Implications of Inertia for Assessing Drug Control Policy: Why Upstream Interventions May Not Receive Due Credit

Background: There is ongoing interest in assessing the effectiveness of various drug control strategies, including policy intended to reduce initiation and prevalence. Compartmental models of trajectories of drug use have been developed that demonstrate that drug “systems” display significant inertia; interventions on systems with high inertia can be difficult to evaluate. Method: The implications of inertia are illustrated by combining a new empirically-derived model of national drug initiation with a compartment model of trends in illicit drug use parameterized for Australia. Results: Typical initiation trajectories seem to display damped oscillation. Lags and interpersonal variation in the trajectories of drug use careers smooth those oscillations for measures of problem use and total drug-related social cost. Even abrupt changes in initiation – as dramatic as a two-year long 50% reduction in initiation – barely ripple the trajectory of drug-related social cost. The benefits of upstream interventions are not diminished but rather are hidden in plain sight by being spread broadly over time so that they leave only the faintest finger-print detectable by before-and-after comparison – even if the intervention has a large effect and a perfect counter-factual is available. Conclusion: Fluctuating initiation is not inconsistent with ever worsening drug problems, and the absence of clear drops in drug-related problems does not imply that an upstream intervention did not produce substantial social benefits. Hence, macro-level evaluation of upstream drug control interventions has inherent limitations. Drug use and related problems are often perceived of as stubborn, even intractable. Some of that perception stems from real limitations in the effectiveness of drug-control interventions. However, inertia inherent in the evolution of drug use can make even cost-effective interventions appear weak (and detrimental interventions appear benign).