Warfarin - Cranberry Juice Interaction: Expert Finds Data Lacking

Henry I. Bussey, Pharm.D.
June, 2006

David J. Greenblatt, M.D. (Chair, Department of Pharmacology and Experimental Therapeutics, Tufts University School of Medicine) recently reviewed the available data which generated a lot of concern that ingestion of cranberry juice might significantly alter the INR in patients taking warfarin. The data are from case reports in the United Kingdom and led the UK Committee on Safety of Medicines to issue a warning that patients taking warfarin should avoid cranberry juice or cranberry products. Dr. Greenblatt's group examined each case report and found "not a single one of these cases plausibly demonstrated that loss of anticoagulant control was caused by cranberry juice." Further, his group performed a study with cranberry juice and flurbiprofen (which is metabolized by the same enzyme that plays a major role in warfarin metabolism, the hepatic CYP2C9 enzyme) and found no effect of cranberry juice on the metabolism of this medication. His group is now conducting a randomized, double-blind, placebo-controlled trial of the effect of cranberry juice on the INR in stable warfarin patients. More data should be forthcoming soon, but Dr. Greenblatt concludes that current evidence does not support a need for concern regarding a warfarin v. cranberry interaction.

Editor's note: Of course, it is possible that cranberry juice might interact with warfarin through some other mechanism; and/or one could postulate that the interaction might be evident only in persons with certain types of 2C9 polymorphisms (who may or may not be included in a randomized controlled trial). But looking for an alternative mechanism for such an interaction or examining the potential that an interaction may occur only in those patients with certain genetic polymorphisms hardly seems justified without more clinical data to suggest that an interaction actually exists. (HIB)

Reference

Greenblatt DJ. Cranberry Juice & warfarin: Is there an interaction? Anticoagulation Forum Newsletter 2006; 10(1):1, 3.