Cranberry juice has been suggested to have an interaction with the blood thinner (anticoagulant) warfarin (Coumadin). However, there are few reported cases of the warfarin-cranberry juice interaction.

Here’s one.

A 78-year-old man was taking warfarin 45 mg each week to treat atrial fibrillation.

• The international normalized ratio (INR) was 6.45 after drinking a half-gallon of cranberry/apple juice in the prior week.
• INR is the test used to measure the time it takes for blood to clot compared to an average.
• There was no bleeding detected.
• After stopping the cranberry juice and maintaining a warfarin dose of 40 over 5 days, the INR returned to the therapeutic range of 2 to 3.

The bottom line?
This person’s experience supports cranberry juice as a possible cause of abnormal INR values, and, by extention, an increased risk of bleeding during warfarin treatment.

However, earlier this year, Drs. David and Antony Pham reported that 2 clinical trials “indicated the lack of an interaction between cranberry juice and CYP isoenzymes 2C9 and 3A,” which are responsible for the metabolism of many drugs, including warfarin. If there’s no interaction here, it’s unclear where the cranberry/warfarin interaction might take place.

OK, but this patient’s experience plus earlier reports of 2 patients with a similar experience suggest that somehow cranberry juice is associated with an increase the INR of patients taking warfarin.

More research is needed, but based on what we know, caution is recommended.

12/7/07 21:47 JR