Xylazine Pharmacokinetics in Patients Testing Positive for Fentanyl and Xylazine 

Clinical Chemistry

This study of xylazine pharmacokinetics used plasma samples from 28 patients who had urine screens positive for xylazine and fentanyl. The patients were being treated for skin lesions, most commonly, then shortness of breath or opioid overdose. At least two subsequent plasma samples were analyzed for xylazine and xylazine metabolites by LC-MS/MS. The median terminal half-life for xylazine in plasma was 12 hours (range 6-21 hours). Animal studies show xylazine to be extensively metabolized, with little unchanged xylazine eliminated in urine. The two most abundant metabolites were oxo-x and sulfone-x, which did not have a window of detection longer than xylazine. Researchers had no information as to the timing or route of xylazine ingestion or if additional xylazine was consumed during the study period. These factors could affect the accuracy of the results.

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