A frail 75-year-old man, T.W., has been admitted to your hospital. He has a fever of 102°F, along with bloody diarrhea and abdominal pain. Prior to his admission, T.W.'s medications included the following: valsartan (Diovan, Novartis ) 80 mg daily, metoprolol 25 mg daily, aspirin 81 mg daily, simvastatin 40 mg daily, warfarin 3 mg on Monday, Wednesday, Friday and 2 mg on Tuesday, Thursday, Saturday, and Sunday. (T.W. suffered a myocardial infarction six months ago.) His last INR was 4.0. Because he had been hospitalized 14 days prior for S. pneumoniae pneumonia that was treated with ceftriaxone, the admitting physician suspected a C. difficile infection, which was subsequently confirmed by the lab. The physician has now ordered metronidazole 500 mg three times daily for T.W., and your computer flags that as a significant drug interaction. What do you recommend to the physician?