High Variation in Purity of Consumer-Level Illicit Fentanyl Samples in Los Angeles, 2023-2025

Background The variation in purity of illicitly manufactured fentanyl has been theorized to be a key driver of overdose. However, data on the purity of illicitly manufactured fentanyl in the United States typically comes from law enforcement seizures and is rarely available at the consumer-level, which is most relevant to overdose risk.

Samples were analyzed from a community-based drug checking program operating at four geographic sites in Los Angeles County, California 2023 Q1 to 2025 Q2. Participants answered an anonymous survey about sample characteristics. Qualitative and quantitative analyses were conducted leveraging directly-observed mass spectrometry (DART-MS) and Liquid chromatography mass spectrometry (LC/MS) respectively. LC/MS quantified a panel of compounds including fentanyl and fluorofentanyl. Composite fentanyl purity was estimated by adding the percent mass of fentanyl and fluorofentanyl.

A total of 353 samples had either fentanyl, fluorofentanyl, or both, quantified. Average purity was 10.0%, SD 11.1%, range 0.1%-64.9%. Samples expected to be fentanyl (n=308) had higher average purity (10.9%) compared to those expected to be heroin (n=24, average purity=2.7%) or other drugs. Powder samples (n=318) had higher average concentration (10.8%) compared to pills (n=11, 1.4%) or tar (n=22, 3.2%). Of expected-fentanyl samples, 42.5% (n=117) had a fentanyl purity of less than 5%, while 17.5% (n=51) had purity over 20%.

We found high variation in fentanyl purity among consumer-level samples sold as fentanyl, which may explain overdose among people with opioid tolerance. Fentanyl concentration was lower among samples sold as heroin, other drugs, or in pill form, and was particularly low among expected non-opioid drugs. Competing Interest Statement The authors have declared no competing interest. Funding Statement This work was supported by the Centers for Disease Control and Prevention as part of Overdose Data to Action: LOCAL (CDC-RFA-CE-23-0003) and the National Institute on Drug Abuse (R01DA057630). Drug Checking Los Angeles has been made possible through an equipment grant from the James B. Pendleton Charitable Trust to the UCLA AIDS Institute and UCLA Center for AIDS Research. CLS received support from the National Institute on Drug Abuse (K01DA050771). JRF received funding from the National Institute on Drug Abuse (DA049644) and the National institute of Mental Health (MH101072). AJK received educational support through the NIH/National Center for Advancing Translational Science (NCATS) UCLA CTSI (TL1TR001883). Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: Study protocols were approved by the UCLA IRB (IRB-22-0760 and IRB-22-1273). I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes Data Availability Data used in this analysis are sensetive and cannot be shared in raw form. However aggregate statistics can be provided by the authors upon reasonable request.