I overstated my point, because in fact, vitamin K levels need to be carefully monitored and followed by physicians for those who take blood thinners (usually for Atrial fibrillation).

Vitamin K in an improper dose (and it has to be regularly checked by specialists) can have the opposite effect of blood thinners - that is, Vitamin K can cause clotting (and in turn strokes and heart attacks).

It's to avoid that, that patients at special risk for that, take blood thinners. They need to be carefully balanced. In that sense Vitamin K can act as a poison for patients who need anticoagulants.