An orally available, long-acting derivative of coumarin with anticoagulant activity. Upon administration, phenprocoumon inhibits the vitamin K epoxide reductase enzyme; inhibition of this enzyme prevents the formation of the reduced, active form of vitamin K (vitamin KH2), which is essential for the carboxylation of glutamate residues of vitamin K-dependent proteins. This prevents the activation of vitamin K-dependent coagulation factors II, VII, IX, and X and the anticoagulant proteins C and S, which abrogates both thrombin production and thrombus formation.