Green Tea and Warfarin Interaction
Green tea does not significantly affect warfarin anticoagulation at modest consumption levels, but excessive intake (approximately 1 gallon per day) may decrease INR and reduce warfarin's anticoagulant effect due to its vitamin K content. 1
Mechanism of Interaction
- Warfarin works by inhibiting vitamin K-dependent clotting factors (II, VII, IX, X) by interfering with the vitamin K cycle 1
- Green tea contains vitamin K (phylloquinones), which can counteract warfarin's anticoagulant effect when consumed in large amounts 1
- Phylloquinones in green tea can be reduced to vitamin KH2 through a warfarin-insensitive pathway, thereby bypassing warfarin's inhibition of vitamin K epoxide reductase 1
Clinical Significance of the Interaction
- At normal consumption levels, green tea does not significantly affect INR or warfarin efficacy 1
- Case report evidence shows that excessive green tea consumption (up to 1 gallon per day) can significantly decrease INR, potentially leading to reduced anticoagulation and increased thrombosis risk 2
- This interaction is dose-dependent - the more green tea consumed, the greater the potential for antagonizing warfarin's effects 2
Management Recommendations
- Patients on warfarin should maintain consistent vitamin K intake, including from green tea, to maintain stable anticoagulation 1
- Modest green tea consumption is generally safe for patients on warfarin therapy 1
- Advise patients to:
- Limit green tea consumption to moderate amounts (less than 24 ounces daily) 1
- Avoid sudden large increases in green tea consumption 2
- Report any significant changes in dietary habits, including starting or stopping regular green tea consumption 3
- Monitor for signs of reduced anticoagulation (such as swelling, pain, or warmth in extremities) if consuming large amounts of green tea 2
Common Pitfalls and Caveats
- Patients may not consider green tea as a "medication" and fail to report its consumption during medication reconciliation 3
- The vitamin K content in green tea may vary depending on preparation methods and brands 4
- Patients with subclinical vitamin K deficiency (estimated at 12% of anticoagulated patients) may be particularly sensitive to even small amounts of vitamin K from sources like green tea 3
- Fluctuations in vitamin K intake from all dietary sources, not just green tea, can affect warfarin stability 5
Other Important Warfarin Interactions to Consider
- Many medications can potentiate warfarin's effect, including antibiotics (trimethoprim-sulfamethoxazole, ciprofloxacin, metronidazole), antifungals (fluconazole), and acetaminophen (>2g/day) 1
- Some medications can reduce warfarin's effect, including rifampin and carbamazepine 1
- Other herbal supplements that may interact with warfarin include St. John's wort (decreases INR by 20%) 1
- Excessive alcohol consumption can increase INR, though modest consumption (approximately 2 ounces/day) appears safe 1