The number of fentanyl overdose deaths that also include xylazine — an animal tranquilizer increasingly mixed into the illicit drug supply — have surged around the country in recent years, new data released by the Centers for Disease Control and Prevention confirm, with xylazine now showing up in more than a quarter of fentanyl deaths from some states.

Xylazine, also known as "tranq," is a powerful sedative that is not approved for any use in humans. It does not respond to overdose-reversing medications like Narcan, and it is known for causing gruesome skin wounds in users.

The new figures, published Thursday in the CDC's Morbidity and Mortality Weekly Report, offer the biggest window to date into the growing toll from xylazine's rapid ascent in the illegal U.S. drug supply through last year.

Across 31 states and the District of Columbia which had enough data reported to the CDC on toxicology reports from overdose deaths, the agency says xylazine was detected in 9% of fatal overdoses involving illicit fentanyl from January 2021 through June 2022.

Of these states, Maryland has the worst statewide rate: 27.7% of fentanyl deaths in the state also involved xylazine. Connecticut is next highest at 26.4% of these deaths, followed by Pennsylvania at 23.3%.

That increase came as drug overdose deaths overall were accelerating nationwide. Overdose deaths have since plateaued at more than 100,000 per month.

"These data show that fentanyl combined with xylazine is increasingly dangerous and deadly. This is why the Biden-Harris Administration recently designated it as an emerging threat," Dr. Rahul Gupta, director of the White House Office of National Drug Control Policy, said in a statement.

Experts have pointed to communities in the mid-Atlantic, from Maryland to Philadelphia, as early hotspots for the drug. CBS Philly reported in January that according to medical toxicologists from Temple Health, xylazine could be found in 90% of that city's dope supply.

A study published in April by the CDC from syringe services programs around Maryland found more than 8 in 10 survey participants had unknowingly taken xylazine, when intending to buy other opioids like fentanyl.

However, the agency cautions that even some states with incomplete data still reported hundreds of fatalities linked to xylazine. 

For example, 735 death certificates in New York and 261 in Florida during this time listed positive xylazine results, though there were too many more deaths missing toxicology results for them to be formally included in this new analysis.

A 276% increase from 2019 to 2022

The CDC data also marks a near tripling in fentanyl deaths involving xylazine, across a smaller subset of health departments that have enough data on testing dating back to 2019 reported to the federal State Unintentional Drug Overdose Reporting System. 

In these 20 states and the District of Columbia, less than 3% of fentanyl deaths in January 2019 were also testing positive for the animal tranquilizer. By mid-2022, xylazine's prevalence had climbed 276% to 10.9% of fentanyl deaths.

Some of that increase might be from increased efforts to test for xylazine, the agency's authors said, though inconsistent approaches to check for it in overdose deaths mean these figures are likely still an underestimate.

Many routine toxicology tests still do not test for xylazine, a finding the Substance Abuse and Mental Health Services Administration described as "alarming" earlier this year. 

A separate CDC arm that analyzes death certificate data, the National Center for Health Statistics, also plans to soon release its first nationwide estimates tracking xylazine overdose deaths from 2018 through 2021.

A cause of death

Not all overdose deaths that spotted xylazine in toxicology reports actually blamed the animal tranquilizer as a cause of death, with a wide range – from 0% to more than 90% – between states blaming xylazine as a cause when detected in fentanyl deaths.

"Medical examiners and coroners might differ regarding whether they consider xylazine to increase fatal overdose risk, or they might be unfamiliar with xylazine and therefore not list it on death certificates," the agency's authors wrote.

Xylazine's increase has been especially worrying since drugs deployed to combat the opioid crisis, like overdose-reversing naloxone and medications to treat withdrawal like methadone, are not designed to work for the animal tranquilizer. 

However, the CDC authors said more needs to be researched about xylazine's effects. 

For example, they did not find an association between naloxone use and xylazine deaths, suggesting the animal tranquilizer so far might not be driving a feared increase in naloxone failures. And compared to fentanyl deaths without xylazine, fewer fatalities with xylazine had no pulse when first responders arrived. 

"Naloxone will not reverse the effects of xylazine. However, because xylazine is often used with opioids like fentanyl, naloxone should still be given," the CDC says. 

Kerry Breen contributed reporting.

In:
• Xylazine
• Drug Overdose
• Fentanyl

Alexander Tin

CBS News reporter covering public health and the pandemic.