Prescription ·Strong evidence ·Reviewed May 2026
Warfarin is an oral vitamin K antagonist anticoagulant used for prevention and treatment of thromboembolic disorders including atrial fibrillation, mechanical heart valves, deep vein thrombosis, and pulmonary embolism. It requires regular INR monitoring and has numerous food and drug interactions. Despite the advent of DOACs, it remains essential for mechanical valve anticoagulation.
The bottom line
Evidence rating strong. Most-documented uses: effective stroke prevention in atrial fibrillation, essential for mechanical heart valve anticoagulation, treatment and prevention of dvt and pe. 12 sources indexed (2017–2025), with 37 interaction records on file.
Core mechanism
Inhibits vitamin K epoxide reductase (VKORC1), blocking the recycling of vitamin K from its epoxide form. This prevents gamma-carboxylation of clotting factors II, VII, IX, and X, as well as anticoagulant proteins C and S. The delayed onset reflects the half-lives of existing circulating clotting factors (factor II has the longest at ~60 hours).
Can be taken with or without food; maintain consistent dietary vitamin K intake; numerous food and drug interactions
Chlorella is high in vitamin K, which antagonizes warfarin's anticoagulant effect. Case report: thrombotest values exceeded therapeutic limit.
Recommendation: Avoid while on warfarin, or maintain very consistent intake with INR monitoring.
21 spontaneous reports of increased INR with glucosamine use in WHO database.
Recommendation: Monitor INR more frequently when starting or stopping glucosamine in warfarin patients.
Berberine displaces warfarin from plasma protein binding sites and inhibits CYP2C9 (warfarin metabolism enzyme). May alter INR unpredictably.
Recommendation: If combining, increase INR monitoring frequency.
Vitamin K2 directly opposes warfarin's mechanism of action. Warfarin works by inhibiting vitamin K-dependent clotting factor synthesis, so supplemental vitamin K2 can restore clotting factor production and significantly reduce warfarin's anticoagulant effect. Even small, consistent doses of K2 can shift INR values unpredictably.
Recommendation: Avoid vitamin K2 supplementation while on warfarin unless directed and closely monitored by your prescriber. If K2 is deemed necessary, maintain a very consistent daily dose and monitor INR frequently.
High-dose vitamin E may potentiate warfarin's anticoagulant effect and increase bleeding risk. Vitamin E at high supplemental doses can interfere with vitamin K-dependent clotting and inhibit platelet aggregation.
Recommendation: Do not start, stop, or increase vitamin E while taking warfarin without discussing it with the anticoagulation clinic or prescriber. Report unusual bruising or bleeding promptly.
Fish oil may potentiate warfarin's anticoagulant effect through its own antiplatelet and antithrombotic properties. While moderate doses (1-2g/day) appear relatively safe, higher doses can increase bleeding time and potentially elevate INR. Clinical bleeding events have been reported in case studies.
Recommendation: If combining fish oil with warfarin, start with low doses (1g/day or less) and monitor INR closely during the first few weeks. Report any signs of unusual bleeding or bruising to your healthcare provider.
Garlic supplements possess antiplatelet properties that may increase bleeding risk when combined with warfarin. Several case reports have documented elevated INR and bleeding episodes in patients taking garlic supplements with warfarin. The effect is more pronounced with concentrated garlic extracts than with culinary amounts.
Recommendation: Avoid concentrated garlic supplements while on warfarin. Moderate culinary use of garlic is generally safe. If you use garlic supplements, inform your prescriber and monitor INR more frequently.
Ginkgo biloba has significant antiplatelet activity that increases bleeding risk when combined with warfarin. Multiple case reports document serious bleeding events including subdural hematoma, intracerebral hemorrhage, and hyphema in patients taking ginkgo with warfarin or other anticoagulants.
Recommendation: Avoid ginkgo biloba while taking warfarin. If already taking both, consult your prescriber about discontinuing ginkgo and monitor INR closely during any changes.
Curcumin may inhibit platelet aggregation and has shown anticoagulant properties in vitro and in animal studies. When combined with warfarin, there is a theoretical and clinically reported risk of enhanced anticoagulation and bleeding. Case reports have documented elevated INR in patients adding turmeric to warfarin therapy.
Recommendation: Use caution when combining turmeric/curcumin supplements with warfarin. Culinary turmeric in food is generally safe, but concentrated curcumin supplements should be discussed with your prescriber. Monitor INR if adding or removing curcumin.
CoQ10 is structurally similar to vitamin K2 (both are quinones) and may partially reduce warfarin's anticoagulant effect. Several case reports have documented decreased INR values in patients who began CoQ10 supplementation while on stable warfarin therapy. The effect is generally modest but clinically relevant.
Recommendation: If you need CoQ10 while on warfarin, start at a low dose and monitor INR closely for 2-3 weeks. Maintain a consistent daily dose to avoid INR fluctuations. Inform your prescriber about CoQ10 use.
St. John's Wort is a potent inducer of CYP3A4, CYP2C9, and P-glycoprotein, which dramatically accelerates warfarin metabolism and reduces its plasma levels. This can cause a clinically significant drop in INR, leading to loss of anticoagulant protection and potential thromboembolic events including stroke.
Recommendation: Do not take St. John's Wort with warfarin. This is a well-established dangerous interaction. If you are currently taking both, consult your prescriber immediately, do not abruptly stop St. John's Wort as INR may rebound sharply.
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.
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.
Numbered references. Citations throughout the page link here.
Fülöp P, Tóth Š, Porubän T et al.. Machine Learning for Warfarin Therapy: A Systematic Review. Pharmaceuticals (Basel, Switzerland). 2025
Mhanna M, Ayyad M, Mortada I et al.. Direct oral anticoagulants versus warfarin in adults with durable left ventricular assist devices: A systematic review and meta-analysis. Current problems in cardiology. 2024
In patients with atrial fibrillation and low stroke risk, DOACs showed comparable efficacy to warfarin with significantly lower bleeding risk, supporting a favorable benefit-risk profile
Patel PH, Ho T, Upadhyay SM. A Systematic Review of Warfarin Use in Post-Bariatric Surgery Patients: Cases Compiled From a Literature Review. The Annals of pharmacotherapy. 2023
Tarar MY, Malik RA, Charalambous CP. Bleeding complications in patients on warfarin undergoing joint injection/aspiration: systematic review and meta-analysis. Rheumatology international. 2023
Apixaban, dabigatran, and rivaroxaban all showed favorable efficacy and safety compared to warfarin for stroke prevention in atrial fibrillation, with apixaban having the best safety profile
Choi S, Oh DS, Jerng UM. A systematic review of the pharmacokinetic and pharmacodynamic interactions of herbal medicine with warfarin. PloS one. 2017
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