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KARI A. MERGENHAGEN, PHARMD., is Clinical Pharmacy Specialist,. Infectious Disease, James J. Peters Veterans Affairs Medical Center (VAMC), Bronx, NY; at the time of writing she was Geriatric Pharmacy Practice Resident, James J. Peters VAMC. OLGA SHERMAN, PHARM.D., is Clinical Pharmacy Specialist, Coagulation/Warfarin/Lipid Clinic, James J. Peters VAMC.

Address correspondence to Dr. Mergenhagen at the James J. Peters Veterans Affairs Medical Center, 130 West Kingsbridge Road (119), Bronx, NY 10468 (kari.mergenhagen2{at}va.gov).

Purpose. A case of increased International Normalized Ratio (INR) values in a patient receiving warfarin who consumed a large amount of cranberry sauce is reported. Summary. A 75-year-old Caucasian man with atrial fibrillation was taking 22.5 mg of warfarin weekly for 10 months and had maintained stable INR values between 2 and 3. During a clinic visit one week after Thanksgiving, the patient’s INR was 4.8. The patient was extensively questioned about any changes in his diet, medications, and health. The patient reported consuming approximately 113 g of cranberry sauce for seven consecutive days before the clinic visit. He reported no other diet or health changes. The patient denied alcohol consumption, had not taken nutritional supplements, and did not miss or double any medication doses during this time period. There were no dosing changes or discernible compliance issues in the 10 months preceding the change in INR. The patient’s complete blood cell count and results of a chemistry panel and liver function tests were within normal limits. He did not have any bleeding or bruising. The only identifiable change was the addition of store-bought cranberry sauce to his diet. The patient’s warfarin dose was held for two days. Subsequently, the patient resumed warfarin at a dosage of 20 mg per week. Seven days after the patient discontinued the cranberry sauce, his INR returned to 2.2 and remained stable over the next month. Conclusion. Consumption of cranberry sauce led to an increase in INR values in a patient receiving warfarin. Index terms: Anticoagulants; Drug interactions; Geriatrics; International normalized ratio; Vaccinium macrocarpon; Warfarin sodium