A recent review of case reports regarding an interaction between cranberry juice and warfarin failed to show any evidence supporting the interaction. A few reports in 2002 suggested that drinking cranberry juice might result in an increase in the international normalized ratio (INR) in patients who were previously stable on warfarin. These reports led the United Kingdom Committee on Safety of Medicines to issue a warning that patients taking warfarin should avoid cranberry juice and cranberry products. When investigators at Tufts University reexamined the data, however, they concluded that none of the cases demonstrated that loss of INR control was caused by cranberry juice. They also performed a study with cranberry juice and flurbiprofen, which is metabolized by the same hepatic isoenzyme (CYP2C9). Cranberry juice had no effect on the metabolism of flurbiprofen. A randomized, double-blind, placebo-controlled trial of the effects of cranberry juice on the INR in stable warfarin patients is currently under way.
Dr. Garrett is a clinical pharmacist practitioner at Cornerstone Health Care in High Point, NC.