Pharmacology  ·  Coagulation

Warfarin — Vitamin K Antagonist

Mechanism, monitoring, interactions, reversal, and indications at a glance

Abbreviations: INR = international normalized ratio  ·  VKORC1 = vitamin K epoxide reductase complex subunit 1  ·  CYP2C9 = cytochrome P450 2C9  ·  4F-PCC = four-factor prothrombin complex concentrate  ·  VTE = venous thromboembolism  ·  AF = atrial fibrillation  ·  APS = antiphospholipid syndrome  ·  TTR = time in therapeutic range  ·  DOAC = direct oral anticoagulant

Mechanism & Onset of Action

Target

Warfarin Inhibits VKORC1

  • VKORC1 recycles vitamin K epoxide → active reduced vitamin K
  • Without active vitamin K: gamma-carboxylation fails
  • Affected proteins: factors II, VII, IX, X; proteins C and S
  • S-warfarin is 3 to 5× more potent; metabolized by CYP2C9

Factor Half-Lives — Clinical Significance

Why Onset Is Delayed

  • Protein C: 6–8 h — falls first → transient procoagulable state; skin necrosis risk in protein C deficiency
  • Factor VII: 4–6 h — early INR rise; NOT true anticoagulation
  • Factor IX: ~24 h  ·  Factor X: ~40 h
  • Prothrombin (factor II): 60–70 h — must fall for true anticoagulation; requires 5 to 7 days

INR Targets & Monitoring

Indication Target INR Notes
VTE treatment/prevention; AF stroke prevention; aortic mechanical valve 2.0–3.0 Standard range for most indications
Mechanical mitral valve; mechanical valve + prior embolism on standard therapy 2.5–3.5 Higher target; add low-dose aspirin if bleeding risk acceptable
INR 4.0–10.0, no bleeding Hold doses Consider oral vitamin K1 1–2.5 mg; recheck in 1–2 days
Life-threatening bleeding Reverse immediately 4F-PCC (weight/INR-dosed) + intravenous vitamin K1 10 mg slow infusion

Key Drug Interactions

CYP2C9 Inhibitors — INR Rises

Reduce Warfarin Dose; Monitor Early

  • Fluconazole: reduce dose 25–50%; check INR in 3–5 days
  • Amiodarone: effect builds weeks to months; reduce 30–50%; weekly monitoring
  • Metronidazole, trimethoprim-sulfamethoxazole
  • NSAIDs: also increase GI bleeding and inhibit platelets — avoid if possible

CYP2C9 Inducers — INR Falls

Major Dose Increases May Be Needed

  • Rifampin: up to 5 to 10× dose increase; very frequent monitoring
  • Carbamazepine, phenytoin, phenobarbital
  • St. John's wort: over-the-counter herbal; strong inducer — ask at every visit
  • Consistent dietary vitamin K is essential; sudden changes alter INR

Warfarin vs. DOAC — When Warfarin Is Required

Warfarin cannot be substituted by a DOAC in: mechanical prosthetic heart valves (all types); AF with moderate-to-severe mitral stenosis; antiphospholipid syndrome with prior arterial thrombosis. DOACs are preferred for non-valvular AF and uncomplicated VTE in most patients. TTR goal is 70% or above; if consistently below 65%, reassess for causes of instability or switch to a DOAC where appropriate.

Suggested References

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