Key interactions

• The cytochrome P450 system in the liver metabolises warfarin

  So warfarin metabolism is subjected to an extensive list of drug interactions of enzyme inducers and enzyme inhibitors
• Enzyme inducers decrease the amount of active warfarin and thus decrease its efficacy and INR:

  • St John's wort
  • Phenytoin
  • Carbamazepine
• Enzyme inhibitors increase the amount of active warfarin and thus increase its potency and INR:

  • Amiodarone
  • Metronidazole
  • Clarithromycin
• Many antibiotics interact with warfarin, and interactions should be checked before issuing an antibiotic prescription
• Key drug interactions:

  • Antibiotics - enzyme inhibitors or inhibition of vitamin K synthesis by intestinal flora
  • Antidepressants (serotonergic agents) - SSRIs and venlafaxine - some inhibit warfarin metabolism and others inhibit platelet function so increased risk of bleeding
  • Antiplatelets
  • Amiodarone - enzyme inhibitor
  • NSAIDs - direct mucosal injury, antiplatelet effects
• Key food interactions:

  • Foods with high vitamin K content - leafy greens - increased vitamin K synthesis antagonises anticoagulant effect of warfarin
  • Cranberry - enzyme inhibitor - increases potency