During the 26-week double-blind treatment period, subjects will record daily assessments including: frequency and timing of bowel movements; sensation of complete bowel emptying; consistency of bowel movements; degree of straining, worst abdominal pain, abdominal discomfort, abdominal bloating, abdominal fullness and abdominal cramping; and use and timing of rescue medication. Subjects will also record weekly assessments including: adequate relief of IBS symptoms, degree of relief of IBS symptoms, IBS severity, and constipation severity.