Is Cranberry Juice Safe for Coumadin (warfarin) Users?

My husband went on Coumadin this week and one of the doctors told him, "NO cranberries." When we read how low the vitamin K is in cranberries, we questioned this.

The nurse researched it for us and was told that cranberries kill some type of bacteria in your body and that the bacteria affects the Coumadin. Is this correct?

The Health meets Food Team Says...

Well, for a long time we sent folks to an article written for for this website back in 2006: Should you avoid cranberry juice on Coumadin (warfarin)?

At the time there wasn't very good science investigating the issue of an interaction between cranberry juice and Coumadin (warfarin). The good news is that we now have pretty good evidence that it's OK to drink cranberry juice.

The feeling has been that somehow cranberry juice interacted with the cytochrome P450 system of enzymes. These enzymes are critical to helping the body clear itself of chemicals such as medications we take. A group of researchers in Boston looked at whether cranberry juice, brewed tea, grape juice or placebo would have an effect on the specific P450 enzyme responsible for clearing warfarin from the body. They used another drug, fluconazole, as a control medication. (Clin Pharmacol Ther 2006;79:125-33.)

Guess what? No effect was seen on the clearance of medications in the body by any of the test substances. Although there was some effect in the test tube with grape juice and tea (not cranberry juice) this wasn't seen when tested on humans.

In another study, a group had ten healthy volunteers drink cranberry juice or water each day for ten days. They tested three different drugs, including warfarin, to see if there was any difference in the drug concentrations or the INR. Again, no difference. (Clin Pharmacol Ther 2007;81:833-39.)

Those faithful readers understand the importance of actual science before making claims. When it comes to something like Coumadin, we were right to be cautious because of the case reports. We now have evidence that it's OK for you to drink cranberry juice (and eat cranberries) while taking warfarin.

Thanks for writing.