It is called "Xylazine" or "Tranq" as it is known on the streets.
Officials say drug suppliers are lacing the fentanyl and heroin supply with the animal tranquilizer because it is cheap and easy to get.
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The use of "Tranq" also comes with disturbing consequences.
Brooke Peder showed us her wounds. She has already had a leg amputated and now she's worried she may lose an arm.
"That's necrotic flesh," she said. "This actually looks phenomenal right now," she said as she points to the open flesh of her arm.
Necrosis is the death of body tissue.
Peder is among a growing number of users who have been affected by "Tranq."
Action News walked the streets of Kensington's open-air drug market to see its impact. We saw many people who use illegal substances walking around with wound care bandages.
"We probably didn't start seeing 'Tranq' until maybe three or four years ago," said Dr. Joseph D'Orzio, an associate professor of Clinical Emergency Medicine.
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Dr. D'Orazio is also an expert in addiction medicine. He said initially substance users like Peder didn't know "Tranq" was in the drug supply, but now they do as it's in almost all of the opioid supply in Philadelphia.
"More than 90% of the bags of heroin fentanyl in Philadelphia are contaminated," he said.
D'Orazio also said Xylazine mixed with an opioid can create a stronger effect.
"It's really cheap," he said. "Here's the thing - Xylazine is an unscheduled veterinary drug. It's easy to obtain and not illegal."
D'Orazio says "Tranq" causes fast-moving necrotic wounds mostly on extremities and not necessarily at injection sites.
"I think we're still in the infancy of understanding this drug," he added.
"Tranq" also causes heavy sedation. People who inject or snort it are often knocked out for hours on end. While experts say it hasn't led to a surge in overdoses, it causes other withdrawal symptoms like extreme anxiety.
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Currently, "Tranq" is heavily concentrated in the supply in the Northeast but D'Orazio said it is spreading nationwide.
Outreach groups like Prevention Point in Kensington and the Philadelphia Department of Public Health are now deploying pop-up medical units to help care for wounds.
Too often they say wound sufferers are hesitant for help due to the stigma, but the loss of limbs is changing minds.
"They're serving upwards of 75 people a month," said Jen Shinefeld with the Philadelphia health department. "I have been doing this work for 12 years and I've never seen wounds like this progress so quickly or as severely as they do with 'Tranq.'"
Brooke Peder said she will continue to care for her wound but that hasn't stopped her from using, despite knowing "Tranq" is in the supply. Her prognosis is still very much uncertain.
"I mean you don't know how hard I've worked to make it get to be this good right now," said Peder.
Doctors also tell us Narcan won't work to revive a person heavily sedated on "Tranq." Still, they say it should be administered since it's the opioids that are linked to the overdoses.