Warfarin works in the bloodstream to make blood less able to form clots, which is all dependent on the vitamin K levels in our daily diets.

Anyone taking warfarin (or a brand of warfarin such as Coumadin®) and who consume inconsistent amounts of vitamin K can potentially interfere with the way warfarin works. If this happens, you may risk a blood clot forming in our body, which can cause stroke.

Many warfarin users have questions about black teas versus green tea and their value to our health and how it might affect warfarin levels. Both green and black teas come from the same tea plant. The two teas differ only in the way they are processed. For example, teas turn black when handled and rolled, this movement releases an enzyme that reacts to oxygen and turns the tea leaves black. Black tea is physically darker and has a bolder flavor than green tea.1

When tea leaves are not rolled or aggressively handled, the leaves do not release the enzyme that reacts with oxygen and the tea is considered green tea.

Interaction with warfarin?

We assume that because green tea leaves are a high source of vitamin K that it will interfere with warfarin blood levels. But, brewed green tea contains low amounts of vitamin K, approximately 0.03 micrograms (ug) of vitamin K per 100 gm of brewed tea.2 The actual concentration of vitamin K in brewed tea will depend on the amount of leaves brewed and the dilution of the tea leaves.3

Black tea's caffeine content may have an effect on platelet function. Caffeine may increase the risk of bleeding when used with warfarin, aspirin, clopidogrel, dipyridamole and ticlopidine. Increases in INR are theoretically possible but significant, consistent interactions have not been documented in humans.4

Finally, use tea bags with actual large tea leaves versus using teas with too much tea dust. Tea bags often times only contain the dust from tea leaves and this concentrates the tea more when brewed. Use the tea bag only once to avoid weakening the bag and possibly letting leaves in your brewed tea. Even drinking one leaf in your tea can increase your vitamin K intake. Healthcare providers have different opinions on pure green tea. You should check with your care team to see what they recommend for you.

  1. 1. Medline Plus. Black tea. National Institutes of Health website. http://www.nlm.nih.gov/medlineplus/druginfo/natural/997.html.December 2013. Accessed March 7, 2014.
  2. 2. T.O. Cheng, Danshen: a popular Chinese cardiac herbal drug, J Am Coll Cardiol 47. 2006. p. 1498.
  3. 3. S.L. Booth, J.A. Sadowski and J.A.T. Pennington, Phylloquinone (vitamin K1) content of foods in the US Food and Drug Administration's Total Diet Study, J Agric Food Chem 43. 1995. pp. 1574-1579.
  4. 4. Parker D, Hoffmann T, Tucker M, Meier D. Interaction Between Warfarin and Black Tea. Ann Pharmacother January 2009 vol. 43 no. 1 150-151.
  5. 5. COUMADIN® is a registered trademark of Bristol-Myers Squibb Pharma Company. Acelis Connected Health is not affiliated or associated with Bristol-Myers Squibb Pharma Company or the COUMADIN® trademark.



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