Older Adults on Cannabis and Interacting Drugs Face Elevated Intoxication Risk, Study Finds

An Ontario cohort study linking cannabis clinic records to administrative health data finds that concomitant medical cannabis and narrow-therapeutic-index drug use in older adults is associated with more than doubled risk of drug-related intoxication, while prescribing patterns for interacting medications showed no adjustment after cannabis authorization.

Why This Matters

Medical cannabis use among older adults is growing rapidly, and this population already carries a disproportionate burden of polypharmacy. Many commonly prescribed medications in seniors, including warfarin and other narrow-therapeutic-index drugs, are metabolized through cytochrome P450 pathways that cannabinoids are known to inhibit or induce. The question of whether prescribers are accounting for these interactions when authorizing cannabis has direct patient safety implications. This study provides some of the first large-scale, real-world outcome data on what happens when that question goes unaddressed.

Clinical Summary

Cannabis interacts with numerous medications through cytochrome P450 enzyme modulation, particularly CYP3A4 and CYP2C9, pathways central to the metabolism of drugs with narrow therapeutic indices such as warfarin, certain anticonvulsants, and immunosuppressants. Published in Age and Ageing in 2025, this longitudinal cohort study by Gomes and colleagues linked clinical cannabis authorization records from Ontario cannabis clinics to provincial administrative health databases (ICES), examining 12,599 seniors who received authorized cannabis prescriptions between June 2014 and January 2019. The study employed two analytical strategies: an interrupted time series analysis to determine whether prescribers adjusted dispensation of interacting co-medications after cannabis authorization, and propensity-score-matched cohort comparisons to estimate the risk of specific adverse outcomes among patients using both cannabis and interacting drugs versus matched controls from the general population.

The interrupted time series revealed no statistically significant change in dispensation trends for drugs at risk of significant cannabis interaction or drugs with a narrow therapeutic index in the year following cannabis authorization compared to the year before, suggesting a systemic prescriber awareness gap. In the matched cohort analyses, concomitant cannabis and narrow-therapeutic-index drug use was associated with a hazard ratio of 2.61 (95% CI 1.42 to 4.79) for drug-related intoxication among 3,926 exposed patients compared to 12,223 controls. The cannabis-warfarin pairing did not reach statistical significance for bleeding (HR 1.19, 95% CI 0.71 to 1.98), though the small exposed group of 378 patients and wide confidence interval cannot exclude a clinically meaningful effect. Cannabis plus levothyroxine was associated with increased heart failure-related emergency visits but not thyrotoxicosis or acute coronary events. The authors emphasize that the observational design and inability to confirm actual cannabis consumption, dosing, or THC-to-CBD ratios preclude causal conclusions, and they call for prospective pharmacovigilance studies and prescriber education interventions.

Dr. Caplan’s Take

This study gets at something I encounter constantly in practice: a patient already on multiple medications, including one or more with a narrow therapeutic window, who has been authorized to use medical cannabis by another provider with no apparent coordination of their medication regimen. The 2.61-fold intoxication risk is striking, but what concerns me equally is the interrupted time series finding. The fact that prescribing patterns did not budge after cannabis authorization tells us that the system is not treating cannabis as a pharmacologically active agent with real interaction potential. Patients asking whether cannabis is safe alongside their existing medications deserve an honest answer, and that answer right now is that we do not have enough data to guarantee safety, and the data we do have points toward real risk.

In my practice, any patient on warfarin, anticonvulsants, immunosuppressants, or other narrow-therapeutic-index drugs who is using or considering medical cannabis gets explicit counseling about interaction risk, closer monitoring of drug levels where applicable, and a documented conversation in the chart. I also reach out to the authorizing cannabis provider when possible to ensure someone is looking at the whole medication list. This is not about opposing cannabis use; it is about basic pharmacological diligence in a polypharmacy population.

Clinical Perspective

This study occupies an important position in the research arc for cannabis-drug interactions: it moves beyond case reports and in vitro pharmacokinetic studies to provide population-level outcome data in a real-world prescribing environment. The intoxication signal with narrow-therapeutic-index drugs is biologically plausible and statistically robust, though it cannot be treated as causal given unmeasured confounding by indication, disease severity, and the fundamental uncertainty about whether authorized patients actually consumed cannabis, at what dose, and in what formulation. The non-significant warfarin-bleeding result should not be interpreted as evidence of safety; the exposed cohort was small, the confidence interval wide, and the direction of effect consistent with concern. For clinicians, this study supports the recommendation that cannabis authorization in older adults should trigger a medication review, but it does not yet provide the granular dose-response or formulation-specific data needed to guide precise adjustment protocols.

From a pharmacological standpoint, both THC and CBD are known inhibitors of CYP2C9, CYP3A4, and CYP2C19, with CBD also inhibiting CYP2D6. These pathways metabolize warfarin, many anticonvulsants, calcineurin inhibitors, and numerous other drugs commonly prescribed to older adults. Clinicians should be aware that even topical or low-dose CBD products can produce systemic levels sufficient to affect hepatic metabolism in some patients, particularly those with hepatic impairment or on enzyme-inhibiting polypharmacy. The single most actionable step clinicians can take now is to implement a structured medication interaction check at the point of cannabis authorization or renewal, with particular attention to INR monitoring in warfarin patients and therapeutic drug level monitoring for anticonvulsants and immunosuppressants.

Study at a Glance

Study Type

Interrupted time series combined with propensity-score-matched longitudinal cohort study

Population

Ontario adults aged 66 and older authorized for medical cannabis (2014 to 2019)

Intervention

Medical cannabis authorization with concomitant use of interacting medications

Comparator

Propensity-score-matched Ontario residents not authorized for cannabis (3:1 ratio)

Primary Outcomes

Drug-related intoxication, bleeding (warfarin cohort), heart failure and thyroid-related events (levothyroxine cohort)

Sample Size

12,599 (ITS); 3,926 exposed vs. 12,223 controls (DNTI cohort); 378 vs. 1,646 (warfarin cohort)

Journal

Age and Ageing

Year

2025

DOI or PMID

Published in Age and Ageing, 2025 (specific DOI not provided in source data)

Funding Source

Not specified in available text

What Kind of Evidence Is This

This is a peer-reviewed observational cohort study combining interrupted time series and propensity-score-matched designs, using linked administrative health data. It sits in the mid-range of the evidence hierarchy, above case reports and cross-sectional surveys but below randomized controlled trials. The single most important inference constraint is that actual cannabis consumption, dosing, formulation, and adherence are not directly measured, meaning the exposure variable reflects authorization rather than confirmed use, which could attenuate or distort the true association.

How This Fits With the Broader Literature

Prior evidence for cannabis-drug interactions in older adults has been largely limited to case reports, pharmacokinetic modeling, and small observational series. Case reports of supratherapeutic INR in patients combining cannabis with warfarin have been published repeatedly, and in vitro data consistently demonstrate cannabinoid inhibition of CYP2C9 and CYP3A4. This study extends that evidence by providing population-level outcome data, confirming that the theoretical interaction risk translates into a measurable clinical signal for drug-related intoxication in a real-world setting. The non-significant warfarin result neither confirms nor contradicts previous case-level concern, given the limited statistical power of the warfarin subcohort. The finding that prescribing patterns did not change after cannabis authorization aligns with surveys showing limited clinician confidence in managing cannabis-drug interactions and echoes findings from Antoniou and colleagues regarding knowledge gaps among Canadian prescribers.

Common Misreadings

The most likely overinterpretation is treating the 2.61-fold intoxication hazard ratio as proof that cannabis directly caused these events. While the association is statistically significant and biologically plausible, this is an observational study using administrative data, and residual confounding by indication, unmeasured disease severity, and the gap between cannabis authorization and actual consumption all limit causal inference. Equally problematic is the opposite error: interpreting the non-significant warfarin-bleeding result as evidence that cannabis is safe with warfarin. The exposed cohort was small, the confidence interval spanned from a protective effect to a near-doubling of risk, and the study was likely underpowered for this specific outcome.

Bottom Line

This study provides the strongest population-level signal to date that concomitant medical cannabis and narrow-therapeutic-index drug use in older adults is associated with elevated intoxication risk, and that prescribers are not systematically adjusting co-medications in response to cannabis authorization. It does not establish causation and cannot guide specific dosing adjustments. For clinical practice right now, it reinforces the need for structured medication interaction review at every cannabis authorization, with enhanced monitoring of drug levels and clinical status in patients on narrow-therapeutic-index agents.

References

1. Gomes T, et al. Medical cannabis and drug interactions in older adults: a population-based cohort study. Age and Ageing. 2025. (Specific DOI not available in source data.)
2. Antoniou T, et al. Knowledge and attitudes of Canadian physicians regarding medical cannabis. Canadian Family Physician. (Cited as contextual reference for prescriber awareness gaps.)