Dosage Forms. Tablet: 1 mg, 2 mg, 2.5 mg, 3 mg, 4 mg, 5 mg, 6 mg, 7.5 mg, 10 mg
Common FDA Label Indication, Dosing, and Titration.
1. Multiple FDA-labeled indications, all dosed similarly, including atrial fibrillation; myocardial infarction; prosthetic cardiac valve component embolism; pulmonary embolism; thrombosis, postmyocardial infarction; venous thromboembolism: Initial, 2-5 mg po daily, adjust dose based on INR; usual maintenance 2-10 mg po daily
1. Antiphospholipid syndrome; cancer—venous thromboembolism: Initial, 2-5 mg po daily, adjust dose based on INR; usual maintenance 2-10 mg po daily
MOA. Warfarin prevents the conversion of vitamin K back to its active form from vitamin K epoxide. This impairs formation of the vitamin K–dependent clotting factors II, VII, IX, and X (prothrombin) and proteins C and S (physiologic anticoagulants).
Drug Characteristics: Warfarin
Medication Safety Issues: Warfarin
Drug Interactions: Warfarin
Adverse Reactions: Warfarin
Efficacy Monitoring Parameters. Measure initial INR after the first 2-3 doses and subsequently at intervals no longer than every 4 wk, once stable dose has been achieved; patients at high risk of bleeding require more frequent monitoring. INR target and therapeutic range depend on indication. Atrial fibrillation/atrial flutter: target 2.5 (range 2-3); prosthetic heart valves: target 2.5 (range 2-3); mechanical mitral or aortic valve: target 3 (range 2.5-3.5); myocardial infarction, ST segment elevation: target 3 (2.5-3.5, with aspirin); venous thromboembolism, prophylaxis, and treatment (including pulmonary embolism, DVT, hip/knee arthroplasty): target 2.5 (range 2-3).
Toxicity Monitoring Parameters. Signs/symptoms of bleeding, CBC, LFT, stool guaiac test.
Key Patient Counseling Points. Report signs/symptoms of hemorrhage, skin and tissue necrosis, and hepatitis. Avoid situations/activities in which cuts, bruising, or injury is likely to occur. Many significant drug-drug interactions, consult healthcare professional prior to new prescription or OTC use. Avoid alcohol, cranberry products, and drastic changes in vitamin K consumption from diet (cruciferous vegetables).
Clinical Pearls. Patients often managed in pharmacist run anticoagulation clinics. Consult local protocols. Excessive anticoagulation with warfarin can be corrected with vitamin K.