Exposure vs Survivability: Interpreting Declines in Overdose Mortality Using Case Fatality Dynamics

This study examines temporal changes in overdose case fatality (CFR), defined as the ratio of fatal overdoses to total overdoses (fatal + nonfatal), to better understand the population-level processes underlying recent declines in U.S. overdose mortality. Specifically, the project evaluates whether observed mortality reductions are more consistent with structural changes affecting overdose exposure — such as shifts in drug supply characteristics — or with behavioral and system-level adaptations that improve survivability, including naloxone diffusion, bystander response, and evolving use practices. Using a longitudinal state-level panel constructed from national surveillance data, the study models CFR dynamics over time while adjusting for seasonality, flexible temporal trends, and persistent geographic differences. By decomposing mortality into its underlying components — overdose incidence (proxied by nonfatal overdoses) and lethality conditional on overdose — the analysis evaluates whether deaths are declining because fewer overdoses are occurring or because overdoses are becoming less likely to result in death. A central objective is to assess whether lethality trends are nationally synchronized, which would suggest broad structural influences, or heterogeneous across states, which would instead indicate the importance of localized behavioral, policy, or health system contexts. Additional models estimate the elasticity between fatal and nonfatal overdoses to evaluate whether the coupling between exposure and death has shifted over time, providing an interpretable indicator of survivability dynamics. This study is descriptive and inferential rather than strictly causal; it is not designed to attribute mortality changes to any single mechanism. Instead, the goal is to evaluate which classes of explanations are most consistent with observed population-level patterns and to provide structured empirical evidence for ongoing debates regarding supply-side versus survivability-based drivers of the overdose transition. Expected contributions include: • Identification of whether mortality declines are primarily incidence-driven or survivability-driven • Evidence on the degree of national versus geographic synchronization in overdose lethality • Quantification of temporal changes in the fatal–nonfatal relationship • Stronger empirical grounding for competing structural and response-based interpretations of the overdose decline By focusing directly on case fatality rather than deaths alone, this study advances a population-health framework for interpreting overdose dynamics and provides a more policy-relevant lens for understanding how mortality reductions emerge.