Warfarin reduces the activity of both vitamin K dependent clotting factors and the natural anticoagulant proteins C and S. The anticoagulant response is usually obtained after 5 to 7 days of treatment. In the treatment of thrombosis, low molecular weight heparin (LMWH) in therapeutic doses is used concomitantly with warfarin. LMWH is not stopped until the INR (international normalised ratio) has been within the therapeutic range for 2 days. Anticoagulant therapy without an existing thrombosis, for example in atrial fibrillation (AF), is started with warfarin alone unless the patient has a known specific thrombophilic disorder. The possibility of bleeding must be borne in mind after trauma, particularly when the head, neck, back or torso have been injured, and monitoring of the clinical condition, INR and blood picture must be arranged.The frequency of INR monitoring should be increased when a new medication is started, an old one is stopped or its dosing changed (INR after 3–5 days), as well as when there is a sudden change in the patient’s condition.In exacerbating heart failure INR has the tendency of rising, and thus the need for warfarin is reduced. In hypertension, warfarin must only be used when normotension has been achieved.A patient who manages warfarin treatment and its monitoring well may go over to self-monitoring; after sufficient guidance he/she can adjust the warfarin dose him-/herself within defined limits.