The recommended intake of vitamin K for adults is 90 micrograms per day, and the best way to meet this guideline is to eat foods that contain vitamin K like green leafy vegetables. Supplements are available without a prescription that range from 10 to 120 micrograms, and vitamin K supplementation generally is safe; however, there are important precautions with regards to possible nutrient and drug interactions. Consult a health care provider before taking dietary supplements.
Toxicity
Vitamin K1, the form most commonly used in supplements, has no known toxicity. A water-soluble form of vitamin K, menadione can lead to liver toxicity and should not be used to treat deficiency. There is no tolerable upper limit for vitamin K.
Precautions
Vitamin K is excreted in breast milk and does cross the placenta, so consult your physician if you are pregnant before taking vitamin K supplements. Vitamin K supplements are not safe for people with a rare metabolic condition known as glucose-6- phosphate dehydrogenase deficiency.
Nutrient Interactions
Oregon University's Linus Pauling Institute notes that both vitamin A and E have been shown to interfere with vitamin K. Large doses of vitamin A are believed to interfere with vitamin K absorption, and vitamin E has been shown to decrease or inhibit vitamin K dependent enzymes. Consult your health care provider if taking either vitamin A or E supplements.
Drug Interactions
Vitamin K reduces the effect of blood-thinning medications like warfarin. Doctors generally recommend avoiding large fluctuations of vitamin K when on these medications. This includes vitamin K-rich foods as well as supplementation. Consult your health care provider if you are taking blood thinners to discuss the amount of vitamin K you should consume.
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