A large-scale study of hospital admissions in Ontario, Canada has shown that there are higher incidences of schizophrenia in patients with cannabis use disorder (CUD) when compared to the rest of the population.
Researchers based in Ontario and Berlin studied data from hospital admissions between 2006 and 2022. The data included people aged 14 – 65 who were paying into the regional healthcare program (97% of the population paid into the Ontario Health Insurance Plan). People who had previously been diagnosed with schizophrenia were excluded from the data.
The study analysed data across three significant periods of cannabis policy change – pre-legalisation (January 2006 to November 2015), liberalisation of medical cannabis (December 2015 to September 2018), and legalisation of non-medical cannabis (October 2018 to December 2022).
Over a 17-year period data from 13,588,681 patient’s medical records were analysed. 118,650 people had at least one emergency department visit for CUD. Incidences of schizophrenia were defined by having a discharge diagnosis from a hospital, psychiatric hospital, or an emergency department.
The study found that 0.7% of the participants without CUD, a total of 91,106 developed schizophrenia throughout the study period. This percentage was over ten times lower than the number of people with CUD who developed schizophrenia over the same period, which was 8.9% (10,853).
Researchers noted that the rates of CUD and schizophrenia were greater in younger males. “Compared with individuals without CUD, individuals with CUD were more likely to be male, to be younger, to live in lower-income neighbourhoods, and less likely to be immigrants to Canada. Individuals with CUD had substantially higher levels of prior outpatient and acute care associated with mental health and substance use.”
Schizophrenia has long been associated with cannabis use. Despite extensive research, conclusive evidence of a causal relationship has been hard to find. Previous results have shown no link between cannabis legalisation and schizophrenia, while other research has suggested that people more likely to develop psychosis may actually seek out cannabis as a way to self-medicate.
While this study does not claim there is a direct link between cannabis legalisation and schizophrenia, researchers say that “the proportion of incident cases of schizophrenia associated with CUD almost tripled during a period encompassing ongoing liberalization of medical and non-medical cannabis.”
“Ongoing research is needed on long-term trends in the incidence of psychotic disorders associated with changes in cannabis policy, particularly the commercialization of the legal cannabis market, which was not well captured in this study.”
