The U.S. Drug Enforcement Administration issued an urgent public health warning Monday about the dangers of xylazine, a powerful sedative approved for veterinary use that has recently been found mixed into large swathes of domestic fentanyl supplies. The agency said that as much as 23% of the fentanyl powder and 7% of the fentanyl pills it had seized in 2022 contained the drug, also known as tranq. “Xylazine is making the deadliest drug threat our country has ever faced, fentanyl, even deadlier,” DEA Administrator Anne Melissa Milgram said. Xylazine is reported to cause serious side effects—including the creation of “severe, necrotic skin ulcerations” that, according to the FDA, can sometimes lead to amputation if left untreated.