Using wastewater analysis to estimate the prevalence of THC and CBD use in a population of Japan during and after the Covid-19 pandemic (2020–2023)

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BACKGROUND: It is illegal to use cannabis products that contain THC in Japan but cannabidiol (CBD) can be legally used for medical purposes. There is a lack of objective evidence on the comparative prevalence of THC and CBD use in Japan. This pilot study used wastewater to estimate the prevalence use of those two substances in the population of a small town in Japan during and after the Covid-19 pandemic period. METHOD: We conducted a longitudinal observational study. Wastewater samples were collected every week for every other two months from October 2020 to March 2023 from the inlet of a wastewater treatment plant serviced a town in Honshu Island, Japan with a population of approximately 120,000 people. Biomarkers of THC and CBD use in wastewater were measured during the monitoring period using liquid-liquid extraction and liquid chromatography-tandem mass spectrometry. RESULTS: The average estimated consumption of cannabis through the proxy of THC in the study population was 5 doses/1000 people/day. We observed a gradual increase of cannabis use from 2020 to 2023 by approximately 27 % per year. The prevalence of CBD use was similar to that of THC although CBD consumption could not be estimated in the absence of pharmacokinetic data. CONCLUSIONS: The prevalence of THC use remained low in Japan compared to other countries despite a steady increase in recent years, but the use of CBD was much more common than in other countries. These findings provide evidence on the value of wastewater surveillance of drug use to differentiate between THC and CBD in Japan.