A 26-Wk Study to Assess Safety & Efficacy of Tenapanor for T/t of Chronic Idiopathic Constipation in Adults

Inclusion Criteria: 1. Patients that are ≥18 to ≤80 years old at the Screening visit (Visit 1). 2. Meet the Rome IV Diagnostic Criteria for functional constipation. 3. Females of non-childbearing potential, or agree to the use of an acceptable means of contraception for up to 30 days following the last dose of study medication. 4. Males must agree to use an appropriate method of barrier contraception (e.g., latex condom with a spermicidal agent) or have documented surgical sterilization. 5. Meets the entry criteria assessed during the Screening period. 6. Ability to understand and provide input on the assessments in the eDiary. 7. Provide written informed consent before the initiation of any study-specific procedures. 8. Must agree to have daily access to eDiary via a computer, tablet or smart phone Exclusion Criteria: 1. Loose or watery stool (BSFS of 6 or 7) in the absence of any medication that is known to affect stool consistency (i.e., prohibited medicine) for \>25% of bowel movements (BMs) during the 3 months prior to the Screening visit 2. Fecal impaction within 6 months prior to the Screening visit 3. Unexplained and clinically significant "alarm symptoms" including lower gastrointestinal (GI) bleeding, iron-deficiency anemia, weight loss or fever of unknown origin 4. Functional diarrhea as defined by Rome IV criteria 5. IBS with constipation (IBS-C), IBS with diarrhea (IBS-D), mixed IBS (IBS-M), or unsubtyped IBS as defined by Rome IV criteria 6. Diagnosis or treatment of any clinically symptomatic biochemical or structural abnormality of the GI tract within 6 months prior to the Screening visit, or active disease within 6 months prior to the Screening visit 7. Patient with central nervous system disorder (e.g., Parkinson's disease, spinal cord injury, or multiple sclerosis) 8. Use of prohibited medications defined by protocol 9. Patient has a history or current evidence of laxative abuse 10. Hepatic dysfunction (ALT \[SGPT\] or AST \[SGOT\] \>2.5 times the upper limit of normal) or renal impairment (serum creatinine \>2 mg/dL) 11. Positive HIV, hepatitis B or hepatitis C test at screening 12. Positive urinary drug test at Screening, with the exception of cannabis and cannabidiol (CBD) 13. Any evidence of or treatment of malignancy within 1 year prior to the Screening visit 14. Any surgery on the stomach, small intestine or colon; appendectomy or cholecystectomy are not excluded unless within 60 days prior to the Screening visit 15. Pregnant women or women planning to become pregnant during trial participation 16. A major psychiatric disorder (DSM-III-R or DSM-IV) including major depression or other psychoses that has required hospitalization in the last year prior to the Screening visit. 17. Alcohol or substance abuse within 1 year prior to the Screening visit 18. Participation in other clinical trials within 1 month prior to the Screening visit 19. Clinical evidence of significant cardiovascular, respiratory, renal, hepatic, gastrointestinal, hematologic, neurologic, psychiatric or any disease that may interfere with the patient's successful completion of the trial 20. Patient is involved in the conduct and/or administration of this trial as an Investigator, Sub-Investigator or other staff member, or is a first-degree family member or relative residing with one of the above persons involved in the trial 21. If, in the opinion of the Investigator, the patient is unable or unwilling to fulfill the requirements of the protocol or has a condition, which would render the results uninterpretable