Wastewater testing in a Japanese town found low but rising THC use and surprisingly high CBD use from 2020 to 2023
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.
Quick Facts
What This Study 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.
Key Numbers
~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 They Did This
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.
Why This Research 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 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.
What This Study Doesn't 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.
Questions This Raises
- ?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?
Trust & Context
- Key Stat:
- 27% annual increase in THC use; CBD use surprisingly matches THC
- Evidence Grade:
- Preliminary: pilot wastewater study in a single town with limited population representativeness and inability to quantify CBD consumption.
- Study Age:
- Published 2026. Data from October 2020 to March 2023.
- Original Title:
- 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).
- Published In:
- Forensic science international, 378, 112687 (2026)
- Authors:
- Thai, Phong K, Zheng, Qiuda, Yagishita, Nanami, Wang, Zhe, Hara-Yamamura, Hiroe, Hall, Wayne, Honda, Ryo
- Database ID:
- RTHC-08658
Evidence Hierarchy
Watches what happens naturally without intervening.
What do these levels mean? →Frequently Asked Questions
Is cannabis use rising in Japan?
Wastewater data from one Japanese town showed a 27% annual increase in THC markers from 2020 to 2023, though use remains low compared to other countries.
Do Japanese people use CBD?
CBD use was surprisingly common, with wastewater detection similar to THC levels. CBD is legal in Japan for medical purposes, unlike THC.
Read More on RethinkTHC
Cite This Study
https://rethinkthc.com/research/RTHC-08658APA
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
MLA
Thai, Phong K, et al. "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, 2026. https://doi.org/10.1016/j.forsciint.2025.112687
RethinkTHC
RethinkTHC Research Database. "Using wastewater analysis to estimate the prevalence of THC ..." RTHC-08658. Retrieved from https://rethinkthc.com/research/thai-2026-using-wastewater-analysis-to
Access the Original Study
Study data sourced from PubMed, a service of the U.S. National Library of Medicine, National Institutes of Health.
This study breakdown was produced by the RethinkTHC research team. We analyze and report published research findings without making health recommendations. All interpretations are based solely on the published abstract and study data.