As you have learned, coumadin or warfarin is a very finicky medication. Unlike many medications whose levels in the blood only depend on taking the medication every day, warfarin levels depend on a number of factors including your diet (specifically the amount of vitamin K in your diet), your other medications (certain antibiotics for example can increase levels of coumadin in the body), and your overall health (certain health problems can increase or decrease your levels). On the other end, warfarin is a life-saving uses with a wide range of uses including prevention of DVTs. Warfarin is so tricky in fact that I don't manage my own patients warfarin dosing and instead rely on specialized pharmacists and nurse practitioners to do it. Often when someone starts taking warfarin they have their blood checked weekly and sometimes more frequently until doctors can find the right dose and patients get in the habit of taking the medication at a scheduled time and control their diet (for example, we routinely tell patients to only eat leafy green vegetables on certain days while taking warfarin and to avoid certain foods like grapefruit completely). After a month or two however the frequency of blood checks are spaced out as patients' levels are repeatedly in the goal range (typically INR 2-3).
Hope that helps.
- Dr. Nundy