warfarinfo

Warfarin Institute of America

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SOY MILK INTERACTION WITH WARFARIN

Cambria-Kiely reported on the case of a man who was stable on warfarin before he started drinking soy milk. Shortly thereafter his INR dropped below the therapeutic range. When he stopped the soy milk, his INR returned to the therapeutic range.

There is possibly one other case reported in a medical journal.

I am interested in getting e-mails from anyone with a similar experience.

Here is an e-mail exchange that I had with a reader of this website who gave permission to use her first name:

Anna: I am currently drinking the Silk brand soy milk, chocolate flavored. I drink two-three 20oz. glasses a day. I didn't know, but I used to drink 6 glasses a tea a day and discontinued drinking the tea. My INR recently was 1.7 and before that it was 4.8. I'm having trouble pinpointing what happened to a stable INR I once had. I know dosage of warfarin is not as important as INR, but isn't a lower dose of warfarin safer? Am I not supposed to have Soy as I cannot even manage dairy?

Warfarinfo: You raise an interesting question. Green, leafy vegetables are rich in vitamin K. Tea, is one of these. However, vitamin K is fat soluble and running hot water over the leaves seems to extract very little of the vitamin K. Soy plants are also green and leafy. Evidently they store the vitamin K in the beans. The soy oil is used to mimic the fat in regular milk. This evidently contains more vitamin K than a similar volume of tea. So it is probably to be expected that drinking soy milk will result in the need for a higher warfarin dose.

Nobody has ever done a study to show that a lower warfarin dose is better for you than a higher dose. Warfarin is metabolized by the liver. If you made an analogy between the liver and forms of transportation, you could say that we are born with a liver like an 18-wheel truck and we can get along with a bicycle. In other words, we use a very small part of our liver's potential. The difference in warfarin that you will need to bring this INR back up is insignificant compared to what the liver can metabolize.

References:

Cambria-Kiely JA. Effect of soy milk on warfarin efficacy. Ann Pharmacother. 2002;36:1893-6.

Cheng TO. Potential interaction between soy milk and warfarin. Am Fam Physician. 2004;70:1231.

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Last updated January 30, 2005