People on certain medications such as Coumadin or Warfarin should not take vitamin K supplements, as stated by the University of Maryland Medical Center. Pregnant or breast-feeding mothers should also be cautious of the vitamin.
Vitamin K is a vitamin that helps the blood regulate its clotting. Because of this, it slows down the benefits of blood thinning medications such as Warfarin. The body makes its own vitamin K naturally, so a supplement is not usually needed. If antibiotics are taken, the production of the vitamin may slow. Pregnant and breast-feeding women should consult a doctor if they feel a supplement is needed because the vitamin can travel through breast milk or the placenta.