[Possible potentiation of phenprocoumon by clarithromycin and roxithromycin]

Two patients, a women of 70 and a man of 75 years old, who were using phenprocoumon chronically, and were monitored by a regional thrombosis service, received a macrolide antibiotic, clarithromycin and roxithromycin respectively, for an airway infection. Both patients developed a serious increase in hypocoagulability, requiring administration of phytomenadione and temporary decreases in phenprocoumon dose. There were no bleeding complications. After the antibiotics were discontinued, the original dosage of phenprocoumon was needed again. It is suggested that in these patients the macrolide antibiotics may have potentiated the effect of phenprocoumon, perhaps as a result of inhibition of phenprocoumon transformation by liver enzymes. In patients receiving chronic treatment with phenprocoumon, coagulation parameters should be regularly checked if they are given a macrolide antibiotic such as clarithromycin, roxithromycin or erythromycin.