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The U.S. Food and Drug Administration announced on Tuesday that it was taking action to restrict unlawful importing of the veterinary drug xylazine, which has been “increasingly found” in the nation’s illicit drug supply. 

The action, an import alert, allows for the detainment of shipments of xylazine and the ingredients used to make it. The alert “aims to prevent the drug from entering the U.S. market for illicit purposes,” the agency said in a news release. An import alert allows the FDA to detain shipments of products that appear to be in violation of the FDA’s laws and regulations. 

The drug, an animal tranquilizer used by veterinarians to sedate large animals like horses, will still be made available for “legitimate uses.” 

When used in people, xylazine can cause “serious and life-threatening effects,” including severe skin wounds and dead tissue, the FDA says. It can also depress breathing, blood pressure and heart rate to “critical levels.” 

Xylazine has been found in overdose deaths across the country, including in California and Pennsylvania. Because xylazine is used in conjunction with other substances, it’s difficult to determine what role the drug plays in overdose deaths. The FDA said that it has been identified as a contaminant “found in combination with opioids,” including the synthetic drug fentanyl. It has also been mixed with stimulants like methamphetamine and cocaine. 

There are concerns that xylazine may not react to treatment options like naloxone, which can reverse opioid overdoses, and routine toxicology screens may not detect its presence, the FDA said in a waning issued in late 2022. People who use drug may also be unaware that they are buying substances contaminated with xylazine, the FDA said at the time. 

“The FDA remains concerned about the increasing prevalence of xylazine mixed with illicit drugs, and this action is one part of broader efforts the agency is undertaking to address this issue,” said FDA Commissioner Robert M. Califf in a news release announcing the import alert. “We will continue to use all tools at our disposal and partner with the Drug Enforcement Administration and other federal, state, local agencies and stakeholders as appropriate to stem these illicit activities and protect public health.”