A probable clinically significant interaction between warfarin and cloxacillin: three case reports.

Three patients were admitted to the Imam Hospital, Tehran, Iran with a diagnosis of bacterial endocarditis. The patients had indications for valve replacement surgery and anticoagulant therapy. The administration of cloxacillin reduced the effect of warfarin, and subsequent increases in warfarin doses were unable to overcome this effect. A decrease in warfarin anticoagulation effects was detected in our three patients following cloxacillin therapy for infective endocarditis. Penicillinase-resistant penicillins remain essential antibiotics in the treatment of severe infections caused by Staphylococcus aureus due to their bactericidal activity, safety, and cost. Thus, in situations where anticoagulants are indicated in patients with infective endocarditis, it would be better to replace warfarin with low-molecular-weight or unfractionated heparin.