Overview
Warfarin is an oral anticoagulant that inhibits vitamin K-dependent clotting factors, used to prevent and treat thromboembolic disorders. It requires careful monitoring via INR (International Normalized Ratio) to balance therapeutic efficacy with bleeding risk. The drug has a narrow therapeutic index and significant drug-drug/food interactions.
Mechanism of Action
Warfarin inhibits vitamin K epoxide reductase, blocking the regeneration of reduced vitamin K needed for γ-carboxylation of clotting factors II, VII, IX, and X, as well as anticoagulant proteins C and S. This results in decreased synthesis of functional clotting factors, prolonging clotting time.
Indications
- Prophylaxis and treatment of venous thrombosis and its extension
- Prophylaxis and treatment of pulmonary embolism (PE)
- Prophylaxis and treatment of thromboembolic complications associated with atrial fibrillation and/or cardiac valve replacement
- Reduction in risk of death, recurrent myocardial infarction, and thromboembolic events after myocardial infarction
Common Doses
- 1 mg
- 2 mg
- 2.5 mg
- 3 mg
- 4 mg
- 5 mg
- 6 mg
- 7.5 mg
- 10 mg
Dosage
Dosing is highly individualized based on INR monitoring. Typical initiation dose is 2-5 mg daily, with maintenance doses adjusted to maintain target INR (usually 2.0-3.0 for most indications, 2.5-3.5 for mechanical heart valves). Loading doses are generally avoided due to increased bleeding risk.
Black Box Warning
Black Box Warning: Warfarin can cause major or fatal bleeding. Risk factors include high INR, age ≥65, history of gastrointestinal bleeding, hypertension, cerebrovascular disease, serious heart disease, anemia, malignancy, trauma, renal impairment, certain concomitant drugs, and long duration of therapy.
Contraindications
- Pregnancy (except in women with mechanical heart valves at high thromboembolic risk)
- Hemorrhagic tendencies or blood dyscrasias
- Recent or contemplated CNS or eye surgery
- Active gastrointestinal, genitourinary, or respiratory tract bleeding
- CNS hemorrhage
- Cerebral aneurysms, dissecting aorta
- Pericarditis and pericardial effusions
- Bacterial endocarditis
- Threatened abortion, eclampsia, preeclampsia
- Spinal puncture or procedures with uncontrollable bleeding risk
- Hypersensitivity to warfarin
- Major regional or lumbar block anesthesia
- Malignant hypertension
- Unsupervised patients with potential for non-compliance
Side Effects
- Hemorrhage (major concern)
- Tissue necrosis
- Calciphylaxis
- Acute kidney injury
- Systemic atheroemboli and cholesterol microemboli
- Hypersensitivity/allergic reactions (urticaria, anaphylaxis)
- Hepatitis, elevated liver enzymes
- Nausea, vomiting, diarrhea, abdominal pain
- Rash, dermatitis, pruritus, alopecia
- Tracheal or tracheobronchial calcification
- Chills
Interactions
- Numerous significant interactions: Antibiotics (especially broad-spectrum), antifungals, antiplatelets (aspirin, clopidogrel), NSAIDs, SSRIs, amiodarone, statins, thyroid hormones, and many others.
- Food interactions: Vitamin K-rich foods (leafy greens) can reduce effect; cranberry juice, alcohol, and certain herbal supplements (St. John's wort, ginseng) can increase bleeding risk.
- Requires careful review of all concomitant medications and diet.
Counseling Points
- Take exactly as prescribed at same time each day.
- Regular INR monitoring is essential - do not miss appointments.
- Report signs of bleeding (unusual bruising, blood in urine/stool, headache, dizziness) immediately.
- Maintain consistent vitamin K intake - avoid drastic changes in diet.
- Inform all healthcare providers about warfarin use before any procedures.
- Avoid alcohol consumption.
- Use soft-bristle toothbrush and electric razor to minimize bleeding risk.
- Carry medical identification indicating anticoagulant use.