Acenocoumarol is a coumarin derivative used as an anticoagulant. Coumarin derivatives inhibit the reduction of vitamin K by vitamin K reductase. This prevents carboxylation of vitamin K-dependent clotting factors, II, VII, IX and X, and interferes with coagulation. Hematocrit, hemoglobin, international normalized ratio and liver panel should be monitored. Patients on acenocoumarol are prohibited from giving blood.