What Is a Vitamin K Antagonist?

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  • Written By: Clara Kedrek
  • Edited By: Jessica Seminara
  • Last Modified Date: 15 November 2019
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A vitamin K antagonist is a type of medication that opposes the effects of vitamin K in the body. These medications are often used as anticoagulants, which means that they are given to patients in order to decrease the ability of the blood to clot. The most commonly used pharmaceutical agent in this class of drugs is warfarin. Patients taking this medication need to have their blood checked regularly to monitor the effect its administration is having on the blood. Pregnant women should not take medications in this class because their developing babies could develop birth defects.

Substances that function as a vitamin K antagonist work by inhibiting the action of vitamin K, a molecule that normally helps facilitate a number of reactions in the body. Perhaps most critically, vitamin K helps to create a number of different proteins that help promote the ability of the blood to clot. This is an important physiologic function because it helps prevent blood loss from cuts or other lesions made to the body. Without the activity of vitamin K in the body, the production of these clotting factors is decreased, and the blood has a decreased ability to clot.


Patients are given vitamin K antagonist medications for a number of different reasons. For example, patients who have mechanical heart valves or pacemakers are given these medications for extended periods, because without them they could have an increased risk for forming blood clots in the heart that could break off and enter the brain, causing a stroke. Patients who have abnormal heart rhythms are also given these medications because they are also at an increased risk for forming blood clots in the heart. Perhaps one of the most common arrhythmias treated with this class of pharmaceutical agents is atrial fibrillation, a condition in which the heart beats in an irregular fashion.

The most commonly used vitamin K antagonist is warfarin, a medication that is also known by the name coumadin. This drug has an interesting history, as its initial use was as a rat poison, as high doses could induce excessive and fatal bleeding in these animals. Humans taking warfarin need to be monitored regularly to measure the effect it is having on the blood's clotting ability, as high concentrations of the substance in the blood can cause excessive bleeding, especially after trauma. A blood test called the international normalized ratio (INR) is checked in these patients, and the target value for this number is typically between 2 to 3.

It is important that pregnant women not take any medication that is a vitamin K antagonist because these medications are known to cause birth defects. These medications are considered to be teratogens, which is the medical term for substances known to harm developing babies. If exposed, especially early in the developmental process, they can develop abnormal skeletons, shortened arms or legs, and mental retardation.


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