This study identified a significant increase in the proportion of patients who used cannabis within 30 days of their first admission to inpatient psychiatry in Ontario, Canada, between 2009 and 2017, compared to 2007. The increase was evident by gender, age, and most diagnostic groups. These findings confirm that cannabis use is more common among persons admitted to inpatient psychiatry compared to general population estimates. For instance, compared to 20% of psychiatric inpatients reporting cannabis use within 30 days of first admission, about 14% of adults in the general population of Ontario reported using cannabis in the prior year in 2017. ³⁷ These differences persist across age groups; for instance, 48% of patients aged 18 to 24 in this study reported cannabis use prior to admission in 2017 compared to 31% of those of the same age group in the Canadian population. ³⁷ Interestingly, the age-adjusted proportion of the Canadian population who reported cannabis use increased by about 7% among males and 6% among females between 2011 and 2017. ¹⁹ Therefore, the increasing proportion of psychiatric patients reporting cannabis use may be one indicator of a broader national trend of increased use of cannabis among adult Canadians.

We found that demographics, health behaviors, clinical symptoms, diagnoses, and life experiences were associated with the use of cannabis prior to admission. The nature and implications of these results may be broad, requiring further research. It will be important to further examine the association of cannabis use and mood disorders, including whether use of cannabis is associated with psychotic features of these disorders. Further research is also needed to examine whether cannabis use is associated with the onset of or attempts to cope with the emotional circumstances related to violent ideation or self-harm ideation. The interaction between gender and psychotic disorders provides new insights into the existing relationship between schizophrenia and cannabis. Although sex differences are known in the clinical presentation and prognosis of schizophrenia, ^38,39 the differences in cannabis use found among men and women with schizophrenia are not well understood. These differences may relate to variations in how cannabis is metabolized and experienced by each sex ⁴⁰ and the potential impact this difference might have on ongoing use of cannabis and the onset or course of schizophrenia. Further research is needed to examine this hypothesis using biological measures of sex that were not available in this study. Finally, the multivariable findings of this study may also be useful in evaluations of cannabis-related policies, particularly as covariates to consider when comparing patterns of cannabis use among different cohorts of psychiatric inpatients over time.

There are a number of questions that arise from the trends observed in this study. First, could these trends relate to changing sentiment around the use of cannabis? Several examples from the United States indicate that reduced social stigma and risk perception has resulted in increased use of cannabis. ^{41 bibr42-0706743720984679–43} These changing social perceptions may account for the narrowing gap in use among males and females. ⁴⁴ Additionally, there may have been changes in the social or clinical norms of reporting or discussing the clinical context of cannabis use between clinicians and patients. We must recognize the possibility then that the proportion of patients using cannabis prior to admission were underestimated. However, given that cannabis use increased across a broad set of individual characteristics, including factors like psychosis that have had longer standing associations with cannabis, it seems more plausible that reporting bias was limited.

Second, are increases in cannabis use prior to first admission to inpatient psychiatry related to increases in the use of cannabis for self-medication of unmet mental health needs among the general population? Self-medication to cope with mental health challenges has become a prominent theory to explain use of cannabis ² . However, with limited evidence supporting the effectiveness of cannabinoids in treating mental health conditions, ¹¹ use of cannabis to self-medicate may actually lead to a vulnerability to addiction or to the development of other mental health conditions. ² This cannot be answered by the data presented in this study because reasons for use were not assessed. Instead, we present this as a hypothesis to be addressed in future research. By definition, all patients in this study have a mental health concern. If patients admitted to inpatient psychiatry were using cannabis to self-medicate prior to admission, an opportunity exists for counselling and education about the risks of that practice. With limited access to mental health care in some areas in Ontario, ⁴⁵ it may be that increases in cannabis use prior to psychiatric admission reflect attempts to cope with mental health conditions.

Third, are increases in the proportions of patients using cannabis prior to first admission to inpatient psychiatry related to changes in cannabis policies? The findings of this study spanned much of the period where access increased to illegal cannabis retail outlets, and medical cannabis policies evolved. ^16,17 For instance, before being repealed and replaced by the nonmedical cannabis policies within Canada’s Cannabis Act in 2018, allowances had been introduced into the Access to Cannabis for Medical Purposes Regulations for home growing of cannabis, including for personal use and on behalf of one with a medical authorization for use. ¹⁶ In other jurisdictions, liberal medical cannabis policies that provided broader opportunities to access cannabis were associated with higher prevalence of mental illness compared to jurisdictions with more restrictive policies. ²⁴ Therefore, it may have been that the liberalization in cannabis policies in Canada was associated with increases in the proportions of individuals using cannabis prior to first admission to inpatient psychiatry.