Warfarin

Vitamin K Antagonist·Haematology - Anticoagulants & Haemostatics

Mechanical heart valves, moderate/severe mitral stenosis, VTE/PE, AF stroke prevention (if DOACs unsuitable)

PBSVKACoumadin, Marevan5 mg tabletS4+1

Target Range

INR 2.0 - 3.0

Half-life

40 h

Bleeding Risk

CRITICAL

Interactions

EXTREME

Rapid Summary

Overview

The classic Vitamin K Antagonist (VKA). Highly effective oral anticoagulant but universally dreaded due to an excruciatingly narrow therapeutic index, massive dietary restrictions, and lethal drug interactions.

Maximum Dose

Titrated strictly via INR blood testing.

Clinical Focus

The ONLY approved oral anticoagulant for patients with Mechanical Heart Valves or severe Mitral Stenosis. DOACs will fail and cause fatal valve thrombosis.

Bottom Line

Warfarin is the only oral anticoagulant for mechanical heart valves, but demands rigorous INR monitoring and has extensive drug/food interactions.

Formulation & Access

Brand Names

Coumadin, Marevan

Oral Routes

Tablets (1, 2, 3, 5 mg). Coumadin and Marevan are NOT bioequivalent and must NEVER be swapped by the pharmacist without strict INR monitoring.

Prescribing & PBS

Unrestricted General Benefit (S4).

Indications & Use

Primary

PBSTGA

Mechanical heart valves, moderate/severe mitral stenosis, VTE/PE, AF stroke prevention (if DOACs unsuitable).

Clinical Place

Superseded by DOACs (Apixaban/Rivaroxaban) for standard AF and DVT due to convenience, but retains absolute monopoly over mechanical valves.

Quick reference

Class & Role

Vitamin K Antagonist — The classic Vitamin K Antagonist (VKA).

Route / Formulation

Tablets (1, 2, 3, 5 mg). Coumadin and Marevan are NOT bioequivalent and must NEVER be swapped by the pharmacist without strict INR monitoring. (Coumadin, Marevan)

Usual Dose & Max

Start PO 3-5 mg OD (strictly taken at 4 PM to align with morning blood tests). Maintenance dose titrated to target INR (usually 2.0 - 3.0 for AF/DVT, or 2.5 - 3.5 for Mechanical Valves). Titrated strictly via INR blood testing.

Key Indication Doses

Anticoagulation: Start PO 3-5 mg OD (strictly taken at 4 PM to align with morning blood tests). Maintenance dose titrated to target INR (usually 2.0 - 3.0 for AF/DVT, or 2.5 - 3.5 for Mechanical Valves). Reversal: Minor bleeding/High INR: Oral/IV Vitamin K. Major life-threatening bleeding: IV Prothrombinex-VF + IV Vitamin K.

Best Uses

Warfarin is the only oral anticoagulant for mechanical heart valves, but demands rigorous INR monitoring and has extensive drug/food interactions.

Avoid / Cautions

Active bleeding, severe uncompliant patient, lack of access to regular INR blood testing. Pregnancy Category D. Highly teratogenic (Fetal Warfarin Syndrome - bone/cartilage defects, fatal fetal bleeding). Must switch to Heparin.

Key Risks

Haematological: Major haemorrhage, fatal intracranial bleeding (especially if INR > 4.0). Tissue: Warfarin-induced skin necrosis (rare, paradoxical massive blood clotting in the skin microvasculature on days 3-8 of therapy, due to rapid depletion of Protein C before Factor II). Metabolic: Calciphylaxis (with chronic long-term use in CKD).

Key Interactions

CYP2C9 inhibitors (Amiodarone, Metronidazole, Fluconazole, Bactrim) massively spike INR and cause bleeding. You MUST halve the warfarin dose if starting these. CYP2C9 inducers (Carbamazepine, Rifampicin, St John's Wort). These destroy Warfarin, dropping the INR to 1.0 and causing massive strokes. NSAIDs, Aspirin, SSRIs (increase bleeding risk without changing INR). Leafy greens (broccoli, spinach) contain high Vitamin K and reverse the drug's effect. Diet must be consistent.

Monitoring

Strict, lifelong INR monitoring (International Normalized Ratio). Usually every 1-4 weeks once stable. Counsel patients on falls risk. A minor head bump in an elderly patient on Warfarin can cause a slow, fatal subdural hematoma.

Clinical Pearl

Because Warfarin depletes Protein C (an anti-clotting factor) faster than it depletes the actual clotting factors, it makes the patient hyper-coagulable for the first 3 days. You MUST 'bridge' them with an injectable blood thinner (like Enoxaparin or Heparin) for the first 5 days until the INR hits 2.0, or they will clot.

Access

Coumadin, Marevan5 mg tablet

Brand: Coumadin, Marevan
Formulation: Tablets (1, 2, 3, 5 mg)

Warfarin

Vitamin K Antagonist·Haematology - Anticoagulants & Haemostatics

Mechanical heart valves, moderate/severe mitral stenosis, VTE/PE, AF stroke prevention (if DOACs unsuitable)

PBSVKACoumadin, Marevan5 mg tabletS4+1

Target Range

INR 2.0 - 3.0

Half-life

40 h

Bleeding Risk

CRITICAL

Interactions

EXTREME

Rapid Summary

Overview

Maximum Dose

Titrated strictly via INR blood testing.

Clinical Focus

The ONLY approved oral anticoagulant for patients with Mechanical Heart Valves or severe Mitral Stenosis. DOACs will fail and cause fatal valve thrombosis.

Bottom Line

Warfarin is the only oral anticoagulant for mechanical heart valves, but demands rigorous INR monitoring and has extensive drug/food interactions.

Formulation & Access

Brand Names

Coumadin, Marevan

Oral Routes

Tablets (1, 2, 3, 5 mg). Coumadin and Marevan are NOT bioequivalent and must NEVER be swapped by the pharmacist without strict INR monitoring.

Prescribing & PBS

Unrestricted General Benefit (S4).

Indications & Use

Primary

PBSTGA

Mechanical heart valves, moderate/severe mitral stenosis, VTE/PE, AF stroke prevention (if DOACs unsuitable).

Clinical Place

Superseded by DOACs (Apixaban/Rivaroxaban) for standard AF and DVT due to convenience, but retains absolute monopoly over mechanical valves.

Quick reference

Class & Role

Vitamin K Antagonist — The classic Vitamin K Antagonist (VKA).

Route / Formulation

Tablets (1, 2, 3, 5 mg). Coumadin and Marevan are NOT bioequivalent and must NEVER be swapped by the pharmacist without strict INR monitoring. (Coumadin, Marevan)

Usual Dose & Max

Key Indication Doses

Best Uses

Warfarin is the only oral anticoagulant for mechanical heart valves, but demands rigorous INR monitoring and has extensive drug/food interactions.

Avoid / Cautions

Key Risks

Key Interactions

Monitoring

Clinical Pearl

Access

Coumadin, Marevan5 mg tablet

Brand: Coumadin, Marevan
Formulation: Tablets (1, 2, 3, 5 mg)