Emergence of xylazine in accidental overdose deaths, impaired drivers, sexual assaults, and public intoxications within San Francisco.

BACKGROUND

Xylazine, a veterinary sedative and alpha-2 adrenergic receptor agonist not approved for human use, has emerged as a significant adulterant in the illicit fentanyl supply across the United States. While its presence has been well documented on the East Coast, systematic reporting from California remains limited.

OBJECTIVE

This study characterizes the emergence, prevalence, and concentration trends of xylazine in forensic casework in San Francisco from 2022 January to 2024 December, and describes polysubstance patterns associated with its use.

DESIGN

We retrospectively analyzed up to thirty-six (36) months of routine comprehensive toxicological testing for over 200 drugs/metabolites performed on all xylazine-positive accidental overdose deaths (AOD), driving under the influence of drugs (DUID), drug-facilitated sexual assault (DFSA), and public intoxication (PI) cases within the City and County of San Francisco, totaling 314 cases.

RESULTS

Xylazine detections increased across all case types from 2023 to 2024 and rose steadily in AOD cases from 2022 through 2024. The majority of xylazine-positive cases were AOD. Mean and peak xylazine concentrations demonstrated year-to-year increases, with peak AOD blood and urine concentrations observed up to 31 and 2100 ng/mL, respectively. All xylazine-positive cases co-occurred with fentanyl, a fentanyl precursor or metabolite, or a fentanyl analog. Additionally, 71% of these cases involved five to eight additional drug classes.

DISCUSSION: Xylazine has rapidly infiltrated the San Francisco illicit fentanyl supply and is frequently present in polysubstance use patterns. Rising prevalence and increasing concentrations indicate evolving market dynamics, with implications for overdose risk and forensic and public health surveillance.