A powerful animal tranquilizer is increasingly showing up in the illicit drug supply, putting unsuspecting users at risk for hard to treat overdoses and dangerous side effects, the Food and Drug Administration warned Tuesday.

The drug, called xylazine, is primarily found in heroin and illicit fentanyl, the FDA said, and overdoses can look similar to an opioid overdose.

But because xylazine isn’t an opioid, these overdoses can’t be reversed using the antidote naloxone, or Narcan.

Xylazine isn’t approved for use in humans. In animals, typically horses, it’s used as a sedative before surgery and as a painkiller.

In humans, it can lead to profound sedation that can last for hours. It’s also linked with gaping skin wounds not seen with other injectable drug use, the FDA said.

“Repeated exposure to xylazine, by injection, has been associated with severe, necrotic skin ulcerations that are distinctly different from other soft-tissue infections (e.g., cellulitis, abscesses) often associated with injection drug use,” it wrote in a letter sent Tuesday to health care professionals nationwide. “These ulcerations may develop in areas of the body away from the site of injection.”

Though mainly mixed with opioids, the agency said, xylazine has also been detected in stimulants such as cocaine and methamphetamine.Medications used to help patients with opioid use disorder won’t work in someone who has become dependent on xylazine.

The agency advised health care workers to consider xylazine poisoning if people who have overdosed don’t respond to naloxone, or when they see severe skin wounds in people known to use injectable drugs.

Because the drug isn’t included in routine toxicology screenings, the FDA added, it may be underdetected in overdoses.

Xylazine is sold on the street under several names, including tranq, tranq dope, sleep-cut, Philly dope and zombie drug, it said.

The advisory only addressed xylazine, but it’s not the only dangerous substance that’s entered the drug supply. Synthetic opioids called nitazenes, too, have been found laced in street drugs. Nitazenes can be up to 10 times more potent than fentanyl.