomidine by the last four months of 2024.

The rise was stunningly rapid: Last May, Philadelphia health officials issued an alert about medetomidine based on the first two positive tests of drug samples collected in the city.

Medetomidine’s spread follows efforts to crack down on xylazine, which just last year was found in 98% of fentanyl sold on the street. By the time of the study, it was detected in 31% of illicit opioids tested.

Perrone and her colleagues drew on data from 165 patients who visited Penn Medicine, Jefferson Health, and Temple Health between September 2024 and January 2025.

In that time period, 91% of those patients required treatment in intensive care units because of their withdrawal symptoms, and 24% needed to be intubated.

That was in part because the doctors found they could treat the severe withdrawal symptoms with a combination of drugs, including dexmedetomidine, that is permitted to be dispensed only in an ICU, Perrone said.

Over the course of the study, 137 patients were treated with dexmedetomidine, and all saw their withdrawal symptoms decrease. But the need for ICU treatment presents an additional challenge for treating patients in medetomidine withdrawal.

“We don’t have a lot of capacity in the ICU,” Perrone said. “Usually, you don’t need to be in the ICU for opioid withdrawal.”

At one health system where Perrone and her colleagues gathered data, patients were tested for medetomidine and its byproducts as it exits the body. All patients with abnormal withdrawal symptoms treated there tested positive for the drug.

Better drug testing at more hospitals could help doctors more quickly identify patients who need treatment for medetomidine withdrawal, Perrone said.

The CDC’s report presented Philadelphia’s experience alongside evidence from other big cities about the threat of medetomidine.

In Chicago, researchers outlined an overdose spike from medetomidine mixed with opioids, suspected to have sickened 178 people in a single week in May 2024.

In Pittsburgh, researchers looked at 23 cases of suspected medetomidine withdrawal at two hospitals between October 2024 and March 2025.