Updated at 10:28 a.m. Feb. 16

About a third of New England’s illicit drug supply is laced with a potent veterinary tranquilizer called xylazine, often known as “tranq,” according to new data released last week by Brandeis University’s Opioid Policy Research Collaborative.

“It’s a very useful medication when you need to sedate a large animal,” said Traci Green, an epidemiologist who leads the Brandeis project. “Its presence in the drug supply right now, especially in things that are thought to be heroin or fentanyl or pain pills, is an unexpected turn.”

The drug is only approved by the Food and Drug Administration for use in animals. In November, the FDA put out a warning about xylazine exposure in humans, saying it can have “serious and life-threatening side effects.”

While the effects of exposure to xylazine are unpredictable and not fully understood, experts say it can slow someone’s breathing and sedate them for hours at a time. Prolonged use can also worsen withdrawal symptoms and cause severe skin infections that are slow to heal.

Doctors and researchers say nonprofits and governments need to change their approach to harm reduction to respond to the increasing prevalence of xylazine in street drugs. At-home tests like fentanyl strips don’t yet exist for this tranquilizer, and while researchers emphasize that opioid-reversal drugs like naloxone are still effective for counteracting opioids in overdoses of drug mixtures, they don’t reverse the effects of xylazine if it’s mixed in with those opioids. Because of that, some addiction specialists say it’s essential to closely study xylazine’s effects in humans and have more cities and states set up supervised consumption sites.

Jessica Taylor, an addiction specialist who works at Boston Medical Center and Boston University, says naloxone is still the first line of defense in an overdose. The opioid-reversal drug is often known by its brand name Narcan.

“In an overdose, the first thing to do is still to give Narcan — especially if someone has what we would consider typical symptoms of an opioid overdose, which would be lower breathing rates, maybe turning blue, not responding, having pupils that are very small,” she said. “But now as opposed to, ‘Give Narcan and call 911,’ we’re saying, ’Give Narcan, call 911 — and really focus on rescue breaths and rescue breathing to support people that may also have xylazine in their system.’”

A person who overdosed from an opioid contaminated with xylazine may not regain consciousness even after being administered naloxone because they remain under xylazine’s sedation, or they may need medical help to recover their breathing.

Taylor argues that supervised consumption sites will promote safety. Often called supervised injection sites, or overdose prevention centers, they are places where people use illegal drugs under medical supervision. New York City opened the nation’s first such site in late 2021, and Somerville is currently exploring setting one up.

“Our drug supply is so poisoned right now that we need to continue to use all of our currently allowed overdose prevention strategies, but also use our advocacy platforms to really insist on other evidence-based strategies that aren't allowable in Massachusetts right now,” she said. “And supervised consumption is really what we need to prevent deaths.”

The ever-more contaminated drug supply has a mix of components, including xylazine, being sold off as one product, like heroin or fentanyl. But it’s not clear why xylazine in particular is being added into drugs — almost exclusively opioids, said Joseph Friedman, a researcher at UCLA. It may have to do with how short-lasting fentanyl’s effects are when compared to other opioids like heroin.

In field work he conducted in Philadelphia, Friedman said some people who were using drugs didn’t want anything that contained xylazine, but they didn’t have a way to avoid it.

“The kind of leading theory right now is that xylazine is being added to fentanyl A) to kind of give it an extra kick, to make it a more psychoactive product. ... And B) to make it last longer,” Friedman said.

In Massachusetts, doctors on the ground say their patients are increasingly asking about the tranquilizer.

“This is something that ... really is front of mind for our patients in the last two to three months in a way that feels different even than last summer,” Taylor said. “People are talking about xylazine, they’re asking us about xylazine. They’re asking if the reason that withdrawal is more uncomfortable or more difficult is related to xylazine.

“So it definitely feels like something that is entering our consciousness in a way that is really important given the contamination levels that we’re talking about,” she continued.

Local organizations like Berkshire Health are distributing wound care kits to try to keep skin lacerations clean.

The hours of sedation can also be dangerous. Health experts warn those exposed to the drug are particularly vulnerable to being robbed or assaulted if they are outside.

“In times like this in February, when we’re worried about cold weather, [then] we’d also worry about things like frostbite or hypothermia if people were not moving in a public place for hours on end,” Green said.

One in three New England street drug samples tested in the latter half of 2022 contained xylazine, up from just one in 10 in 2020, according to the Brandeis findings made in collaboration with the state and local groups.

In prior decades, xylazine use in the United States was generally limited to Puerto Rico, Friedman explained. But in recent years, Philadelphia’s opioid supply has been increasingly laced with xylazine — more than 90% of illicit opioids there tested in 2021 contained the tranquilizer.

“The epicenter of the prevalence is still the Northeast, especially Philadelphia, but it’s spreading now to other parts of the East,” said Friedman, whose research last year tracked overdose deaths where xylazine was involved. “So we found very high prevalence in Maryland, Connecticut, Alabama and now Chicago. And so you can kind of see xylazine spreading out from where it initially was concentrated and basically following in the footsteps of fentanyl.”

Jessica Kelly, the supervisor of Berkshire Health System’s harm reduction program in North Adams, said xylazine is less common in her region. There it's present in one out of every five or seven samples.

“Xylazine is definitely something that’s new on the radar,” she said. “It’s definitely being used as a cutting agent in the heroin supply in Western Mass. It’s a lot more commonly used in the Boston area — but with all things, the more popular it is there, the more popular it’s going to get here sooner or later.”

Friedman also says understanding the scope of the problem and educating users on what is in their drugs should be a priority. The Center for Disease Control and Prevention’s mortality database doesn’t currently track xylazine in overdose deaths, unlike fentanyl and other drugs.

“A) We need better data,” Friedman said. “We have to actually be tracking xylazine on a federal level in, kind of, official national statistics. B) We need to help people who use drugs understand what is in their drugs. Not just because of xylazine but because of all of these really scary combinations of drugs that are being mixed together and sold.

“Unfortunately, it seems like xylazine is here to stay,” he added. “So this is something we’re going to have to kind of learn to manage.”

This story was updated to correct a typo.