Fentanyl in British Columbia’s unregulated drug supply : potency, policy, and implications for public health

Background: As North America’s unregulated drug toxicity crisis continues, British Columbia, Canada remains one of the most affected jurisdictions, with persistently elevated rates of drug-related morbidity and mortality. The unregulated opioid supply has undergone marked transitions, first from heroin to fentanyl, and more recently toward increasingly complex polysubstance formulations involving fentanyl analogues and other sedatives. The nature of these changes remains poorly understood, as do their implications for population health. The overarching objective of this dissertation is to examine how characteristics of the unregulated drug supply can be measured and incorporated into population-level health outcomes research. Methods: To assess how population-level drug supply data are used in health research, a scoping review was conducted that described methodological approaches to incorporating drug supply measures as independent variables. Then, longitudinal community-based drug checking data from two Canadian settings were used to perform a controlled interrupted time-series analysis examining whether British Columbia’s decriminalization policy was associated with changes in fentanyl concentrations in unregulated opioids. Finally, using descriptive and analytical modelling approaches, an evaluation of whether geography-specific ecological measures of fentanyl strength were associated with drug-related mortality was conducted. Results: The review found that drug supply measures have been used to predict health outcomes, but remain inconsistently conceptualized and applied across studies. In the quasi-experimental analysis, although secular trends in fentanyl concentrations were observed, trajectories in Vancouver, British Columbia and Toronto, Ontario did not meaningfully diverge, providing no evidence that decriminalization was associated with changes in fentanyl strength. Analyses leveraging province-wide drug checking data showed that higher monthly median fentanyl concentrations were associated with higher drug-related mortality rates, with an estimated average increase of 0.072 deaths per 100,000 population per month (p = 0.029) for each one-percent increase in fentanyl concentration. Conclusions: While the unregulated drug supply may appear unpredictable, these findings suggest that its volatility may be structured rather than random. Population-level drug supply measures represent an important, underutilized dimension of substance use epidemiology. Future research and policy evaluation should consider how shifts in drug supply composition act as both drivers of population health risk and potential sources of confounding when assessing interventions.