Researchers from the University of Sydney in Australia investigated the possible impact of two commonly used herbal medicines, garlic and cranberry, on the anticoagulant, warfarin (Coumadin).

First, the details.

  • 12 healthy men of known CYP2C9 and VKORC1 genotype were randomly assigned to each treatment.
  • 1 dose of warfarin 25 mg administered alone
  • 1 dose of warfarin 25 mg administered after 2 weeks of pretreatment with either garlic or cranberry.
    • CYP2C9 and VKORC1 are genetic variants that affect warfarin metabolism.

And, the results.

  • Cranberry significantly increased the area under the INR-time curve by 30% when taken with warfarin compared to warfarin alone.
    • INR (International normalized ratio) is a measure of the ability of blood to clot where increased values reflect less clotting
  • Garlic had no effect on warfarin.

The bottom line?
The authors concluded, “Cranberry alters the pharmacodynamics of warfarin with the potential to increase its effects significantly. Co-administration of warfarin and cranberry requires careful monitoring.”

Both herbal medicines showed some evidence of VKORC1 genotype-dependent interactions with warfarin, which the authors think is worthy of further investigation.

This study supports earlier reports in individual patients.

6/14/08 20:41 JR

Hi, I’m JR

John Russo, Jr., PharmD, is president of The MedCom Resource, Inc. Previously, he was senior vice president of medical communications at, a complementary and alternative medicine website.