Warfarin is an anticoagulant (commonly referred to as a blood thinner) which aims to prevent thrombus formation. Warfarin belongs to the class of vitamin K antagonists, which also includes acenocoumarol and phenindione. Common indications for oral anticoagulation include: Treatment of venous thromboembolism (VTE): deep vein thrombosis (DVT) and pulmonary embolism (PE) Stroke prevention in atrial fibrillation (AF) Prevention of thrombosis in patients with mechanical heart valves (warfarin preferred) This article explains how warfarin works, why monitoring with INR is needed, and how drug and food interactions affect its use. It also covers when bridging therapy is required and how warfarin can be reversed safely. Mechanism of Action Warfarin prevents thrombus formation where the thrombus consists of a fibrin web enmeshed with platelets and red blood cells. Anticoagulants target thrombi in the veins. In contrast, thrombi in faster flowing arteries are mainly composed of platelets, making antiplatelet medication would be more effective. Warfarin inhibits the enzyme vitamin K epoxide reductase, reducing the regeneration of vitamin K1. Vitamin K1 is responsible for synthesising vitamin K dependent clotting factors II, VII, IX, X, therefore warfarin decreases clotting ability. Vitamin K1 also synthesises anticoagulant proteins C and S. After taking a vitamin K antagonist, it takes 48-72 hours for it to take effect; this is due to the half lives of the vitamin K dependent clotting factors: Factor II: ~60 hours Factor VII: 4–6 hours Factor IX: 24 hours Factor X: 48–72 hours At the start of treatment with warfarin there can be a prothrombotic effect (especially in patients with a protein C and S deficiency) because proteins C and S have shorter half-lives (8 hours and 30 hours respectively). The prothrombotic effect lasts until the vitamin K dependent clotting factors have circulated out of the body. For this reason, patients who are at high risk thrombosis will receive bridging therapy often with low molecular weight heparin (LMWH) until the international normalised ratio (INR) is within a therapeutic range. Fig 1: the clotting cascade and actions of anticoagulants Monitoring Warfarin has a narrow therapeutic window/index, requiring monitoring with the international normalised ratio (INR). The dose of warfarin is usually adjusted based on the INR. An INR measures how long it takes for your blood to clot A normal INR = 1 Common targets: INR 2.5 (range 2–3) for AF, DVT, or PE Higher or variable targets for mechanical prosthetic heart valves INR below range → risk of thrombosis INR above range → risk of bleeding Interactions Warfarin is a mixture of enantiomers: R-warfarin is metabolised by CYP1A2 and CYP3A4 S-warfarin is metabolised by CYP2C9 Metabolism by CYP enzymes gives rise to several drug interactions. Drugs that ↓ warfarin levels Drugs that ↑ warfarin levels CYP3A4 and CYP2C9 inducers Carbamazepine Oral contraceptives Rifampicin Phenytoin Azathioprine St Johns Wort Reduce absorption of warfarin Cholestyramine CYP3A4 and CYP2C9 inhibitors Ketoconazole Fluconazole Clarithromycin Omeprazole Unknown mechanism Cranberry juice Glucosamine Reversal Warfarin’s effect can be reversed with vitamin K (oral/IM depending on urgency). Patients should be counselled on consuming a similar amount of vitamin K daily through diet e.g. liver, broccoli, brussels sprouts and green leafy vegetables, to avoid INR fluctuations. Summary Table Drug/Class Mechanism of Action Monitoring Key Interactions Special Populations Reversal Agent Warfarin (Vitamin K antagonist) Inhibits vitamin K epoxide reductase → ↓ synthesis of factors II, VII, IX, X, proteins C & S Requires INR monitoring (narrow therapeutic index) Many CYP interactions: ↓ levels: carbamazepine, rifampicin, phenytoin, St John’s wort, cholestyramine ↑ levels: ketoconazole, fluconazole, clarithromycin, cranberry juice Prothrombotic at initiation (bridging with LMWH often needed) Vitamin K Do you think you’re ready? Take the quiz below Pro Feature - Quiz Warfarin Question 1 of 1 Submitting... Skip Next Rate question: You scored 0% Skipped: 0/1 More Questions Available Upgrade to TeachMePharmacy Pro Challenge yourself with over 200 multiple-choice questions to reinforce learning. Learn More Rate This Article