Approximately 520 patients will be entered into this study taking place throughout the US and Canada. This study aims to determine if an investigational drug is safe and effective for treating symptoms of C. difficile-associated diarrhea (CDAD) and lowering the risk of repeat episodes of CDAD. The investigational drug will be evaluated in comparison to current standard antibiotic treatment, so all patients will receive active medication. All study related care is provided including doctor visits, physical exams, laboratory tests and study medication. Total length of participation is approximately 6 weeks.

Inclusion Criteria:

• 18 years of age
• The presence of CDAD at the time of enrollment with no other likely etiology for the diarrhea
• Less than or equal to 48 hours of treatment with metronidazole, vancomycin or other antibacterial therapy specific for CDAD
• Baseline serum potassium > 3.0 mmol (meq)/L
• Patient considered sufficiently stable clinically to likely complete a 6 week study period