"I'm deeply concerned about what this threat means for the nation," Dr. Rahul Gupta, the White House director of the Office of National Drug Control Policy, said in May when he warned that xylazine is a major emerging health threat.

According to Gupta's office, xylazine-positive overdose deaths increased tenfold in the southern U.S. from 2020-2021, sevenfold in the western U.S. and fivefold in the Midwest. Most of those drug deaths also involved fentanyl.

Over the years, xylazine has occasionally turned up in street drugs, but over the past 12 months, it suddenly went viral. Overdose deaths and severe medical complications — including terrible flesh wounds caused by the chemical — have soared in the South and West, where xylazine had been almost unknown.

Xylazine also makes it much harder to revive people after fentanyl overdoses, and the chemical adds another layer of intense addiction and cravings.

"People who are in this are getting sucked further and further and further in," said KC, who uses street fentanyl in Dover, Delaware. She said xylazine is making addiction deadlier and harder to escape. "It just feels kind of hopeless right now."

Drug experts say they only have theories about who's adding xylazine into street drug cocktails — and why.