Changes in cannabis potency and cannabis-related harms: A 22-year ecological study in Denmark

Background: In the past two decades, increased potency of cannabis resin has been reported across the U.S. and Europe, with Denmark reporting the highest THC concentration in seized cannabis resin in Europe. The increased potency raises concerns as high levels of THC have been associated with increased risk of cannabis-related harms. A study from the Netherlands found time-dependent associations between changes in THC concentration in herbal cannabis and first-time admissions to cannabis treatment. More studies are needed to examine if potency is linked with cannabis-related harms such as admission to cannabis treatment, as well as other outcomes such as incidence of cannabis-induced psychotic disorder and dual diagnosis (schizophrenia and cannabis use disorder).

Methods: Data on THC concentration in seized cannabis resin samples were obtained from the three forensic departments in Denmark (2000-22). Data on admissions to cannabis treatment and incidence of cannabis-induced psychotic disorder and dual diagnosis (2000-22) were obtained from national registers in regions overlapping with police seizures. We used mixed effects linear regression models to examine time-dependent associations between THC concentration in cannabis resin samples and first-time cannabis admissions to specialist drug treatment; incidence of cannabis-induced psychotic disorder; and incidence of dual diagnosis. Candidate time lags were 0-10 years.

Results: THC concentration increased almost 4-fold from a mean of 8.3% to 31.2% from 2000-22. After adjusting for age, sex, and non-cannabis drug treatment admissions, THC was positively associated with cannabis treatment entry at lags of 0-6 (lags 0-6, p<0.0001), with the strongest association at 0 years (z=8,691, p<0.0001). After adjusting for age, sex, and other substance-induced psychosis, THC was positively associated with incidence of cannabis-induced psychosis at lags of 0-4 (lags 0-2, p<0.0001; lags 3-4, p<0.01), with the strongest association at 0 years (z=5,124, p<0.0001). After adjusting for age, sex, and other substance use disorder, THC was positively associated with incidence of dual diagnosis at lags of 0-1 (lags 0-1, p<0.01), with the strongest association at 0 years (z=3,051, p<0.01). We did not find positive associations in the unadjusted models or models only adjusted for age and gender.

Conclusions: The potency of seized cannabis resin increased almost 4-fold from 2000-22. We found positive time-dependent associations between changes in potency and three measures of cannabis-related harms in fully adjusted models. Our findings corroborate and extend previous findings suggesting cannabis potency is associated with an increased risk of cannabis-related harms, but that other factors (e.g. related to the treatment system) also play a role.

This submission was prepared in collaboration with Janna Cousijn, Francesca Filbey, Emily Kenyon and Sarah W. Feldstein Ewing who have submitted related studies.