Medical Cannabis Patients Had Higher Rates of ER Visits for Drug Poisoning Than Matched Controls

What the researchers found

Among 18,653 medical cannabis patients matched to 51,243 controls, poisoning incidence was 4.71 per 1,000 person-years for cannabis patients vs 1.73 for controls (adjusted HR: 2.45, 95% CI: 1.56-3.84). Mental/behavioral disorder incidence was 8.89 vs 5.01 per 1,000 person-years (adjusted HR: 2.27, 95% CI: 1.66-3.11). No sex differences were observed.

Why it matters

While the companion study (RTHC-04336) found low absolute rates of cannabis-specific harms, this study reveals that medical cannabis patients face elevated risk for broader substance-related problems compared to the general population, raising questions about pre-existing vulnerability versus cannabis-related risk.

The numbers in context

18,653 cannabis patients matched to 51,243 controls; poisoning: 4.71 vs 1.73 per 1,000 person-years (aHR 2.45); mental/behavioral disorders: 8.89 vs 5.01 per 1,000 (aHR 2.27); median 243 days follow-up; no sex differences

How the study worked

Retrospective cohort study matching 18,653 adult medical cannabis patients from Ontario clinics to 51,243 population-based controls. Outcomes were ER visits or hospitalizations for psychoactive drug poisoning and substance-related mental/behavioral disorders. Conditional Cox proportional hazards regressions used with median follow-up of 243 days.

What this study cannot tell us

Cannot establish causation. Medical cannabis patients differ from the general population in ways beyond cannabis use (chronic pain, mental health conditions) that could explain higher ER visit rates. Short follow-up period. Matching may not fully control for confounding by indication.

How to read the evidence

Large matched cohort study with administrative data, but confounding by indication remains a concern

When this study was published

2022 study

The bigger picture

This elevated risk could reflect that medical cannabis patients have underlying conditions (chronic pain, mental health disorders) that independently increase substance-related risks, rather than cannabis itself causing the excess events. Disentangling these factors is crucial for medical cannabis policy.

Questions still open

• Does the elevated risk reflect pre-existing vulnerability in people who seek medical cannabis, or does cannabis itself increase risk? Would more comprehensive matching on comorbidities reduce the observed difference? How do these rates compare to patients on long-term opioid therapy?

Common questions

Read the original research

Substance Use Disorders and Psychoactive Drug Poisoning in Medically Authorized Cannabis Patients: Longitudinal Cohort Study.

Canadian journal of psychiatry. Revue canadienne de psychiatrie, 67(7), 544-552

Citation

Zongo, Arsène; Lee, Cerina; El-Mourad, Jihane; Dyck, Jason R B; Hyshka, Elaine; Hanlon, John G; Eurich, Dean T. (2022). Substance Use Disorders and Psychoactive Drug Poisoning in Medically Authorized Cannabis Patients: Longitudinal Cohort Study.. Canadian journal of psychiatry. Revue canadienne de psychiatrie, 67(7), 544-552. https://doi.org/10.1177/07067437211060597

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