A 40-year-old male is well controlled on warfarin for a factor V Leiden deficiency and recurrent deep vein thrombosis. He presents today with a community-acquired pneumonia and is placed on erythromycin. Three days later, he develops bleeding and his INR is 8.0 (indicating an increased time for blood clotting to occur, where INR is international normalized ratio). Which of the following best explains why this bleeding occurred?