Administration of concomitant antibiotics (CA) is a known risk factor for treatment failure in the treatment of CDI, as well as for recurrence of CDI. Recent data suggested that among patients receiving CA, fidaxomicin is superior vancomycin. While these data are encouraging, many clinicians remain unclear on how to apply these data to patient care. Additionally, patients were excluded from the trials presented to the FDA if it was expected that they would require ≥ 7 days of CA. Therefore, the clinical question still remains of how to apply these data to the real world patient who requires a long course of CA and develops CDI while on therapy. We therefore propose an open label, comparative and prospective study of fidaxomicin 200 mg twice daily vs oral vancomycin 125 mg four times daily for the treatment of CDI among patients who are receiving a long course of CA.