Based on FDA | Is it true that consuming soy products, especially fermented soy like natto, can interfere with warfarin and lower INR levels?

Yes certain soy products can interfere with warfarin, but the effect depends on the type of soy and the mechanism involved. Fermented soy foods like natto are strongly associated with lowering the INR (reducing warfarin’s effect) because they are very rich in vitamin K2 (menaquinones), while non‑fermented soy items have less predictable and generally smaller effects; consistency and monitoring are key. ^{[1] [2]}

Why natto is high‑risk

• Natto contains large amounts of vitamin K2 produced during fermentation, which directly counteracts warfarin, a vitamin K antagonist. This can lower your INR and increase clotting risk. ^[3]
• Clinical and animal observations show natto intake can antagonize warfarin’s effect, raising clotting test values (i.e., lowering INR), likely due to increased vitamin K from Bacillus natto in the gut and from absorbed vitamin K in blood. ^[3]
• Even relatively low daily intakes of vitamin K2 (menaquinone‑7) in supplement form (10–45 μg/day) have been shown to lower INR in many individuals on vitamin K antagonists, suggesting natto’s high K2 content can be clinically significant. ^[2]

Other soy products: what we know

• Non‑fermented soy foods (e.g., tofu, soy milk, edamame) are typically much lower in vitamin K than natto; their effects on INR are less consistent and often minimal when eaten in a steady, habitual pattern. ^[1]
• Some clinical resources note that “soy products” might increase INR in certain situations, but this is not consistent across studies and may relate to non–vitamin K mechanisms (e.g., modulation of drug transporters/enzymes) whose clinical relevance remains uncertain. ^{[4] [5]}
• Official labeling for warfarin emphasizes that botanicals and foods can interact in unpredictable ways and recommends more frequent INR checks when starting or stopping such items. ^{[6] [7]}

Practical guidance for people on warfarin

• Avoid natto: Because of its high vitamin K2 content and well‑documented antagonism of warfarin, natto is generally best avoided while on warfarin. ^{[3] [2]}
• Keep diet consistent: The most important principle with vitamin K intake on warfarin is consistency; sudden increases in vitamin‑K–rich foods can lower INR. ^[1]
• Be cautious with vitamin K2 supplements: Even low doses of MK‑7 can reduce INR, so these supplements should be avoided unless your clinician specifically manages your dose around them. ^[2]
• Monitor when making changes: If you introduce, remove, or substantially change amounts of soy products, ask for an INR check within about 3–7 days, since warfarin’s effect can shift and may require a dose adjustment. ^{[6] [7]}

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Summary table: soy products and warfarin

Soy item	Fermented?	Vitamin K content tendency	Expected effect on INR with typical intake	Recommendation
Natto	Yes	Very high (K2)	Likely to lower INR (reduce warfarin effect)	Avoid while on warfarin. [3] [2]
Miso	Yes	Variable; lower than natto but can contain K	Effect uncertain; usually smaller than natto	If used, keep intake consistent and monitor INR with changes. [5] [6]
Tempeh	Yes	Variable; may contain K	Effect uncertain; likely less than natto	Maintain consistent intake; monitor with changes. [6]
Tofu, soy milk, edamame	No	Generally low to moderate	Typically minimal if intake is consistent	Allowed with steady intake; check INR if changing amounts. [1] [6]
MK‑7 (vitamin K2) supplement	N/A	Defined dose (10–45 μg+)	Lowers INR in many users on VKAs	Avoid unless supervised. [2]

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Key takeaways

• Natto can significantly counteract warfarin and lower INR due to high vitamin K2 best to avoid. ^{[3] [2]}
• Non‑fermented soy foods usually have a modest impact; the priority is to keep intake consistent and monitor INR when diet changes. ^[1]
• When starting or stopping any botanical or diet pattern that could interact, plan closer INR monitoring with your care team. ^{[6] [7]}