Vitamin K Side Effects and Warnings

Safety Issues

Vitamin K is quite safe at the recommended therapeutic dosages.

Note: Vitamin K directly counters the effects of the anticoagulant warfarin (Coumadin) . If you are taking warfarin, you should not take vitamin K supplements or alter your dietary intake of vitamin K without doctor supervision. ¹ (One study suggests a novel way of using this effect deliberately. ² Researchers gave people on warfarin a fixed daily dose of vitamin K in order to override the changes in warfarin action caused by the natural variation in day-to-day dietary vitamin K consumption. The results were positive: INR values—the standard measurement of warfarin’s blood thinning effect—became more stable. Needless to say, however, this method should not be used except under close physician supervision.)

Newborns are commonly given vitamin K 1 injections to prevent bleeding problems. Although some have suggested that this practice may increase the risk of cancer, ³ enormous observational studies have found no such connection (one such trial involved more than one million participants). ⁴

Interactions You Should Know About

If you are taking:

• Warfarin (Coumadin) : Do not take vitamin K supplements or eat foods high in vitamin K except under the supervision of a physician. (You will need to have your medication dosage adjusted.)

• Cephalosporins or other antibiotics : You may need more vitamin K if you are already deficient in this nutrient.

• Anticonvulsants—such as phenytoin (Dilantin) , carbamazepine , phenobarbital , and primidone (Mysoline) —and are pregnant: You may need more vitamin K.