A medical cannabis patient who increased his THC dose by 7.35 mg experienced a dangerous elevation in warfarin levels (INR 5.2), likely due to both drugs competing for the same liver enzymes.
Medical cannabis patients taking blood thinners, pharmacists, prescribers managing anticoagulation therapy
A 7.35 mg THC dose increase caused INR to spike to 5.2
What the researchers found
A 67-year-old man on warfarin developed a supratherapeutic INR of 5.2 after self-titrating his THC dose upward by 7.35 mg. Warfarin was held for 2 days until INR normalized. The interaction scored "probable" on the Naranjo scale and is explained by shared CYP450 metabolism.
Why it matters
Warfarin is one of the most commonly prescribed blood thinners, and this interaction could cause life-threatening bleeding. As more older adults use medical cannabis, this drug interaction needs wider awareness.
The numbers in context
Patient age 67; THC dose increase of 7.35 mg; INR rose to 5.2 (therapeutic range typically 2-3); Naranjo score 8 (probable); no bleeding occurred; INR normalized after holding warfarin 2 days
How the study worked
Case report of a single patient presenting at a cannabis dispensary in Buffalo, NY, with documented INR elevation temporally linked to THC dose increase.
What this study cannot tell us
Single case report cannot establish definitive causation. Patient was self-titrating, introducing imprecision. Other potential contributing factors (diet, other medications) not fully detailed.
How to read the evidence
Single case report with probable Naranjo causality score
When this study was published
Published in 2021. Awareness of THC-warfarin interactions remains limited among many clinicians.
The bigger picture
THC and warfarin are both metabolized by cytochrome P450 enzymes, and this case adds to growing evidence that cannabinoids can cause clinically significant drug interactions, especially with medications that have narrow therapeutic windows.
Questions still open
- What THC dose threshold triggers this interaction? Should medical cannabis patients on warfarin have more frequent INR monitoring? Are other anticoagulants (DOACs) also affected by cannabis?
Common questions
Can cannabis interact with blood thinners?
What should patients on warfarin know about cannabis?
Read the original research
Δ-9-tetrahydrocannabinol dose increase leads to warfarin drug interaction and elevated INR.
Journal of the American Pharmacists Association : JAPhA, 61(1), e57-e60
Citation
Brown, Geoffrey W; Bellnier, Terrance J; Janda, Maria; Miskowitz, Kyle. (2021). Δ-9-tetrahydrocannabinol dose increase leads to warfarin drug interaction and elevated INR.. Journal of the American Pharmacists Association : JAPhA, 61(1), e57-e60. https://doi.org/10.1016/j.japh.2020.07.028
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