NutriStack
Green Tea Extract and Warfarin, a conflict.
Green tea contains vitamin K1 which can antagonize warfarin's anticoagulant effect. Concentrated green tea extract supplements provide substantially more vitamin K than brewed tea. High-dose consumption has been associated with decreased INR values in warfarin-treated patients.
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At a glance
- Substances
- Green Tea Extract and Warfarin
- Pair type
- Conflict
- Evidence (highest tier)
- Moderate
- Source citations
- 4 sources
- Stack Score effect
- −10 to your Stack Score (per scored conflict row).
- Scope
- Supplement × Prescription
- Last verified
- May 30, 2026
Conflict · Moderate evidence
Conflict
What is happening. Green tea contains vitamin K1 which can antagonize warfarin's anticoagulant effect. Concentrated green tea extract supplements provide substantially more vitamin K than brewed tea. High-dose consumption has been associated with decreased INR values in warfarin-treated patients.
Mechanism. Green tea leaves contain approximately 1428 mcg vitamin K per 100g dry weight. Concentrated extracts can deliver significant vitamin K doses that promote gamma-carboxylation of clotting factors, directly opposing warfarin's mechanism.
Recommendation. If taking warfarin, avoid high-dose green tea extract supplements. Moderate consumption of brewed green tea (1-2 cups/day) is generally acceptable if kept consistent. Monitor INR when changing tea consumption habits.
Sources (4)
- Taylor JR, Wilt VM. Probable antagonism of warfarin by green tea. Ann Pharmacother. 1999;33(4):426-428. PMID 10332534
- Booth SL et al. Relationships between dietary intakes and fasting plasma concentrations of fat-soluble vitamins in humans. J Nutr. 1997;127(4):587-592. PMID 9109608
- Wang M, Zeraatkar D, Obeda M, Lee M et al.. Drug-drug interactions with warfarin: A systematic review and meta-analysis. British Journal of Clinical Pharmacology. 2021. PMID 33769581
- He B, Kang S, Su R et al.. Chemoprophylaxis Effect of EGCG on the Recurrence of Colorectal Cancer: A Systematic Review and Meta-Analysis. Current Pharmaceutical Design. 2024. PMID 38988171
Stack Score
How this pair moves the number.
Effect on the composite score
If both Green Tea Extract and Warfarin are in the same stack, this pair applies −10 to your Stack Score (per scored conflict row).
The full algorithm, the clamping rules, and four worked stacks are documented at /methodology/stack-score.
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