Xylazine, a veterinary tranquilizer known on the street as "tranq," is a cheap and powerful sedative that's infiltrating the nation's drug supply and is increasingly being found in opioid overdose deaths.

The epicenter of this growing public health crisis is the Kensington neighborhood of Philadelphia, already known as the largest open-air drug market in the Northeast. "Tranq" is detected in over 90% of dope samples in Philadelphia. A record 1,276 unintentional overdose deaths happened in Philadelphia in 2021, according to data from Philadelphia's Department of Health and xylazine was found in over a third of them.

While "tranq" has been detected in Philadelphia for at least eight years, it is now increasingly found in illicit drugs beyond the streets of the city.

Overdose deaths in the United States overall involving xylazine were 35 times higher in 2021 than in 2018, according to the Centers for Disease Control and Prevention. In 2023, "tranq" was detected in at least 48 states by the Drug Enforcement Administration.

The Food and Drug Administration approved xylazine for veterinary use only but it began entering the drug supply when dealers across the country started mixing it into common street drugs like heroin, cocaine, and, most commonly, fentanyl to increase profits, according to the DEA.

"Tranq" is colloquially referred to as the "zombie drug," as many appear to be in a trance-like stupor when using the drug. Regardless of whether it's smoked, snorted, or injected, the damage it causes can be severe. People using "tranq" often develop severe wounds on their skin that can occur most often on their limbs. If left untreated, the wounds can become necrotic and require amputation, according to the DEA.

Katie Mowrey and Stephanie Klipp are nurses responding to people who have become "tranq"-dependent in the Kensington area.

"Our overdose response has had to change significantly," Klipp told "Nightline."

Day after day, they pack their supplies and set out to aid those affected as volunteer nurses with the non-profit organization Savage Sisters Recovery. They say the effects of xylazine are like nothing either of them has ever seen.

"Nightline" reporter Ashan Singh witnessed the nurses' daily process of administering treatment, often providing wound care out of the trunk of their cars or at the Savage Sisters outpost in the neighborhood.

"We can't just pop Narcan on people now. We have to do lots of rescue breathing because of the tranq that's coming. So that's going to change the way overdose looks across the country when it becomes infiltrated," Klipp said.

If someone overdoses on an opioid containing xylazine, the overdose reversal drug Narcan will counteract the opioids. But Narcan does not reverse the effects of xylazine, so the person might still appear sedated and need additional care, according to the CDC.

Sarah Laurel is the executive director and founder of Savage Sisters Recovery, a program that has nine recovery houses for both women and men. They offer trauma therapy, holistic therapy, yoga, kickboxing, and mindfulness workshops - all things that helped Sarah personally during her recovery.

Laurel, who once lived on these same streets while struggling with a heroin addiction before entering recovery six years ago, told "Nightline" it's been hard to watch the way xylazine has gripped her community.

"Xylazine was originally put into the supply to extend the euphoria and the high feeling for individuals consuming that dope," Laurel said. "Nobody asked for this. Nobody knew that this was being adulterated into our supply."

Even those who do develop terrible wounds from prolonged "tranq" use say they find it difficult to stop. They become unable to break free from an incessant cycle where exceptionally painful withdrawals send them running back to the drug for relief, according to those who spoke to "Nightline" for this report.

Jose Castillo was once homeless in Kensington, battling an opioid addiction. Now a harm reduction specialist for Savage Sisters Recovery, he says detoxing from drugs that contained "tranq" was extremely brutal.

"It was to the point where I didn't think I was gonna make it through it," Castillo told "Nightline."

"The sickness lasts longer, or you're just in the middle of going through withdrawal, and there's no type of medicine they're giving you that's helping it. I don't know, it's a whole different ballgame," he added.

The White House declared the fentanyl-xylazine mixture an "emerging threat" to the U.S. in April, and cities across the country are beginning to face their own struggles as members of their communities become impacted by the substance. Most notably in the South, where xylazine-positive overdose deaths have climbed by an alarming 1127%, according to the DEA.

In rural Greenville, North Carolina, 77-year-old Diannee Carden is the founder and executive director of the nonprofit needle-sharing organization ekiM, which is the only needle exchange in the area for hundreds of miles. Carden created the organization four years after her son Michael died of a heroin overdose in 2012. Carden says her son made notable contributions to New York's harm reduction community in the early 2000s and still inspires her work today.

"[Michael] helped me to understand that it's not just enough to say, you know, 'Let me get you some help.' You know, 'Where would you like to go? Would you rather do something else?' You have to be able to take care of the whole person." Carden said. "I feel [like] I'm contributing something to an underserved population here, and at the same time, I'm honoring my son."

Carden serves nearly 100 people who use drugs every week. She says they are alarmed at the "tranq" creeping into their supply.

Her organization offers drug testing through the University of North Carolina - Chapel Hill so people can confidently understand what they are taking. The demand for it has been so great even neighboring counties send samples to Carden's group for testing at the university.

Dr. Nabarun Dasgupta, a leading drug overdose researcher, launched the Street Drug Analysis Lab at UNC - Chapel Hill in 2020. The kits that Carden and her team send get processed here, along with samples of street drugs from North Carolina and 28 other states and counting.

"We work directly with harm reduction programs and health departments who can message the results back and provide the individuals with support services," Dasgupta told "Nightline."

Results are added to the lab's website daily to keep states, community groups and health departments up to date with the amount of "tranq" on the street.

"We already see it showing up with methamphetamine. We see xylazine spreading from the opioids to stimulants. And, you know, we expect that will happen more and more," Dasgupta said.

Dasgupta says the work is crucial to getting ahead of the threat "tranq" poses because the information helps people who use drugs know what they are putting into their bodies.

"I think we were caught flat-footed with xylazine," says Dr. Dasgupta. "[Because] the wounds are [so] dramatic looking, a lot of drug treatment centers won't let people in… so then you have folks caught in this Catch-22."

There is still a lot that remains unknown about "tranq" and its effects on the human body.

"We do know that people are dying with xylazine in their bodies at the time of autopsy. We don't know what a toxic amount of xylazine is," Dasgupta said.

While the medical community is still researching the harm "tranq" poses, experts like Dasgupta agree that it's complicating an already dire overdose crisis.

"As a society, we've had too many empty seats at Thanksgiving. We've had too many people missing from our lives," Dasgupta said. "We have to admit that 100,000 people a year dying is not okay. We need to be open to new solutions, or else we're just not going to get out of this."

If you or someone you care about is struggling with thoughts of suicide, substance abuse or other mental health crises, text or call the Crisis Lifeline at 988. Free help is available 24/7.

ABC News' Anneke Ball and Nicole Wetsman contributed to this report.