Warfarin Therapy Management

Warfarin Therapy Management: Other Explanations for INR Changes

By: Andrew M McKinney, PharmD

Warfarin Therapy management - When adding any new prescription medication, over the counter medication, herbal or supplement product, make sure your doctor checks your INR for any adjustment needed in your warfarin.

Finding the cause of INR’s that are not within the goal range is often more difficult than simply identifying a new medication or an easily identified change in dietary Vitamin K intake. This problem is further complicated by the fact that many patients are poor historians and often omit critical pieces of information about their health, diet, over-the-counter medications, herbals, and other supplements.

Case reports and studies may provide conflicting information on the effect of some medications and supplements on warfarin therapy so discouraging unnecessary use of any medication or supplement is the best approach. Acute illnesses and exacerbations of chronic conditions can also have a major affect on INR and should always be considered when a medication or dietary explanation of INR change is not found.

The focus of this guide is to cover some of the possible causes of INR variation that are easily overlooked as well some that are unlikely to have an effect. Some overlooked sources of Vitamin K are also included.

Possible Increased Effect:

  • Acetaminophen – (still drug of choice with warfarin)
  • Acute alcohol consumption
  • Caffeine
  • Chamomile
  • Charbroiled foods – (questionable)
  • CHF exacerbation
  • Cranberry juice/products – studies now suggest no interaction
  • Dark Chocolate
  • Diarrhea
  • Fever
  • Fish Oils
  • Flax seed
  • Flu vaccine
  • Glucosamine/Chondroiton
  • Grapefruit juice
  • Iodine – (over long period)
  • Licorice
  • Marijuana
  • Mexican vanilla (tonka bean derived)
  • Ophthalmic erythromycin
  • Topical salicylates – sports/arthritis creams, etc.
  • Vomitting


Possible Decreased Effect:

  • Cooking oils – soybean, canola, vegetable (soybean oil 25mcg/tbsp)
  • Co-Q10
  • Ginseng
  • Green tea, black tea
  • Mayonnaise
  • Menthol cough drops
  • Nutrition bars – (ex. Luna Bar can contain up to 65mcg Vitamin K)
  • Nutrition Supplement Drinks – Boost, Ensure, etc. (30mcg per 8 oz)
  • Smokeless Tobacco
  • Smoking
  • Soy milk/products
  • Viactive Calcium Soft Chews - (40mcg Vit K per chew, rec. dose – 3 per day)


Ann Pharmacother. 2009 Apr;43(4):754-60. Epub 2009 Mar 31. Fatal intracranial bleed potentially due to a warfarin and influenza vaccine interaction.
Ann Pharmacother. 2009 Jul;43(7):1347-53. Epub 2009 Jun 16. Probable interaction between warfarin and marijuana smoking.
Pharmacotherapy. 2008 Apr;28(4):540-8. Potential glucosamine-warfarin interaction resulting in increased international normalized ratio: case report and review of the literature and MedWatch database.Knudsen JF, Sokol GH.
Pharmacotherapy. 2008 Apr;28(4):536-9. Potential interaction between warfarin and high dietary protein intake. Hornsby LB, Hester EK, Donaldson AR.
Ann Pharmacother. 2004 Apr;38(4):595-7. Epub 2004 Feb 6.Possible warfarin failure due to interaction with smokeless tobacco.Kuykendall JR, Houle MD, Rhodes RS.
Pharmacotherapy. 2009 Aug;29(8):1002-6. Potential interaction between pomegranate juice and warfarin. Komperda KE.
Ann Pharmacother. 2002 Dec;36(12):1893-6.Effect of soy milk on warfarin efficacy. Cambria-Kiely JA.
Ann Pharmacother. 2000 Jun;34(6):729-33.Potentiation of warfarin anticoagulation associated with topical methyl salicylate. Joss JD, LeBlond RF.
Am J Health Syst Pharm. 2007 Mar 1;64(5):490-4.Interaction potential between cranberry juice and warfarin. Pham DQ, Pham AQ.
Pharmacotherapy. 2010 Jan;30(1):110.Probable warfarin interaction with menthol cough drops.Coderre K, Faria C, Dyer E.