Xylazine-Fentanyl Positivity and Concentration in Urine Drug Tests

The US opioid crisis is currently driven by illicitly manufactured fentanyl combined with an evolving mix of stimulants and adulterants, such as the veterinary sedative xylazine, which further complicate clinical and public health responses. Using urine drug testing (UDT) to understand changes in the geographic spread and relative concentration of xylazine over time may provide clarity on the changing dose of exposure for xylazine and future emerging adulterants.

To differentiate spatiotemporal trends in xylazine detection and relative concentration in UDT specimens from 2023 to 2025.

This retrospective cross-sectional study analyzed UDT specimens collected from US patients (1 specimen per patient) aged 18 years or older from March 2023 and September 2025. Tests were ordered by behavioral health, primary care, or substance use treatment practitioners. If more than 1 specimen was provided by a patient during the study period, only the first specimen collected was included. Data were analyzed from June to September 2025.

UDT specimens were analyzed for fentanyl and xylazine (primary exposure) using liquid chromatography with tandem mass spectrometry. Specimens were included in the study if fentanyl was detected.

Differences in xylazine positivity by patient and practitioner characteristics, copositivity for other substances, and fentanyl concentration were assessed using logistic regression. Spatiotemporal trends in monthly percentages of xylazine positivity and concentration in UDT, stratified by East and West Census regions, are described.

A total of 42 307 specimens (24 339 [57.52%] from male patients; 15 583 [36.83%] from patients aged 26-35 years) were analyzed. In both regions, xylazine detection increased between the first and last months (East: 8.52 percentage points [from 11.11% to 19.63%]; West 13.46 percentage points [from 0.0% to 13.46%]) while concentration decreased (East: -37.76 ng xylazine per 1 mg creatinine [from 61.77 to 24.00 ng xylazine per 1 mg creatinine]; West: -7.98 ng xylazine per 1 mg creatinine [from 28.62 to 20.64 ng xylazine per 1 mg creatinine]). Odds of xylazine detection were 79% lower in the West (AOR vs East, 0.21 [95% CI, 0.19 to 0.22]; P < .001), and 120% higher in samples with heroin positivity (AOR vs heroin negativity, 2.20 [95% CI, 2.07 to 2.35]; P < .001) or with higher fentanyl concentrations (AOR per 10-fold increase, 1.55 [95% CI, 1.53 to 1.57]; P < .001).

In this cross-sectional study of xylazine-fentanyl in UDT specimens, xylazine detection increased but concentration decreased over time. These findings suggest UDT complements existing drug surveillance and provides unique insights into emerging fentanyl adulterants.