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Philadelphia was once known for having the "purest heroin in the nation," city health officials say, but now the city's opioid supply has been overtaken by an animal tranquilizer linked to thousands of drug overdoses across the nation.

Why it matters: Xylazine, known on the streets as "tranq," can be lethal, when mixed with heroin or fentanyl, and causes nasty wounds and sores that can result in amputations.

Between the lines: Advocates say open sores and wounds can delay people from getting addiction treatment because many rehab facilities won't accept anyone with untreated lesions for fear of spreading infectious diseases.

State of play: First detected on the city streets about a decade ago, xylazine went dormant for years before making a vicious comeback, health officials said.

The big picture: Kensington, the epicenter of the city's opioid crisis, is considered the hub of the xylazine boom, health officials said.

What they're saying: Local drug traffickers use xylazine as a cutting agent to extend supplies of heroin and fentanyl, inspector Jamill Taylor, of the Philadelphia police narcotics unit, told Axios.

Shawn Westfahl, an overdose prevention coordinator for Prevention Point, the city's only needle exchange, told Axios the combination of xylazine and fentanyl can put a person to sleep for hours, making it harder to determine whether someone is experiencing an overdose.

Westfahl has observed that some are becoming dependent on the xylazine-fentanyl combination and seeking out "tranq dope" as they experience withdrawal symptoms.

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