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.

Methods 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.

Results 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%.

Conclusions 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.

The authors have declared no competing interest.

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).

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Study protocols were approved by the UCLA IRB (IRB-22-0760 and IRB-22-1273).

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