Causes of Supratherapeutic INR despite Full-Dose Warfarin Administration

 Hereditary

• Polymorphisms in P450 CYP2C9*2 and CYP2C9* → ↓ metabolism → ↑ Warfarin t_1/2

  Acquired

• Non-compliant patient 
• Drugs

Acetaminophen	Cephalosporins	Tramadol
Allopurinol	Fluoroquinolones	Capecitabine
Amiodarone	Macrolides (Clarithromycin, Erythromycin)	5-FU
Androgens	Penicillin except Nafcillin, Dicloxacillin	Imatinib
Methyldopa	Trimethoprim-Sulfamethoxazole	Cholesterol lowering drugs less Cholestyramine
Testosterone	Metronidazole	Cimetidine
Oxandrolone	Fluconazole	SSRIs
Methyltestosterone	Voriconazole	Sitaxentan
Tamoxifen	Miconazole (oral)	PPIs
Glucosamine	Clofibrate	Sulfinpyrazone