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Diet and Warfarin

By Dr Graham Stuart

I am on warfarin. Is there any food or drink that I cannot take? I have heard that I should not drink cranberry juice as it interferes with warfarin in some way. Also, how much alcohol is it safe for me to drink each night?

Warfarin is an extremely useful drug that prevents the formation of blood clots. This is necessary for patients who have an artificial heart valve, a history of blood clots, pulmonary hypertension or who have undergone a Fontan operation. However warfarin has a 'narrow therapeutic margin'. This means that any change in the amount of warfarin in your bloodstream may change the way warfarin affects you. Thus, too much warfarin can cause excessive bleeding, while too little reduces the drug's ability to prevent clots. Certain drugs, natural health products (such as herbal remedies and vitamins) and foods change the level of warfarin in the bloodstream.


Many drugs interact with warfarin. It is wise to ask the pharmacist (or your cardiologist/GP) if there is a warfarin interaction before starting any new drug. Common drugs that can cause this include antibiotics, analgesics/anti-inflammatories, antidepressants and cholesterol-lowering agents.


Certain foods and 'natural' health products can affect warfarin levels. Some increase the level (for example: ginko bilboa) while others reduce the warfarin effect (ginseng). Vitamin K, in particular, decreases the effect of warfarin. There are large amounts of vitamin K in certain foods such as liver. broccoli. brussels sprouts, and green leafy vegetables such as spinach, coriander and cabbage.


The interaction between alcohol and warfarin is complex. Alcohol acts as a mild anticoagulant. It also can affect the metabolism of the liver, which s important tor metabolising warfarin. As a rule of rhumb, two to three drinks per day - glasses of wine. beer etc -(typical cardiologist input) are unlikely to affect your warfarin levels. Intermittent binge drinking (typical medical student input) leads to an increase in INN (international ratio, the measurement used to check warfarin levels in the blood) due to warfarin being metabolised more slowly. Chronic heavy alcohol intake (Harley Street cardiologist input just joking!) results in a lower warfarin level because the alcohol increases the metabolism of warfarin.

Natural health products/ supplements that affect warfarin levels or blood clotting

Here is a list of foods and natural health products which affect warfarin levels or which have an intrinsic effect on blood clotting. This list is not exhaustive. If you take warfarin, you should avoid sudden changes in your daily intake of these foods.

You can minimise your risk of warfarin interactions by taking your warfarin medication at the same time each day and testing your blood regularly (ideally with a home monitor). Talk to your doctor or pharmacist before trying any new 'natural' remedies or supplements.

If you are already taking one of these products and warfarin, do not change your routine unless you have discussed it with your health professional, and go easy on the foods that affect warfarin.

Finally, if you have any unusual bruising or bleeding. contact your health care professional for advice right away.
  • chondroitin plus glucosamine
  • coenzyme 010 - also known as ubiquinone, ubidecarenone
  • danshen (salvia miltiorrhiza)
  • devil's claw (harpagophytum procumbens)
  • dong quai -- also known as danggui. Chinese angelica (angelica sinensis)
  • feverfew (tanacetum parthenium)
  • fenugreek together with boldo (peumus boldus)
  • fish oil supplements that contain eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)
  • ginkgo biloba
  • ginseng (panax ginseng) - also known as Asian ginseng. Chinese ginseng. Japanese ginseng, Korean ginseng
  • American ginseng (Panax quinquefolius)
  • green tea (camellia sinensis)
  • horse chestnut (aesculus hippocastanum)
  • lycium barbarum also known as Chinese wolfberry, Di GU Pi, Goji berry. Gou Qi Li
  • papaya extract (containing papain)
  • certain brands of quilinggao - also known as 'essence of tortoise shell'
  • St. John's Wort (hypericum perforatum)
  • vitamin A
  • vitamin K
  • wintergreen (used on the skin) - also known as methyl salicylate

Foods that affect warfarin or blood clotting

  • avocado
  • cranberry juice
  • flax (flaxseed)
  • garlic
  • ginger
  • mango
  • onions
  • papaya
  • seaweed (sushi wrap)
  • soy protein products (including soymilk and tofu)
Dr Graham Stuart is a GUCH Consultant at Bristol Royal Infirmary.

Printed in GUCH News - Winter 2005
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