Potentiation of Warfarin Anticoagulation by Acetaminophen

To the Editor.— Acetaminophen is not thought to interact with warfarin, and patients receiving this oral anticoagulant are frequently told that they can safely use this analgesic-antipyretic in place of aspirin. We recently admitted a patient who was receiving warfarin to our hospital with bleeding closely associated with concurrent acetaminophen ingestion. Report of a Case.— A 66-year-old woman presented to Sunnybrook Health Science Centre, North York, Ontario, on June 8 with a 3-day history of hematuria and gingival bleeding. Her past pertinent medical history included a stroke in 1986 for which warfarin was prescribed. She has been followed up in the Anticoagulation Clinic on a monthly basis since then. Her average daily warfarin dose has been 3 mg/d to maintain a prothrombin time between 15 and 19 seconds; her highest prothrombin value over this time was 23 seconds. In late May 1990, an acetaminophen-codeine combination was prescribed for a recurring