Fentanyl Detection in Biological Samples From Individuals Reporting Synthetic Cannabinoid Use in São Paulo, Brazil.

INTRODUCTION: Brazil has historically exhibited low illicit opioid prevalence, though forensic data indicate rising fentanyl seizures, often in combination with other substances. People who use Synthetic cannabinoid receptor agonists (SCRA) are particularly vulnerable to unintentional fentanyl exposure due to the unpredictable chemical composition of these products. This study aimed to investigate the discordance between self-reported drug use and toxicological detection of fentanyl among individuals reporting recent SCRA use recruited from addiction treatment and psychosocial care centres in São Paulo. METHODS: Cross-sectional study with 65 participants reporting SCRA use within the preceding 4 days, recruited from a specialised addiction treatment centre and two community-based psychosocial care centres in São Paulo. Oral fluid samples collected by independent research staff using the Quantisal device were analysed by liquid chromatography-tandem mass spectrometry. Sociodemographic characteristics, self-reported substance use and clinical symptoms were assessed by structured questionnaire. Exploratory multivariable logistic regression investigates factors associated with fentanyl detection. RESULTS: Fentanyl was detected in 32% of participants (21/65; 95% CI 21, 44), none of whom reported intentional opioid use. Fentanyl-positive participants were more likely to be experiencing homelessness (52.4% vs. 13.6%, p = 0.002), to report longer SCRA use duration (p = 0.001), and to present with nausea/vomiting (90.5% vs. 43.2%, p < 0.001) and myalgia (61.9% vs. 11.4%, p < 0.001). No independent factors associated with fentanyl detection were identified in exploratory multivariable analyses. DISCUSSION AND CONCLUSIONS: A substantial discordance between self-reported drug use and toxicological findings was observed. While the small sample size limits generalizations, these findings provide preliminary evidence that the historical assumption of negligible illicit opioid exposure in this population warrants reassessment. Whether this reflects changes in the local drug supply, underreporting of opioid use, or other exposure pathways cannot be determined from the available data. These results underscore the need for systematic toxicological surveillance and drug checking to monitor the evolving drug supply in Brazil.