Application of Wastewater‐Based Epidemiology in Japan to Estimate the Prevalence of Substance Use Including Methamphetamine and Ephedrine

Estimating substance use in Japan traditionally depends on police seizure records and national self‐reporting surveys, which face substantial challenges due to social stigma. This study applies wastewater‐based epidemiology to overcome these challenges of monitoring illicit drug and legal stimulant use in a Japanese metropolitan area serving ∼1.8 million residents. Influent wastewater samples collected over 4 weeks in 2021 were analyzed for 29 substances. Methamphetamine was the only illicit drug consistently detected (consumption at 17.0 ± 5.1 mg/day/1000 people), reflecting its prevalence in Japan's drug scene and aligning with patterns in other East Asian countries. Ephedrine and pseudoephedrine, known precursors of methamphetamine were detected at high levels, suggesting their common use in legal medicines. Nicotine and methamphetamine consumption remained stable across weekdays and weekends. Higher ephedrine consumption during the lockdown period likely resulted from increased use of Kampo medicines to manage COVID‐19 symptoms. This study provides the first objective evidence of population‐level methamphetamine use in a large urban catchment of Japan, which is relatively low compared to other countries. The findings demonstrate the feasibility of wastewater analysis as a complementary surveillance approach to the existing system, enabling more responsive and evidence‐based drug policy.