Preliminary Results of a Longitudinal Study Assessing Cognition in Medical Cannabis Patients

Understanding the Risks for Medical Cannabis Patients

Access to medical cannabis is on the rise in a number of countries around the world as legislation and both political and public opinion continues to evolve. It is also worth noting that unauthorised use of cannabis for medicinal purposes remains extremely prevalent. The high usage rate of cannabis for medicinal purposes – whether obtained legally or illicitly – potentially also increases the risk of cannabis intoxication, poisoning, and withdrawal symptoms.

Data collected from the Alberta National Ambulatory Care Reporting System in Canada revealed that emergency department visits and cannabis-related calls to poison control centres increased following the 2018 law change. Similar results were demonstrated by a US study which found that the prevalence of hospitalisations for cannabis abuse and dependence increased significantly between 2002 (0.52%) and 2011 (1.34%), although these findings related mainly to recreational users. As evidence consistently demonstrates that medical and recreational user populations have significant differences, it is crucial to better understand the incidence and predictors of cannabis-associated risks among medical cannabis patient populations.

The authors of a recent study aimed to assess rates of emergency department visits and hospitalisations due to cannabis poisoning and cannabis-related mental health disorders among medically authorised cannabis patients. The study focused on patients in Ontario, Canada, where medical cannabis has been legally accessible since 2001. Canada also became the second country in the world to legalise regulated recreational cannabis in 2018, potentially leading to a rise in the number of patients self-medicating with recreational products.

Design and Methods of the Study

Few past studies assessing the prevalence of cannabis-associated hospitalisations, emergency department visits, and other risk factors have focused solely on medical cannabis populations. Therefore, the researchers and authors of the study aimed to assess the incidence rates of emergency department visits or hospitalisation due to (1) poisoning by cannabis and (2) mental and behavioural disorders due to use of cannabis among patients who were medically authorised to use cannabis. These were defined according to coded medical records. Factors that are associated with these outcomes were also assessed. 

This retrospective longitudinal study recruited a cohort of patients who received an authorisation to use cannabis for several health conditions in Ontario, Canada. All participating patients were aged 18 or over and accessed medical cannabis products from various clinics across Ontario between April 24, 2014 and March 31, 2017. The researchers assessed data collected in participating cannabis clinics, in addition to data from the Discharge Abstract Database (DAD) and the National Ambulatory Care Reporting System (NACRS) which contains all data on hospitalisations and emergency department visits, respectively.

Potential Predictors

In addition to the primary outcomes, the researchers also assessed three groups of potential predictors, including sociodemographic variables (e.g., age, sex, area of residence), comorbidities (including diabetes, congestive heart failure, COPD, asthma, cancer, pain, neurologic disorders, etc), and previous drug poisoning or substance use disorders (emergency department visits or hospitalisations specific to drug poisoning or mental and behavioural disorders).

Results of the Study

A total of 23,091 patients were included in the study, of which 53% were male and the majority were aged 31 to 60 years. The most common comorbidities were musculoskeletal disorders (48.87%), asthma (23.51%), neurological disorders (21.82%) and metabolic diseases (21.52%).

For the first outcome (emergency department visit or hospitalisation with a main diagnosis code for cannabis/cannabinoid poisoning), during a median follow-up of 240 days, 14 patients visited the emergency department or were hospitalised for cannabis poisoning for a total of 17,353.8 person-years, giving an incidence rate of 8.06 per 10,000 person-years. For the second outcome (emergency department visit or hospitalisation with a main diagnosis code for mental or behavioural disorders due to the use of cannabis), a total of 26 patients visited the emergency department or were hospitalised for mental or behavioural disorders associated with cannabis use, giving an incidence rate of 15.0 per 10,000 person-years. 

By considering the secondary diagnosis codes in the definition of the outcomes for the predictors assessment only, the total events increased to 18 for cannabis poisoning and to 82 for mental/behavioural disorders associated with cannabis use. Nonetheless, the data collected for this study show that rates of emergency department visits or hospitalisations for poisoning by cannabis and cannabis-related mental and behavioural disorders were low among patients with a medical cannabis authorisation. 

Prior poisoning by narcotics or hallucinogens was identified as a predictor for cannabis poisoning, while seven predictors were identified for cannabis-related mental and behavioural emergency department visits and hospitalisations. These included prior poisoning by other psychoactive drugs (antidepressants and other); mental and behavioural disorders due to psychoactive drugs, or alcohol use; other mental and behavioural disorders; younger age; diabetes; and COPD. The researchers note, therefore, that patients with a history of drug poisoning or substance use disorder, and more generally patients with mental and behavioural disorders should be regarded as patients with higher susceptibility to potential risks. 


The main finding of this study suggests that the incidence of cannabis poisoning and/or mental and behavioural disorders is low in medically authorised patients according to coded hospital records. 

These findings correlate with past findings and support the introduction of systematic assessments among patients seeking medical cannabis to identify potential risks associated with cannabis use disorder (CUD), drug addiction, and cannabis use. Nonetheless, the researchers acknowledge a number of limitations. Firstly, this study was unable to identify CUD/cannabis poisoning and intoxication that did not result in emergency department visits or hospitalisations. Secondly, the cohort was not fully representative of all patients who use cannabis for medicinal purposes, as evidence shows a large number of patients self-medicate with unauthorised cannabis products in Canada.

The researchers conclude that the data assessed in this study suggest a low incidence of cannabis poisoning among patients who are authorised by healthcare providers for the use of medical cannabis. It is recommended that future research should “specifically assess the patterns of cannabis use that are more likely to be associated with CUDs.” Such research could help prevent potential drug-cannabinoid interactions and help clinicians to identify and intervene among patients at risk of problematic cannabis use.