Oral anticoagulants are ušed in the primary and secondary prophylaxis of venous and arterial thrombosis. Warfarin, a coumarin glycoside, has been widely ušed in the Czech Republic. Its complex mechanism of action and pharmacological properties require regular prothrombin time evaluation using an International Normalized Ratio (INR) index. Even the correct management of warfarin therapy does not guarantee elimination of bleeding risk or thrombotic recurrence. But these risks increase with wrong practice. The length of anticoagulant therapy depends on the type and location of thrombosis. It is also important to establish whether it is the first incident or a recurrence of thrombosis and to consider other risk factors including thrombophilia.

        Key words: warfarin, tromboembolic disease, prothrombin time, INR, drug interactions