Warfarin, which is often sold under the brand name Coumadin, is an anticoagulant medication that is widely used to prevent or treat blood clots. Finding the right dose of warfarin can be difficult, and the challenge is made harder by the interaction of many foods and herbal products, including soy products. As with any health supplement, ask your doctor before consuming soy products while taking warfarin.
Soy products are one of the foods that may cause potentially dangerous interactions with warfarin. In at least one reported case, consuming soy milk reduced the effectiveness of warfarin, reports the New York University Langone Medical Center. Reducing the activity of warfarin increases the risk of blood clot formation, which can increase the risk of severe health effects such as stroke and pulmonary embolism.
The effectiveness of warfarin is assessed by a measurement called the international normalized ratio, or INR. Ingesting soy milk caused reductions in INR to ineffective levels in one patient who had been receiving a stable dose of warfarin, explains an article published in the December 2002 issue of the journal "Annals of Pharmacotherapy." After the patient stopped drinking soy milk, his INR levels returned to the effective range.
Warfarin slows blood-clotting by inhibiting the action of vitamin K, a vital player in the series of biochemical reactions that result in blood clotting. Soy products contain high levels of vitamin K. Eating foods that contain high levels of vitamin K, such as soy products, can counteract the activity of warfarin, possibly necessitating an increased dose of warfarin, advises HealthFinder.gov.
Although additional research is needed to confirm the effect of soy products on warfarin INR levels, you may still want to use caution with this combination. If you are taking warfarin, you do not need to completely avoid soy products or other foods high in vitamin K, you just need to make sure you consume roughly the same amount of vitamin K each day, explains the Agency for Healthcare Research and Quality. Avoid consuming a large amount of vitamin K-rich foods in a single sitting, or eating two vitamin K-rich foods at once.