Warfarin

Vitamin K Antagonist·Haematology - Anticoagulants & Haemostatics

Mechanical heart valves, AF with valvular disease, VTE/PE

PBSANTICOAGULANTCoumadin, Marevan (Do NOT interchange brands without intense INR monitoring).1 mg tabletS4+1

Target INR

2.0 - 3.0

Half-life

40 h

Bleeding Risk

CRITICAL

Interactions

VERY HIGH

Rapid Summary

Overview

The original oral anticoagulant. A Vitamin K antagonist with an extremely narrow therapeutic window, volatile kinetics, and hundreds of drug interactions.

Maximum Dose

Titrated entirely based on INR results.

Clinical Focus

The only oral anticoagulant approved for mechanical heart valves or moderate-to-severe mitral stenosis.

Bottom Line

Warfarin is the only oral anticoagulant for mechanical heart valves and valvular AF, but requires meticulous INR monitoring and has extensive drug and dietary interactions.

Formulation & Access

Brand Names

Coumadin, Marevan (Do NOT interchange brands without intense INR monitoring).

Oral Routes

Tablets (1 mg, 2 mg, 3 mg, 5 mg).

Prescribing & PBS

Unrestricted General Benefit (S4).

Indications & Use

Primary

PBSTGA

Mechanical heart valves, AF with valvular disease, VTE/PE.

Clinical Place

Superseded by DOACs for standard AF/VTE, but remains absolute mandatory therapy for metallic heart valves and antiphospholipid syndrome.

Quick reference

Class & Role

Vitamin K Antagonist — The original oral anticoagulant.

Route / Formulation

Tablets (1 mg, 2 mg, 3 mg, 5 mg). (Coumadin, Marevan (Do NOT interchange brands without intense INR monitoring).)

Usual Dose & Max

Start PO 5 mg OD (at 16:00). Check INR daily. Adjust dose using a validated nomogram. Titrated entirely based on INR results.

Key Indication Doses

Initiation: Start PO 5 mg OD (at 16:00). Check INR daily. Adjust dose using a validated nomogram. Maintenance: PO 1-10 mg OD. Guided entirely by INR (Target 2.0-3.0 for most, 2.5-3.5 for mechanical valves). Elderly / Frail: Start at 2.5 mg or lower. Extremely sensitive.

Best Uses

Warfarin is the only oral anticoagulant for mechanical heart valves and valvular AF, but requires meticulous INR monitoring and has extensive drug and dietary interactions.

Avoid / Cautions

High fall risk, active bleeding, severe non-compliance. Pregnancy Category D. Teratogenic (fetal warfarin syndrome). Must switch to LMWH.

Key Risks

Haematological: Fatal haemorrhage (intracranial, GI). Dermatological: Warfarin-induced skin necrosis (rare, occurs in the first few days due to rapid depletion of Protein C, causing paradoxical micro-clotting). Other: Purple toe syndrome.

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. Assess for bruising, bleeding gums, melaena, haematuria.

Clinical Pearl

If a patient on Warfarin has a life-threatening bleed, Vitamin K IV alone is too slow. You MUST give Prothrombinex-VF (Factor II, IX, X) immediately alongside Vitamin K to restore clotting factors.

Access

Coumadin, Marevan (Do NOT interchange brands without intense INR monitoring).1 mg tablet

Brand: Coumadin, Marevan (Do NOT interchange brands without intense INR monitoring).
Formulation: Tablets (1 mg, 2 mg, 3 mg, 5 mg)

Warfarin

Vitamin K Antagonist·Haematology - Anticoagulants & Haemostatics

Mechanical heart valves, AF with valvular disease, VTE/PE

PBSANTICOAGULANTCoumadin, Marevan (Do NOT interchange brands without intense INR monitoring).1 mg tabletS4+1

Target INR

2.0 - 3.0

Half-life

40 h

Bleeding Risk

CRITICAL

Interactions

VERY HIGH

Rapid Summary

Overview

The original oral anticoagulant. A Vitamin K antagonist with an extremely narrow therapeutic window, volatile kinetics, and hundreds of drug interactions.

Maximum Dose

Titrated entirely based on INR results.

Clinical Focus

The only oral anticoagulant approved for mechanical heart valves or moderate-to-severe mitral stenosis.

Bottom Line

Warfarin is the only oral anticoagulant for mechanical heart valves and valvular AF, but requires meticulous INR monitoring and has extensive drug and dietary interactions.

Formulation & Access

Brand Names

Coumadin, Marevan (Do NOT interchange brands without intense INR monitoring).

Oral Routes

Tablets (1 mg, 2 mg, 3 mg, 5 mg).

Prescribing & PBS

Unrestricted General Benefit (S4).

Indications & Use

Primary

PBSTGA

Mechanical heart valves, AF with valvular disease, VTE/PE.

Clinical Place

Superseded by DOACs for standard AF/VTE, but remains absolute mandatory therapy for metallic heart valves and antiphospholipid syndrome.

Quick reference

Class & Role

Vitamin K Antagonist — The original oral anticoagulant.

Route / Formulation

Tablets (1 mg, 2 mg, 3 mg, 5 mg). (Coumadin, Marevan (Do NOT interchange brands without intense INR monitoring).)

Usual Dose & Max

Start PO 5 mg OD (at 16:00). Check INR daily. Adjust dose using a validated nomogram. Titrated entirely based on INR results.

Key Indication Doses

Best Uses

Warfarin is the only oral anticoagulant for mechanical heart valves and valvular AF, but requires meticulous INR monitoring and has extensive drug and dietary interactions.

Avoid / Cautions

High fall risk, active bleeding, severe non-compliance. Pregnancy Category D. Teratogenic (fetal warfarin syndrome). Must switch to LMWH.

Key Risks

Key Interactions

Monitoring

Strict, lifelong INR monitoring. Assess for bruising, bleeding gums, melaena, haematuria.

Clinical Pearl

If a patient on Warfarin has a life-threatening bleed, Vitamin K IV alone is too slow. You MUST give Prothrombinex-VF (Factor II, IX, X) immediately alongside Vitamin K to restore clotting factors.

Access

Coumadin, Marevan (Do NOT interchange brands without intense INR monitoring).1 mg tablet

Brand: Coumadin, Marevan (Do NOT interchange brands without intense INR monitoring).
Formulation: Tablets (1 mg, 2 mg, 3 mg, 5 mg)