As he does on most days, Kenzo, a twentysomething Dunkin’ Donuts manager, recently injected a $10 stamp bag of fentanyl, the opioid that can be 50 times stronger than heroin. Kenzo—the internet pseudonym of someone who spoke under the condition of anonymity for fear of professional consequences—prefers plain-old heroin, but can’t get it anymore in the Kensington neighborhood of Philadelphia where he buys drugs. Dealers only offer fentanyl-based mixes, he told The Daily Beast.

On this occasion, he instantly blacked out and fell into a “stress dream” in which his store was full of people and he didn’t have any doughnuts, he recalled. He woke up on the floor of his apartment to the sight of his girlfriend holding an empty canister of Narcan, the overdose reversal drug. His limbs were twisted, his lips a greyish blue.

“I had to figure out that she had Narcanned me,” Kenzo explained in an interview.

A 10-year opioid user, he had been revived by naloxone (the generic of the drug sold as Narcan) nasal spray before. This didn’t feel the same. “Narcan makes you instantly sick from withdrawal. The Narcan didn’t have any effect [this time],” he said. His senses were dulled and his mind hazy for the next few hours. “It was scary as hell,” he added.

It was the man’s first encounter with “tranq dope,” in this case a fentanyl mixture heavily laced with xylazine that law enforcement officials, experts, and users say is increasingly prevalent on the street. Xylazine, a common tranquilizer for animals, puts users in hours-long sleepy stupors—and a surge in its proliferation is just the latest wrinkle in America’s deadly opioid crisis.

Kenzo said he was aware dealers sold the stuff. In Kensington, a notorious open air drug market, they shout “tranq dope” from street corners, he recalled. He’s also read about it online on harm-reduction forums.

In fact, he said, he was actively trying to avoid it. “I’m not looking to get knocked out,” he explained. “I want to go to get high and not get sick and go to work.”

But in the post-heroin, fentanyl-soaked illicit drug market, one never knows what exactly they are buying.

In some areas of the U.S., they are increasingly buying xylazine. National data is not available because medical examiners don’t always test for xylazine. But in Philadelphia, the drug’s prevalence in overdose deaths has increased over the past three years, according to a DEA spokesperson. Last year, two urban counties in Ohio issued public warnings about the distribution of xylazine-heavy drug batches. Public health officials near Dayton detected the daze-inducing mixture in one OD fatality toxicology report, and officials near Columbus linked xylazine to three fatalities.

When tallying up state OD fatalities for 2019, Connecticut’s chief medical examiner, James Gill MD, found 71 cases where xylazine was present, of 1,200 deaths total. “It was surprising,” Gill told The Daily Beast. “I don’t think we saw it in any notable quantity before.”

On the black market of late, xylazine is almost always sold in a mixture with fentanyl, the synthetic opioid of unpredictable potencies that has become the most common drug in overdose deaths in the U.S. Public health officials are especially concerned about xylazine because the drug makes the batch “Narcan-resistant.” Xylazine is a clonidine analogue, not an opioid, so it doesn’t respond to naloxone, while also depressing the muscular and nervous systems. In other words, it lessens the effectiveness of one of paramedics’ and users’ best tools for preventing an OD death.

The odd thing is despite decades of mass abuse of legal pharmaceuticals, xylazine is not on any schedule of controlled substances in the U.S. It is widely used, and widely available, in veterinarian clinics and horse and cattle farms. Its intended purpose is easing the handling and surgery of animals; its primary effects are sedation, anesthesia, muscle relaxation, and analgesia.

The drug is “fairly easy to get,” said Patrick J. Trainor, a supervisory special agent and public information officer for the DEA. “I have heard of packages of it being intercepted by commercial shipping carriers for several years now, so it’s been an issue.”

It has long been used as a diluting agent, according to Trainer—an additive to make fentanyl and heroin mixtures less strong.

The “tranq dope” trend has upped the xylazine content and made the drug a feature in its own right. Stamp bags of it go by names like “bad dream” and “flat line” (with a faltering electrocardiogram signal as a logo), according to Kenzo.

“If you only have $5 and want to stay high, it’s the best bang for your buck,” he said.

Xylazine gained traction as a heroin substitute in Puerto Rico in the early 2000s. After 9/11, drug trafficking became more difficult and dealers were more invested in getting their most profitable heroin to the mainland, according to Rafael A. Torruella, a research psychologist who studied the phenomenon for the National Development and Research Institutes, Inc.

To fulfill local demand, dealers on the island sold heroin heavily diluted with xylazine, and some users started injecting “anestesia de caballo” straight, Torruella said. Puerto Rico’s equestrian industry made supplying the drug relatively easy.

The result was gruesome.

“It is a horse tranquilizer,” Torruella told The Daily Beast, “so literally the person looks high and also groggy and sleepy.” Zombie users roamed streets, unresponsive to other people. Open ulcers, the result of constant injection and general neglect of the body, became an identifier of a xylazine user.

Eventually even the drug dealers couldn’t stomach the damage they were causing. “They said they weren’t going to sell that shit anymore,” said Torruella, “and they told the other dealers, ‘We’re going to kill you if you sell that shit.’”

Torruella called Puerto Rico’s flirtation with xylazine an expression of “the failure of the war on drugs.”

“You can’t get opium from what was Taliban-controlled Afghanistan or cocaine from South America, so you just sell whatever shit is around,” he said, adding, “That’s what we were left with, the worst shit.”

Kenzo feels a similar sense of resignation about the drug market in Philadelphia.

In his writings on forums, he’s made clear that getting hooked was an awful mistake. He bemoans “the unhealthy lifestyle that goes along with that, like having an awful diet, spending way too much” and “[b]eing tethered to my city due to physical dependence.”

But at least heroin offered some predictability. Now he feels like he’s at the mercy of drug manufacturers and the increasing capriciousness of their product lines.

“Over the last few years, it’s been mainly designer drugs, all these synthetic mixes,” he said. “If it were up to me, I would still be doing heroin, not fentanyl, not fentanyl and whatever random chemicals, not fentanyl with elephant tranquilizer or horse tranquilizer.”