A 12-Week Study With a 4-Week Randomized Withdrawal Period to Evaluate the Efficacy and Safety of Tenapanor for the Treatment of IBS-C

- 18 to 75 years old

- Females must be of non-childbearing potential; If of child-bearing potential, must have negative pregnancy test and confirm the use of one of the appropriate means of contraception.

- Males must agree to use an appropriate method of barrier contraception or have documented surgical sterilization

- Subject meets definition of IBS-C using Rome III Criteria for the Diagnosis of IBS

- A colonoscopy based on AGA guidelines; every 10 years at ≥ 50 years old, or the occurrence of any warning signs

- Functional diarrhea as defined by Rome III criteria

- IBS with diarrhea (IBS-D), mixed IBS (IBS-M), or unsubtyped IBS as defined by Rome III criteria

- Diagnosis or treatment of any clinically symptomatic biochemical or structural abnormality of the GI tract within 6 months prior to screening, or active disease within 6 months prior to screening; including but not limited to cancer, inflammatory bowel disease, diverticulitis, duodenal ulcer, erosive esophagitis, gastric ulcer, pancreatitis (within 12 months of screening), cholelithiasis, amyloidosis, ileus, non-controlled GERD, gastrointestinal obstruction, ischemic colitis or carcinoid syndrome.

- Subject has a history or current evidence of laxative abuse (in the clinical judgment of the physician)

- Hepatic dysfunction (ALT [SGPT] or AST [SGOT] >2.5 times the upper limit of normal) or renal impairment (serum creatinine > 2mg/dL)

- Any evidence of or treatment of malignancy (other than localized basal cell, squamous cell skin cancer or cancer in situ that has been resected) within the previous year

- Any surgery on the stomach, small intestine or colon, excluding appendectomy or cholecystectomy (unless within 60 days of screening visit)