The animal sedative medetomidine is increasingly showing up in the U.S. illegal drug supply, according to three reports released Thursday.

Medetomidine, which is used to sedate pets and is somewhat similar to xylazine, was first detected in illegally manufactured opioids in North America in 2022. Investigators say the drug is being mixed in with other illicit drugs, mainly fentanyl, the synthetic opioid behind most overdose deaths.

On Thursday, the Centers for Disease Control and Prevention published three reports about recent clusters of medetomidine cases in Chicago, Philadelphia and Pittsburgh.

In Chicago, health officials last May were puzzled by a surge in overdoses. The patients had taken fentanyl, but the overdose-reversing drug naloxone didn't seem to work. After an investigation, the city's health department reported 12 confirmed cases of medetomidine-involved overdose — the largest to date — as well as more than 160 probable or suspected cases including a possible death.

The two other reports focused on medetomidine withdrawal in patients in Philadelphia and Pittsburgh.

In Philadelphia, medetomidine was detected in 72% of illegal opioid samples tested late last year, overtaking xylazine, which has complicated the U.S. response to the opioid crisis.

More than 160 people were hospitalized for an unusual version of fentanyl withdrawal that was resistant to medications that helped against fentanyl and xylazine. However, another drug — dexmedetomidine — did work, health officials said.

Pittsburgh reported 10 similar cases during a similar time period.