Vitamin K 
When you are prescribed Warfarin one of the first things you will be told is to be mindful of the amount of Vitamin K in your diet. So why is this?
Put simply,
Vitamin K is a vital part of the blood clotting process. So, for people at risk of blood clots forming, Warfarin is prescribed to control the action of Vitamin K.
It therefore follows that the amount of Vitamin K in your body will affect how well Warfarin is able to control the blood clotting process. So eating large amounts of Vitamin K containing foods can put you at higher risk of blood clots occurring.
You should not try and avoid eating foods containing Vitamin K altogether. It is an important nutrient and has a number of valuable health benefits.
Added to this, your correct dosage of warfarin will be worked out taking into account your normal consumption of Vitamin K.
So, the important thing is to be consistent in the amount of Vitamin K you are eating over the course of the week and try not to make big, or sudden, changes.
Remember,
An increased amount of Vitamin K can lower your INR making Warfarin less effective and potentially increasing the risk of blood clots
A decreased amount of Vitamin K can increase your INR making Warfarin more effective and putting you at risk of bleeding 
It is, therefore, important to have an awareness of the type of foods that have a high Vitamin K content. In general these fall into 3 main groups,
Dark green leafy vegetables, such as spinach, kale or broccoli
Some Oils, such as rapeseed and olive oil
Certain Fast Foods, such as crisps and fast food fries