Update:

Antibiotics for treatment of mild or moderate CDI include oral vancomycin or oral fidaxomicin; metronidazole has been an alternative agent.

Oral vancomycin is suggested for the initial treatment of mild or moderate CDI; fidaxomicin has been associated with a lower recurrence rate but is substantially more expensive than vancomycin

Use of metronidazole has been associated with treatment failure; the reasons are poorly understood. One contributing factor may be that stool drug levels in patients taking oral metronidazole decrease as colonic inflammation subsides, where stool drug levels in patients taking oral vancomycin or fidaxomicin remain high throughout the course of therapy.