Phenprocoumon is the dominant anticoagulant in clinical use in several continental European countries. It used for the prevention and treatment of thromboembolic disease including venous thrombosis, thromboembolism, and pulmonary embolism as well as for the prevention of ischemic stroke in patients with atrial fibrillation. Phenprocoumon inhibits vitamin K reductase, resulting in depletion of the reduced form of vitamin K (vitamin KH2). Bleedings are the most important side -effects of oral anticoagulants. The gastrointestinal and the urinary tract are often affected; the most dangerous are intracerebral hemorrhages. A great number of drugs increase the risk of bleeding of oral anticoagulants. Enzyme inhibitors (e.g. allopurinol, androgens, cimetidine, ciprofloxacin, co-trimoxazole, certain anti-inflammatory agents, fibrates, imidazoles, macrolide antibiotics, etc.) reinforce, and enzyme inducers (e.g. barbiturates, rifampicin) and oral contraceptives reduce, the anticoagulant action.