What Is the Connection between Warfarin and Vitamin K?

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  • Written By: Marlene Garcia
  • Edited By: Daniel Lindley
  • Last Modified Date: 10 October 2019
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Warfarin and vitamin K both regulate blood clots, with warfarin prolonging the process and vitamin K vital to the chemical process that causes clotting in the blood. Warfarin acts against vitamin K by increasing the time it takes for blood to clot. Patients generally receive a warning about the interaction between warfarin and vitamin K from doctors who prescribe the drug.

An anti-coagulant medication, Warfarin reduces blood clotting. It is typically prescribed for patients at risk of clots blocking the flow of blood to the brain or heart. If warfarin and vitamin K are consumed together, it decreases the effectiveness of the drug. Doctors typically recommend patients using warfarin keep vitamin K levels consistent from day to day.

A spike of the vitamin might render warfarin useless. If vitamin K levels decrease sharply, the effects of warfarin could increase. Patients using this drug typically require testing of warfarin and vitamin K levels once a month to measure blood clotting times. If the levels of warfarin and vitamin K are off, medication or diet can be adjusted to achieve the desired balance. Alcohol and cranberry juice might also skew levels of warfarin and vitamin K in the blood.


Foods highest in vitamin K include cooked spinach, turnip greens, kale, collard greens, Swiss chard, and mustard greens. Doctors advise no more than one serving a day of these vegetables. Other vegetables contain moderate levels of vitamin K, with three or less servings a day suggested. They include raw spinach, Brussels sprouts, broccoli, romaine and green leaf lettuce, and raw turnip greens. Other foods contain smaller amounts of the nutrient, including coffee and tea.

Warfarin and vitamin K imbalances are not the only concerns when taking this medication. People taking warfarin might bleed excessively from an accidental cut or injury, since their blood won't clot as easily. They may also bruise easily and notice black stools from rectal bleeding. In rare cases, skin tissue becomes damaged, leading to gangrene and potential amputation of the affected body part.

This medication is often prescribed for people whose blood clots too easily. It might also be used by patients with prosthetic heart valves, which are prone to clogging. Others take the drug after a heart attack or open heart surgery as a preventative measure.

Physicians typically explain the effects of warfarin and vitamin K to patients and advise them about bleeding concerns. The medication should be avoided before any dental or medical procedure that could cause bleeding, including routine vaccinations. Warnings might be issued about dietary supplements containing vitamin K, and certain herbs that affect how the drug works.


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