Warfarin inhibits vitamin K dependent coagulation factors. It is used to thin the blood in patients who have had deep vein thrombosis (DVT) or pulmonary embolism or have a high risk of forming blood clots in the heart secondary to atrial fibrillation or mechanical heart valve replacement. Almost all overdoses occur because of errors in dosing and/or drug-drug interactions. The overdose may cause no symptoms and only be recognized on a blood test that demonstrates a high Prothrombin Time (PT) or high INR (International Normalized Ratio). However, overdoses can lead to life threatening bleeding.