Enhanced Anticoagulant Effect of Warfarin When Co-administered With Quercetin.

Abstract

A 79-year-old man on stable warfarin therapy for atrial fibrillation presented with an international normalized ratio (INR) of 7.5. The patient started quercetin supplementation several days prior to a routine INR monitoring appointment, taking 1 capsule daily. The supplement was made by Natural Factors and came in a dosage strength of 250 mg quercetin and 500 mg of Vitamin C. The patient previously had a stable INR between 2 and 3 for months, with an INR of 2.5 five days beforehand. The patient had no recent changes to contributing factors. The patient was taking stable amiodarone therapy for 4 months prior to this episode with no other critical INR values. No adverse bleeding events were reported by the patient. The patient had been taking 1 warfarin 7.5 mg tablet once daily with a weekly dose of 52.5 mg. After the INR of 7.5, the patient stopped taking the quercetin supplement indefinitely and restarted warfarin therapy after missing one day. Five days after the initial INR, the INR was found to be 2.5. The patient’s most recent labs were red blood cell (RBC) count of 4.48 103/ mm3, hemoglobin (HGB) of 13.7 g/dL, aspartate aminotransferase (AST) of 26 IU/L, alanine aminotransferase (ALT) of 18 IU/L, and a calculated creatinine clearance (CrCl) of 91 mL/min.