National amphetamine consumption in Germany estimated by wastewater analysis: a stratified extrapolation with methamphetamine cross-contribution correction

Background: Wastewater-based epidemiology (WBE) provides objective, population-level estimates of illicit drug consumption, yet methods for scaling city-level measurements to national totals remain underdeveloped. This study applies a stratified extrapolation framework to estimate national pure amphetamine consumption in Germany, incorporating an empirical correction for the metabolic cross-contribution of methamphetamine. Methods: Population-normalised mass loads (PNML) for amphetamine and methamphetamine were obtained from 13 German wastewater treatment plants participating in the 2025 EUDA/SCORE campaign (12,053,422 inhabitants; 14.3% of the national population). Consumed amphetamine was back-calculated using a correction factor CF = 2.77 (f _excr = 0.361). A wastewater-derived methamphetamine correction (α = 0.09) was applied. National extrapolation employed five city-size strata: three with direct observations and two—comprising 69% of the population—requiring median-based imputation. Uncertainty was assessed through an 18-scenario matrix varying CF (2.50–3.33), imputation strategy, and methamphetamine correction. Results: Methamphetamine-corrected per-capita consumption rates ranged from 0.057 to 0.569 mg/person/day across 12 sites with non-zero estimates; one site (Chemnitz) corrected to zero. Five eastern German catchments required correction, with reductions of 2–100%. The central national estimate was 20.6 kg/day (approximately 7,500 kg/year; ~7.5 tonnes/year). The full scenario range was 3.6–12.5 tonnes/year. Roughly 63% of the national total derived from the two fully imputed strata. Conclusions: Stratified WBE extrapolation yields an approximate, order-of-magnitude national estimate of amphetamine consumption for Germany. Structural uncertainty arising from non-representative sampling dominates the total uncertainty budget. Methamphetamine correction is critical for eastern German catchments, and expanding monitoring to small-town and rural treatment plants is the single most effective step toward reducing estimation uncertainty.