Temporal and regional associations between fentanyl concentrations in the unregulated drug supply and drug-related mortality in British Columbia, Canada

BACKGROUND: British Columbia (BC), Canada, continues to experience persistently high rates of unregulated drug toxicity. While the presence of fentanyl in the drug supply is well-established, less is known about how fluctuations in its concentration may influence mortality at the population level. This study describes geographic variation and temporal trends in fentanyl concentrations across BC and assesses if they are associated with unregulated drug toxicity rates. METHODS: Using a validated machine learning model that quantifies fentanyl and fluorofentanyl concentrations in unregulated opioid samples, we estimated concentrations in samples submitted to BC drug checking services from October 2018 to June 2025. We derived monthly median concentrations by geographic health service delivery area to determine historic monthly typical fentanyl strength. We then examined the relationship between fentanyl concentrations and unregulated drug toxicity mortality rates using a generalized additive mixed model (GAMM), accounting for regional heterogeneity, temporal trends, and autocorrelation. RESULTS: Median fentanyl concentrations varied geographically and temporally across the province, peaking provincially at 11.0% in mid-2023 before declining to 5.1% in early 2025. The GAMM estimated that, on average, each 1-percentage point increase in median fentanyl concentration was associated with a 0.072 increase in the monthly drug-related mortality rate per 100,000 population (p = 0.029). CONCLUSIONS: Quantifying fentanyl concentrations from point-of-care drug checking data enables detection of geographic and temporal patterns in the unregulated drug supply and their associations with drug-related mortality. This approach offers a tool for harm reduction, drug supply monitoring, and policy response during the ongoing drug toxicity crisis.