Changes in cannabis potency and cannabis-related psychiatric harms: a 23-year ecological study in Denmark
Kristine Rømer Thomsen, B. Thylstrup, Morten Hesse, Christian Lindholst, L. Reitzel, T. S. Eriksen, A. Erlangsen, Amir Englund, Tom P. Freeman, C. Hjorthøj
Psychological Medicine May 4, 2026 DOI: 10.1017/s0033291726104036 (opens in new tab)
Study at a glance
AI-extracted from the abstract| Characteristics | Observational study using national register data and forensic samples, with mixed-effects linear regression Peer reviewed |
|---|---|
| Population | Seized cannabis resin samples from Denmark (2000–2022) and national register data on people admitted to cannabis treatment, diagnosed with cannabis-induced psychosis, or with dual diagnosis of schizophrenia and cannabis use disorder |
| Duration | 2000–2022; time lags of 0–10 years examined |
| Topics | Cannabis |
| Key findings | Average THC concentration in seized Danish cannabis resin rose from 8.3% to 31.2% between 2000 and 2022. In fully adjusted models, higher THC was positively associated with first-time cannabis treatment entry, cannabis-induced psychosis, and dual diagnosis, with the strongest associations at year 0. No positive associations were found in unadjusted models. |
Abstract
Abstract Background In recent decades, the potency of cannabis resin increased globally, raising concerns, as higher potency has been associated with increased risk of psychiatric harms at the individual level. The aim here was to examine whether changes over time in the potency of seized cannabis resin samples were associated with psychiatric harms at the population level.
Methods: Data on ∆-9-tetrahydrocannabinol (THC) concentration in seized cannabis resin were obtained from forensic departments in Denmark (2000–2022), the country reporting the highest potency in Europe. Data on admissions to cannabis treatment, incidence of cannabis-induced psychosis, and dual diagnosis (schizophrenia and cannabis use disorder) were obtained from national registers. Time-dependent associations between potency and the outcomes were examined with mixed-effects linear regression models and associations across age and sex were explored. Candidate time lags were 0–10 years.
Results: THC concentration increased almost fourfold: mean 8.3–31.2% from 2000 to 2022. In fully adjusted models, THC was positively associated with first-time cannabis treatment entry at lags of 0–6, strongest at year 0 (p < 0.0001); incidence of cannabis-induced psychosis at lags of 0–4, strongest at year 0 (p < 0.0001); and incidence of dual diagnosis at lags of 0–1, strongest at year 0 (p < 0.01). No positive associations were found in unadjusted models. Subgroup analyses indicated associations in older patients and women.
Conclusions: Potency of seized cannabis resin increased almost fourfold from 2000 to 2022. Changes in cannabis potency were positively associated with psychiatric harms at the population level across all outcomes.