NutriStack
Cannabis (THC-Dominant) and Warfarin, a caution.
THC-dominant cannabis can raise INR and bleeding risk in people taking warfarin. Case reports describe supratherapeutic INR after recreational or medical cannabis exposure, and THC can inhibit CYP2C9, the main pathway for the more potent S-warfarin enantiomer. The risk is highest when cannabis dose, route, or frequency changes suddenly.
One pair, every claim cited. The two substances, the type, the mechanism, the recommendation, and the primary literature.
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At a glance
- Substances
- Cannabis (THC-Dominant) and Warfarin
- Pair type
- Caution
- Evidence (highest tier)
- Moderate
- Source citations
- 3 sources
- Stack Score effect
- −5 to your Stack Score (per scored caution row).
- Scope
- Supplement × Prescription
- Last verified
- May 30, 2026
Caution · Moderate evidence
Caution
What is happening. THC-dominant cannabis can raise INR and bleeding risk in people taking warfarin. Case reports describe supratherapeutic INR after recreational or medical cannabis exposure, and THC can inhibit CYP2C9, the main pathway for the more potent S-warfarin enantiomer. The risk is highest when cannabis dose, route, or frequency changes suddenly.
Mechanism. Delta-9-THC can inhibit CYP2C9-mediated warfarin metabolism, increasing S-warfarin exposure and INR. Cannabinoid products may also vary widely in THC/CBD content, making the pharmacokinetic effect hard to predict.
Recommendation. Do not start, stop, or sharply change THC-dominant cannabis use without telling your anticoagulation clinic. Ask for an INR check within 3-7 days after any change and again after the pattern stabilizes. Seek urgent care for black stools, vomiting blood, severe headache, weakness, or bleeding that does not stop.
Sources (3)
- Damkier P, Lassen D, Christensen MMH, Madsen KG, Hellfritzsch M, Pottegård A. Interaction between warfarin and cannabis. Basic Clin Pharmacol Toxicol. 2019;124(1):28-31. PMID 30326170
- Greger J, Bates V, Mechtler L, Gengo F. A Review of Cannabis and Interactions With Anticoagulant and Antiplatelet Agents. J Clin Pharmacol. 2020;60(4):432-438. PMID 31724188
- Smythe MA, Wu W, Garwood CL. Anticoagulant drug-drug interactions with cannabinoids: A systematic review. Pharmacotherapy. 2023;43(12):1327-1338. PMID 37740600
Stack Score
How this pair moves the number.
Effect on the composite score
If both Cannabis (THC-Dominant) and Warfarin are in the same stack, this pair applies −5 to your Stack Score (per scored caution row).
The full algorithm, the clamping rules, and four worked stacks are documented at /methodology/stack-score.
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