Increasing THC dose caused a dangerous spike in blood-thinning medication levels
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.
Quick Facts
What This Study 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.
Key Numbers
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 They Did This
Case report of a single patient presenting at a cannabis dispensary in Buffalo, NY, with documented INR elevation temporally linked to THC dose increase.
Why This Research 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 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.
What This Study Doesn't 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.
Questions This Raises
- ?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?
Trust & Context
- Key Stat:
- A 7.35 mg THC dose increase caused INR to spike to 5.2
- Evidence Grade:
- Single case report with probable Naranjo causality score
- Study Age:
- Published in 2021. Awareness of THC-warfarin interactions remains limited among many clinicians.
- Original Title:
- Δ-9-tetrahydrocannabinol dose increase leads to warfarin drug interaction and elevated INR.
- Published In:
- Journal of the American Pharmacists Association : JAPhA, 61(1), e57-e60 (2021)
- Database ID:
- RTHC-03027
Evidence Hierarchy
Describes what happened to one person or a small group.
What do these levels mean? →Frequently Asked Questions
Can cannabis interact with blood thinners?
Yes. This case report documents a probable interaction between THC and warfarin that caused dangerous blood-thinning levels. Both substances are processed by the same liver enzymes (CYP450).
What should patients on warfarin know about cannabis?
Changes in cannabis dose, especially THC increases, may alter warfarin levels. Patients should inform their anticoagulation providers about cannabis use and have their INR monitored more frequently when changing doses.
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Cite This Study
https://rethinkthc.com/research/RTHC-03027APA
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
MLA
Brown, Geoffrey W, et al. "Δ-9-tetrahydrocannabinol dose increase leads to warfarin drug interaction and elevated INR.." Journal of the American Pharmacists Association : JAPhA, 2021. https://doi.org/10.1016/j.japh.2020.07.028
RethinkTHC
RethinkTHC Research Database. "Δ-9-tetrahydrocannabinol dose increase leads to warfarin dru..." RTHC-03027. Retrieved from https://rethinkthc.com/research/brown-2021-9tetrahydrocannabinol-dose-increase-leads
Access the Original Study
Study data sourced from PubMed, a service of the U.S. National Library of Medicine, National Institutes of Health.
This study breakdown was produced by the RethinkTHC research team. We analyze and report published research findings without making health recommendations. All interpretations are based solely on the published abstract and study data.