NutriStack
Vitamin K1 and Warfarin, a conflict.
Vitamin K1 (phytonadione) directly reverses warfarin's anticoagulant effect. Even small, regular doses found in multivitamins (25 mcg/day) have been shown to lower the INR and require warfarin dose increases, particularly in patients with depleted baseline vitamin K stores. Inconsistent K1 intake from supplements is one of the most common causes of unstable INR.
One pair, every claim cited. The two substances, the type, the mechanism, the recommendation, and the primary literature.
Same shape as the other 1,729 pairs in the public database.
From the interaction database
What the row says.
Every entry follows the same shape: what is happening, the mechanism, the recommendation, and the primary literature.
At a glance
- Substances
- Vitamin K1 and Warfarin
- Pair type
- Conflict
- Evidence (highest tier)
- Strong
- Source citations
- 2 sources
- Stack Score effect
- −10 to your Stack Score (per scored conflict row).
- Scope
- Supplement × Prescription
- Last verified
- May 30, 2026
Conflict · Strong evidence
Conflict
What is happening. Vitamin K1 (phytonadione) directly reverses warfarin's anticoagulant effect. Even small, regular doses found in multivitamins (25 mcg/day) have been shown to lower the INR and require warfarin dose increases, particularly in patients with depleted baseline vitamin K stores. Inconsistent K1 intake from supplements is one of the most common causes of unstable INR.
Mechanism. Warfarin inhibits vitamin K epoxide reductase (VKORC1), preventing the recycling of vitamin K needed to gamma-carboxylate clotting factors II, VII, IX, and X. Supplemental vitamin K1 directly replenishes the substrate pool, bypassing warfarin's blockade and restoring clotting factor activity.
Recommendation. Avoid vitamin K1 supplements unless your prescriber has specifically directed you to take them. If a multivitamin containing vitamin K1 is unavoidable, keep the dose and brand identical every day and ask your anticoagulation clinic to re-check INR within 1-2 weeks of starting.
Sources (2)
- Kurnik D, Loebstein R, Rabinovitz H, et al. Over-the-counter vitamin K1-containing multivitamin supplements disrupt warfarin anticoagulation in vitamin K1-depleted patients. A prospective, controlled trial. Thromb Haemost. 2004;92(5):1018-24. PMID 15543329
- Holbrook AM, Pereira JA, Labiris R, et al. Systematic overview of warfarin and its drug and food interactions. Arch Intern Med. 2005;165(10):1095-106. PMID 15911722
Stack Score
How this pair moves the number.
Effect on the composite score
If both Vitamin K1 and Warfarin are in the same stack, this pair applies −10 to your Stack Score (per scored conflict row).
The full algorithm, the clamping rules, and four worked stacks are documented at /methodology/stack-score.
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