Green Tea and Warfarin Interaction

Mechanism of Interaction

Warfarin works by inhibiting vitamin K-dependent clotting factors (II, VII, IX, X) by interfering with the vitamin K cycle, according to the American College of Cardiology 3
Green tea contains vitamin K (phylloquinones), which can counteract warfarin's anticoagulant effect when consumed in large amounts, as noted by the American College of Cardiology 4, 5
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, as reported by the American College of Cardiology 4

At normal consumption levels, green tea does not significantly affect INR or warfarin efficacy, as found in studies published in Circulation: Arrhythmia and Electrophysiology 1, 2

Patients on warfarin should maintain consistent vitamin K intake, including from green tea, to maintain stable anticoagulation, as recommended by the American College of Cardiology 4, 5
Modest green tea consumption is generally safe for patients on warfarin therapy, according to Circulation: Arrhythmia and Electrophysiology 1, 2
The American College of Cardiology advises patients to limit green tea consumption to moderate amounts (less than 24 ounces daily) 1, 2

Many medications can potentiate warfarin's effect, including antibiotics (trimethoprim-sulfamethoxazole, ciprofloxacin, metronidazole), antifungals (fluconazole), and acetaminophen (>2g/day), as noted by the National Comprehensive Cancer Network 6, 2
Some medications can reduce warfarin's effect, including rifampin and carbamazepine, according to the American College of Cardiology 7, 6
Other herbal supplements that may interact with warfarin include St. John's wort (decreases INR by 20%), as reported in Circulation: Arrhythmia and Electrophysiology 1, 2
Excessive alcohol consumption can increase INR, though modest consumption (approximately 2 ounces/day) appears safe, as found in Circulation: Arrhythmia and Electrophysiology 2