This is a multi-center, Phase 2, randomized, double-blind, placebo controlled study of RDX5791 in subjects with IBS-C. Patients who are 18 to 75 years old, meeting the definition of IBS-C as defined by the Rome III Criteria for the Diagnosis of IBS will undergo a battery of screening procedures to determine eligibility for the trial.

The study will consist of a 2-week treatment-free screening period, a 4-week blinded treatment period, and a 2-week treatment-free follow-up period.

Inclusion Criteria:

• Subject meets Rome III criteria for IBS-C
• If > 50 years old, colonoscopy evaluation within 10 years
• All ages, negative colonoscopy if any "warning symptoms"
• Active disease during 2-week screening period
• Compliant with IVRS

Exclusion Criteria:

• Subjects has IBS-D, IBS-M or un-subtyped IBS according to Rome III criteria
• Diagnosis or treatment of any clinically symptomatic biochemical or structural abnormality of the GI tract
• Use of medications that are known to affect stool consistency
• Subject has any protocol-excluded or clinically significant medical or surgical history that could confound the study assessments or prevent completion of study