OBJECTIVE: To report a case of warfarin-cranberry juice interaction, which resulted in an international normalized ratio (INR) elevation on 2 separate occasions.
CASE SUMMARY: A 46-year-old female was receiving a total weekly dose of 56 mg of warfarin. During the 4 months prior to the incident INR, her average INR was 2.0, with a range of 1.6-2.2, while taking the same weekly dose of warfarin. Her INR increased to 4.6 after drinking approximately 1.5 quarts (1420 mL) of cranberry juice cocktail daily for 2 days. Her INR 14 days later without cranberry juice cocktail consumption was 2.3. For the next 3 months, while taking warfarin 56 mg per week, her average INR was 2.1, with a range of 1.4-2.5. At a subsequent visit, after drinking approximately 2 quarts (1893 mL) of cranberry juice cocktail daily for 3-4 days, her INR had increased to 6.5. Her INR after holding warfarin for 3 days was 1.86. Her INR 7 days after resuming the weekly dose of warfarin 56 mg was 3.2. During both of the elevated INR episodes, no other factors were identified that would have resulted in an elevated INR, such as drug, herbal, disease, or other food interactions. An objective causality assessment revealed the interaction was highly probable.
DISCUSSION: Warfarin is the most commonly used anticoagulant for chronic therapy. There have been several case reports of cranberry juice or cranberry sauce potentiating the effects of warfarin by elevating the INR; however, clinical trials evaluating this interaction have failed to demonstrate a significant effect on an INR.
CONCLUSIONS: Our case report describes INR elevations in a patient previously stable on warfarin after ingestion of cranberry juice cocktail daily for several days. This elevation occurred on 2 separate occasions, which distinguishes our case from other published literature.