NICOUMALONE [ACITROM] REDUCES THE CONCENTRATION OF PROTHROMBIN IN BLOOD AND INCREASES THE PROTHROMBIN TIME BY INHIBITING THE FORMATION OF PROTHROMBIN IN THE LIVER. THE DRUG ALSO INTERFERES WITH THE PRODUCTION OF FACTORS VII, IX, X, AS WELL AS PROTEIN C, AND ITS COFACTOR PROTEIN S SO THEIR CONCENTRATION IN THE BLOOD IS LOWERED DURING THERAPY.