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Vitamin K & Warfarin Reversal

By Helen Messina

Warfarin, an oral anticoagulant, hinders the normal clotting effects of vitamin K. Discovered in 1942 at the University of Wisconsin, warfarin decreases the action of clotting factors to a level that is medically beneficial. By blocking vitamin K, warfarin slows the clotting mechanism, a desirable effect to treat or prevent a number of conditions such as atrial fibrillation, stroke, heart attack and valve replacement. In the event of excess anticoagulation or overdose with warfarin, synthetic vitamin K is the medication of choice to prevent and correct bleeding abnormalities.

Vitamin K Antidote

Vitamin K, the antidote for warfarin excess or overdose, can be used with fresh frozen plasma, which also reverses the effects of warfarin. Occasionally both are given. The synthetic form of vitamin K is called phytonadione or vitamin K-1. When blood tests, specifically a prothrombin test, reveal excess anticoagulation, phytonadione administration begins, either orally or by injection, until the prothrombin time is restored to the desired level. Warfarin action lasts longer than vitamin K; consequently, repeated doses are necessary, especially with continued bleeding or overdose.

Warfarin Reversal

While a significant medication to stop blood clotting, warfarin can also cause bleeding and requires repeated blood test monitoring until desired levels are achieved. An outright risk of bleeding is actually low, even with an extended clotting time as reflected by blood test results. The first choice for treating excessive anticoagulation is the withdrawal of warfarin and other medications that enhance warfarin 's effects. Frequent lab test monitoring is necessary, as an active bleed or very high risk to bleed requires treatment with vitamin K.


Superwarfarins, a term applied to the compounds found in rodenticides, or rat poison, are similar in action to the anticoagulation of warfarin, only at an extreme level. The anticoagulation effects of rodenticides, such as brodifacoum or indanedione, can last for weeks, even months, after only a single dose. When proof of anticoagulation is determined by an elevated PT or active bleeding, phytonadione is given to correct the deficit in clotting factors. Treatment might require extended observation and additional large doses of vitamin K because of prolonged action of superwarfarins.


The prothrombin time blood test, or PT, is used to monitor response to warfarin therapy as well as to monitor therapy reversing the effects of warfarin excess. No food or fluid restrictions apply to test preparation. The International Normalized Ratio or INR results are included with the PT test. The INR reflects the relationship between the individual's prothrombin time and the degree of physical coagulation.

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