BACKGROUND/OBJECTIVES: The pharmacology of fentanyl and xylazine is not characterized in persons regularly exposed to illicit fentanyl. This case series presents individual-level urine pharmacokinetics of fentanyl, norfentanyl, and xylazine in persons with opioid use disorder (OUD). METHODS: Participants (N = 11) provided urine samples (n = 95) for quantitative analysis. Terminal half-lives and predicted detection times were calculated. RESULTS: Urinary half-life ranges were: fentanyl 1.5-28.7 h, norfentanyl 5.2-27.4 h, and xylazine 1.1-25.2 h. Predicted detection window ranges were: fentanyl 13.0-130.9 h (0.5-5.5 days), norfentanyl 64.8-255.9 h (2.7-10.7 days), and xylazine 13.8-123.4 h (0.6-5.1 days). DISCUSSION/CONCLUSIONS: Persons are presenting with a high magnitude of drug exposure that lends to an extended duration of urinary detection. SIGNIFICANCE: First characterization of urinary excretion in persons with OUD.