A wastewater surveillance study in a Japanese town of 120,000 found cannabis use at about 5 doses per 1,000 people per day, increasing 27% annually from 2020 to 2023, with CBD use prevalence similar to THC despite cannabis being illegal.
Drug policy researchers, epidemiologists using wastewater-based epidemiology, and anyone interested in cannabis use trends in Asia.
27% annual increase in THC use; CBD use surprisingly matches THC
What the researchers found
Average estimated cannabis consumption was 5 doses/1,000 people/day, with a 27% annual increase from 2020 to 2023. CBD use prevalence was similar to THC use. Cannabis use remained low compared to other countries despite the steady increase. CBD was much more commonly detected than in other countries' wastewater studies.
Why it matters
Japan has some of the strictest cannabis laws globally. Wastewater analysis provides objective population-level data that bypasses the severe underreporting expected in self-report surveys in a country where cannabis possession carries up to 5 years in prison.
The numbers in context
~120,000 population served. 5 doses/1,000 people/day estimated cannabis use. 27% annual increase from 2020-2023. CBD use prevalence similar to THC. CBD detection much higher than in other countries' wastewater. Weekly sampling every other 2 months for 2.5 years.
How the study worked
Longitudinal wastewater surveillance study from October 2020 to March 2023. Weekly samples collected every other two months from the inlet of a wastewater treatment plant serving approximately 120,000 people on Honshu Island, Japan. THC and CBD biomarkers measured by liquid-liquid extraction and LC-MS/MS.
What this study cannot tell us
Pilot study in a single town that may not represent Japan overall. CBD consumption could not be quantified due to absent pharmacokinetic data. Wastewater cannot identify individual users or use context. The 27% annual increase covers a period including COVID-19 disruptions.
How to read the evidence
Preliminary: pilot wastewater study in a single town with limited population representativeness and inability to quantify CBD consumption.
When this study was published
Published 2026. Data from October 2020 to March 2023.
The bigger picture
This is one of the first wastewater studies to separately measure THC and CBD in a country where only THC is illegal. The finding that CBD use matches THC use suggests a population actively distinguishing between legal and illegal cannabinoids, even as overall cannabis use grows.
Questions still open
- Is the rising THC use in Japan driven by younger demographics? How does CBD use compare to self-reported surveys in Japan? Will the steady increase in THC use influence Japan's cannabis policy debate?
Common questions
Is cannabis use rising in Japan?
Do Japanese people use CBD?
Read the original research
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).
Forensic science international, 378, 112687
Citation
Thai, Phong K; Zheng, Qiuda; Yagishita, Nanami; Wang, Zhe; Hara-Yamamura, Hiroe; Hall, Wayne; Honda, Ryo. (2026). 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).. Forensic science international, 378, 112687. https://doi.org/10.1016/j.forsciint.2025.112687