The oral anticoagulant warfarin inhibits the vitamin K-dependent clotting factors II, VII, IX, and X and also inhibits the synthesis of proteins C and S. Vitamin K is the fat-soluble substance in our bodies that promotes blood clotting by helping make these factors in the liver.

Small initial daily doses (such as 5–7.5 mg) is the preferred way to start warfarin. Peak effect of the drug occurs 36–72 hours after it is started. The response to warfarin varies among individuals, and so each patient is monitored to prevent underdosing or overdosing. Patients must not take any new drugs or natural compounds without consulting their physician, since they may affect the effects of warfarin.

The test used to monitor the patient’s response to warfarin is the one-stage PT (prothrombin time) test. After the initial dosage of warfarin 5–7.5 mg/day for the first 2 days, the daily dose is adjusted according to a standard ratio (INR). Heparin is stopped after the 4th or 5th day of warfarin therapy if the INR is in the therapeutic range (2–3). Monitoring is necessary because some people are either fast or slow metabolizers. When the anticoagulant effect and dose requirements are stable, INR monitoring continues through the course of warfarin treatment.

Long-term warfarin treatment for DVT reduces recurrent clotting from 47% to 2%. The less intense but carefully controlled warfarin (INR 2–3) reduces risk of bleeding from 20% to 4% while still being effective.

Exception to long-term cumadin therapy:

  1. Adjusted-dose unfractionated heparin by injection for pregnant women
  2. Adjusted-dose unfractionated heparin or unmonitored LMWH for those whose oral anticoagulant therapy is difficult to control or stabilize