Cranberry juice and warfarin

• there has been reported cases suggesting an interaction between warfarin and cranberry juice resulting in increased INR values. Since 1999 the Committee on Safety of Medicines (CSM) had received 12 reports suggesting this interaction
• the interaction is biologically plausible because cranberry juice contains various antioxidants including flavonoids, which are known to inhibit cytochrome P450 activity, and warfarin is predominantly metabolised by the P450 isoform CYP2C9
• CSM has reviewed these cases and concluded that there is now sufficient evidence of an interaction for formal advice to be issued (2)
  • it is not possible to define a safe quantity or brand of cranberry juice, therefore patients taking warfarin should be advised to avoid this drink unless the health benefits are considered to outweigh any risks. Increased medical supervision and INR monitoring should be considered for any patient taking warfarin and a regular intake of cranberry juice
  • it is not known whether other cranberry products, such as capsules or concentrates, might also interact with warfarin. Therefore similar caution should be observed with these products

Reference:

1. Current Problems in Pharmacovigilance (2003);29: 8.
2. Current Problems in Pharmacovigilance (2004);30:1-12.