The incidence of cannabis-induced psychosis in Denmark rose steadily from 2.8 to 6.1 per 100,000 person-years between 2006 and 2016, paralleling increases in THC potency and cannabis use.
Psychiatrists, public health officials, and policymakers evaluating cannabis regulation.
Cannabis-induced psychosis: 2.8 to 6.1 per 100,000 over 10 years
What the researchers found
Cannabis-induced psychosis incidence more than doubled from 2.8 per 100,000 in 2006 to 6.1 per 100,000 in 2016. A corresponding increase occurred in dual diagnoses of schizophrenia with cannabis use disorder. In contrast, alcohol-induced psychosis decreased over the same period, and other substance-induced psychoses showed no trend.
Why it matters
The selective increase in cannabis-induced psychosis (while alcohol-induced psychosis declined and others remained stable) suggests this trend is specific to cannabis rather than reflecting general changes in diagnostic practices or healthcare access.
The numbers in context
Cannabis-induced psychosis: 2.8 per 100,000 (2006) to 6.1 per 100,000 (2016). Alcohol-induced psychosis decreased. Other substance-induced psychoses showed no trend. Dual diagnosis (schizophrenia + CUD) also increased.
How the study worked
National registry study using the Danish Psychiatric Central Research Register. Analyzed absolute incidence and incidence rates per 100,000 person-years for cannabis-induced psychosis, other substance-induced psychoses, and dual diagnosis (schizophrenia + CUD) from 1994 to 2016.
What this study cannot tell us
Registry data depends on clinical diagnostic accuracy. Cannot directly link individual THC exposure levels to psychosis. Changes in help-seeking behavior or cannabis availability could partially explain trends.
How to read the evidence
National registry covering entire Danish population with consistent methodology over 22 years. Strong for trend detection.
When this study was published
2021 study analyzing Danish registry data from 1994-2016.
The bigger picture
Denmark has seen both rising cannabis use and increasing THC potency during this period. The parallel rise in both cannabis-induced psychosis and schizophrenia with CUD dual diagnoses suggests a real clinical impact that tracking systems are capturing consistently.
Questions still open
- How much of the increase is attributable to higher THC potency vs. more widespread use? Are countries with different cannabis policies seeing similar trends? Would availability of lower-potency products slow the increase?
Common questions
Could changes in diagnosis explain the increase?
Did schizophrenia with cannabis use also increase?
Read the original research
Annual incidence of cannabis-induced psychosis, other substance-induced psychoses and dually diagnosed schizophrenia and cannabis use disorder in Denmark from 1994 to 2016.
Psychological medicine, 51(4), 617-622
Citation
Hjorthøj, Carsten; Larsen, Maria Oku; Starzer, Marie Stefanie Kejser; Nordentoft, Merete. (2021). Annual incidence of cannabis-induced psychosis, other substance-induced psychoses and dually diagnosed schizophrenia and cannabis use disorder in Denmark from 1994 to 2016.. Psychological medicine, 51(4), 617-622. https://doi.org/10.1017/S0033291719003532
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