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DAVID Q. PHAM, PHARM.D., BCPS, is Assistant Professor of Pharmacy Practice, Arnold & Marie Schwartz College of Pharmacy and Health Sciences, Long Island University, Brooklyn, NY, and Internal Medicine Pharmacotherapy Specialist, Kings County Hospital Center, Brooklyn. ANTONY Q. PHAM, B.S., B.A., is a Pharm.D. degree candidate and Pharmacy Intern, School of Pharmacy, University of California San Francisco, San Francisco.

Address correspondence to Dr. David Q. Pham at Arnold & Marie Schwartz College of Pharmacy and Health Sciences, Long Island University, 75 DeKalb Avenue, Brooklyn, NY 11201-5497 (david.pham{at}liu.edu).

Purpose. The interaction potential between warfarin and cranberry juice is discussed. Summary. Reports from the United Kingdom have raised concern over the interaction potential between cranberry juice and warfarin. Warfarin is the most commonly prescribed oral medication for anticoagulation therapy. Cranberry juice is a flavonoid, which has been shown to induce, inhibit, or act as a substrate for the biosynthesis of several cytochrome P-450 (CYP) isoenzymes. Specifically, cranberry juice may inhibit the activity of CYP2C9, the primary isoenzyme involved in the metabolism of S-warfarin. A search of the medical literature identified three peer-reviewed case reports and two peer-reviewed, prospective, randomized, placebo-controlled clinical trials using metabolic surrogates of warfarin (flurbiprofen and cyclosporine) that described possible interactions between cranberry juice and warfarin. Two case reports suggested that cranberry juice increased the International Normalized Ratio (INR) of patients taking warfarin, but neither clearly identified cranberry juice as the sole cause of INR elevation. One case report appeared to show a correlation between the effects of cranberry juice and warfarin metabolism. Both clinical trials indicated the lack of an interaction between cranberry juice and CYP isoenzymes 2C9 and 3A, both of which are necessary in warfarin metabolism. More studies are required to determine the potential interaction between cranberry juice and warfarin. Conclusion. The available data do not seem to show a clinically relevant interaction between cranberry juice and warfarin; however, patients taking warfarin with cranberry juice should be cautioned about the potential interaction and monitored closely for INR changes and signs and symptoms of bleeding. Index terms: Anticoagulants; Drug interactions; International Normalized Ratio; Juices; Metabolism; Toxicity; Vaccinium macrocarpon; Warfarin