Cranberry and Warfarin Don’t Mix

Jacob Schor ND FABNO

September 22, 2008

We are fast approaching cranberry season.  If you haven’t purchased your kids Halloween costume already, you’d better hurry, as the stores will sell out.  Before trick or treating even happens, Thanksgiving  decorations will be up and the groceries will start selling fresh cranberries as loss leaders.


Domestic cranberry production was about 5 million hundred pound barrels back in 1997.  Current production has increased to 6. 9 million barrels in 2007.


A strong cautionary warning appeared in the British Journal of Pharmacology in August 2008.  Cranberries interact with the drug warfarin, commonly sold as Coumaden. This drug is an anticoagulant and used to prevent blood clotting.  It is a serious drug to take and requires routine monitoring to keep drug levels in certain specified ranges. 


Abdul Mohammed and his colleagues on the Pharmacy faculty at the University of Sidney in Australia tested the effect of cranberries on warfarin in healthy volunteers.  What they found was disturbing.  Cranberries made the drug action stronger, about 30% stronger.


This would be ok if people taking warfarin ate the same amount of cranberries year round.  Then they could get away with taking almost a third less of the drug.  It doesn’t work that way though.  Cranberry consumption is seasonal.  The majority of the crop will be eaten between Thanksgiving and Christmas.  People taking warfarin should be hesitant with cranberries, avoiding second helpings if possible.  As far as drinking cranberry juice, either drink it regularly, the same amount each day, or avoid it.  The later suggestion makes the most sense.


 Br J Pharmacol. 2008 Aug;154(8):1691-700.

    Pharmacodynamic interaction of warfarin with cranberry but not with garlic in healthy subjects.

    Mohammed Abdul MI, Jiang X, Williams KM, Day RO, Roufogalis BD, Liauw WS, Xu H, McLachlan AJ.

    Faculty of Pharmacy, University of Sydney, Sydney, New South Wales, Australia.

    BACKGROUND AND PURPOSE: Patients commonly take complementary medicines in conjunction with warfarin yet evidence supporting the safety or the risk of a herb-drug interaction is lacking. The aim of this study was to investigate the possible impact of two commonly used herbal medicines, garlic and cranberry, on the pharmacokinetics and pharmacodynamics of warfarin in healthy male subjects. EXPERIMENTAL APPROACH: An open-label, three-treatment, randomized crossover clinical trial was undertaken and involved 12 healthy male subjects of known CYP2C9 and VKORC1 genotype. A single dose of 25 mg warfarin was administered alone or after 2 weeks of pretreatment with either garlic or cranberry. Warfarin enantiomer concentrations, INR, platelet aggregation and clotting factor activity were measured to assess pharmacokinetic and pharmacodynamic interactions between warfarin and herbal medicines. KEY RESULTS: Cranberry significantly increased the area under the INR-time curve by 30% when administered with warfarin compared with treatment with warfarin alone. Cranberry did not alter S- or R-warfarin pharmacokinetics or plasma protein binding. Co-administration of garlic did not significantly alter warfarin pharmacokinetics or pharmacodynamics. Both herbal medicines showed some evidence of VKORC1 (not CYP2C9) genotype-dependent interactions with warfarin, which is worthy of further investigation. CONCLUSIONS AND IMPLICATIONS: Cranberry alters the pharmacodynamics of warfarin with the potential to increase its effects significantly. Co-administration of warfarin and cranberry requires careful monitoring.