The anticoagulant effect of warfarin can be reduced from sporadic high intake of foodstuffs containing high levels of vitamin K, e.g. leafy green vegetables and vegetable oils such as canola, olive and sunflower. Total daily intake of such foodstuffs should therefore be kept as consistent as possible to avoid major fluctuations of dietary vitamin K.

Concurrent ingestion of complementary and alternative medicines having an antiplatelet effect, e.g. fish oils, garlic, ginger, Ginkgo biloba, devil's claw, feverfew, horse-chestnut and willow, should be avoided because of substantially increased bleeding risk with warfarin.

Depending on how alcohol is used, the INR can increase or decrease. Moderate daily intake, namely a recommended maximum of two standard drinks per day, usually has little impact. One standard drink is:

  • 300mL ordinary strength beer
  • 60mL fortified wine (sherry, martini, port)
  • 30mL spirits (whisky, gin, vodka)
  • 100mL table wine

Binge drinking can result in dramatically increased INRs with increased bleeding risk, and chronic heavy intake can result in decreased INRs with increased risk of thromboembolism.

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