The Plecanatide Irritable Bowel Syndrome With Constipation Study (IBS-C)

Inclusion Criteria: * Male or female aged 18-75, inclusive * Body Mass Index = 18-35 kg/m2, inclusive * Meets modified Rome III criteria for irritable bowel syndrome with constipation which includes abdominal pain or discomfort for at least 3 days/month in the last 3 months with symptom onset for at least 6 months. * Less than 3 CSBMs and less than 6 SBMs per week during the last 3 months. * Hard or lumpy stools ≥ 25 % of defecations * Patient has average abdominal pain intensity scores ≥ 3 (scale 0-10)for the combined 2 week pre-treatment period * Patient is willing to discontinue use of supplemental fiber, laxatives, prescription and nonprescription medications, herbal or dietary supplements intended to treat constipation during the screening, pre-treatment, treatment and 2-week post-treatment periods * Willing to maintain a stable diet during the study. * Patients with hemorrhoids and/or diverticulosis (NOT diverticulitis) CAN be entered into the study. Exclusion Criteria: * Loose stool (mushy) or watery stool in the absence of any laxative or prohibited medicine for \> 25% of BMs during the 3 months prior to screening visit OR during the 14 day pre-treatment period * Patient has diarrhea-predominant or mixed ( diarrhea and constipation cycling or diarrhea and normal cycling) IBS. * Active peptic ulcer disease not adequately treated or not stable * History of cathartic colon, laxative, enema abuse, or ischemic colitis. * Fecal impaction within 3 months of screening * Patient has had /has any: structural abnormality of the GI tract or gastric bypass surgery, pelvic floor dysfunction, pseudo-obstruction, active infectious gastritis, diverticulitis, anal fissures or any disease or condition that can affect GI motility or defecation or can be associated with abdominal pain * Unexplained and clinically significant "alarm symptoms" including lower GI bleeding, iron-deficiency anemia, weight loss or systemic signs of infection or colitis. * Major surgery within 60 days of screening.