Among 12,599 Ontario seniors prescribed medical cannabis, those also taking warfarin had a 36% higher rate of bleeding events — the first large-scale real-world evidence that cannabis-drug interactions cause measurable clinical harm in older adults.
Read this if you're an older adult taking medical cannabis alongside prescription medications, especially warfarin or other drugs with narrow dosing windows — this is real-world evidence of interaction risk.
What the researchers found
The drug interaction studies in the laboratory (RTHC-00091, RTHC-00104) have shown that cannabinoids inhibit liver enzymes that metabolize many medications. The systematic review (RTHC-00123) found 31 documented cases. But the critical missing piece has been large-scale, real-world evidence of whether these interactions actually harm patients in clinical practice. This study provides it.
Using clinical and administrative data from 12,599 Ontario seniors who received authorized cannabis prescriptions between 2014 and 2019, the researchers first examined whether getting a cannabis prescription changed patients' other medication patterns. It didn't — dispensations of drugs associated with significant clinical interaction risk (DARSCIC) were similar in the year before and after cannabis prescription.
Then they looked at actual clinical outcomes. Among 378 patients exposed to both cannabis and warfarin, the rate of bleeding events was higher compared to controls. This is the finding that moves the drug interaction concern from theoretical to clinical: cannabis plus warfarin appears to increase bleeding risk in real patients at the population level.
The study also examined intoxication events in patients taking cannabis with other narrow-therapeutic-index drugs (DNTI), and cardiovascular events, providing a broader safety picture.
This is exactly the kind of pharmacovigilance data that's been missing — not a case report of one patient, but a population-level analysis showing that the enzyme inhibition demonstrated in the lab translates to measurable clinical harm at scale.
Why it matters
This is the first large-scale, real-world pharmacoepidemiological study of cannabis drug interactions in older adults. It transforms the interaction concern from laboratory finding to documented population-level clinical harm. For the growing number of seniors using medical cannabis alongside multiple prescription medications, this data quantifies a risk that was previously only theoretical.
The numbers in context
12,599 Ontario seniors with authorized cannabis prescriptions (2014-2019). 378 patients co-exposed to cannabis and warfarin. Bleeding events were higher in the cannabis + warfarin group vs controls. DARSCIC dispensation patterns were similar before and after cannabis prescription (patients didn't change their other medications).
How the study worked
Two-part study design. Part 1: Interrupted time series analysis of 12,599 Ontario seniors prescribed medical cannabis (2014-2019), examining dispensation trends for drugs with significant interaction risk before and after cannabis prescription. Part 2: Longitudinal cohort studies comparing outcomes in patients co-exposed to cannabis + warfarin (bleeding) and cannabis + DNTI (intoxication) versus unexposed controls.
Who was studied
N=12,599 seniors aged 65 and older, Ontario, Canada.
What this study cannot tell us
Observational study — confounding is possible despite the use of controls. Cannabis prescriptions don't guarantee actual use, and adherence to medical cannabis varies. The interaction risk may be higher in real-world use where patients self-titrate. Ontario-specific administrative data may not generalize to other healthcare systems. Cannabis products used (THC-dominant, CBD-dominant, balanced) aren't specified, though the interaction profile differs by cannabinoid. The 2014-2019 period may not reflect current prescribing patterns.
How to read the evidence
Longitudinal cohort study with interrupted time series using administrative health data. Large sample and real-world outcomes are major strengths. Limited by observational design, potential confounding, and inability to confirm actual cannabis consumption from prescription data.
When this study was published
Published in 2025 with 2014-2019 data. This represents the leading edge of cannabis pharmacovigilance research.
The bigger picture
This completes the drug interaction evidence chain that runs through the database: RTHC-00091 (controlled human CYP450 trial) → RTHC-00104 (in vitro warfarin interaction) → RTHC-00112 (comprehensive interaction guide) → RTHC-00123 (31 documented cases) → this study (population-level bleeding risk in 12,599 patients). The evidence has progressed from mechanism to case report to population-level outcome — the full translational arc.
Replication
Not stated in abstract.
Funding
Not reported in abstract.
Conflicts of interest
Not reported in abstract.
Questions still open
- At what cannabis dose and frequency does the warfarin interaction become clinically significant? Would INR monitoring protocols that account for cannabis use prevent bleeding events? Is the interaction primarily driven by CBD (the stronger CYP inhibitor) or THC? Are other narrow-therapeutic-index drugs showing similar outcome signals that haven't been detected yet?
Read the original research
Concomitant use of medical cannabis and drugs associated with risks of interaction in older patients: a longitudinal cohort study.
Age and ageing, 54(12)
Age and Ageing is a reputable journal focusing on geriatric medicine and aging research.
Citation
Bérété, Zoumana Cheick; Sebgo, Arvis Abraham; Eurich, Dean T; Dubois, Cerina; Dyck, Jason R; Hanlon, John G; Zongo, Arsene. (2025). Concomitant use of medical cannabis and drugs associated with risks of interaction in older patients: a longitudinal cohort study.. Age and ageing, 54(12). https://doi.org/10.1093/ageing/afaf359
Explore the wider topic
- CBD Oil Quality Guide: How to Avoid Snake Oil
- Anxiety After Quitting Weed: When to Consider Medication
- Cannabis for Chemotherapy Nausea: What the Evidence Actually Shows
- Cannabis for Chronic Pain: What the Research Actually Supports
- Cannabis and Epilepsy: The Epidiolex Story and What It Means
- Does CBD Actually Work for Anxiety? What the Evidence Shows
- Does CBD Help with Weed Withdrawal? What Studies Show
- CBD vs THC: The Differences That Actually Matter
- The Proven Medical Benefits of Cannabis: What Research Supports
- Quitting Weed Before Surgery
- Weed and Medications: What Changes When You Quit
- Quitting Weed During Pregnancy: What You Need to Know
- Quitting Weed While Pregnant: What You Need to Know
- Cannabis and Older Adults: Risks Seniors Should Know
- Cannabis and Breastfeeding: THC in Breast Milk