Inclusion Criteria:

• Patient must provide written or verbal informed assent and the parent/guardian/LAR must provide written informed consent before the initiation of any study-specific procedures.
• Patient is a male or female outpatient, 12 to 17 years of age inclusive, at the time the patient provides assent for the study and parent/guardian/LAR has provided signed consent.
• Patient is able to read and understand the assessments in the eDiary.
• Female patients of childbearing potential must have a negative serum pregnancy test at Visit 1 (screening) and a negative urine pregnancy test at Visit 3 (randomization) prior to dosing.
• Female patients who have had their first menstrual period and are sexually active must agree to use a reliable form of contraception. Reliable contraception is defined as:
  • Hormonal contraception (eg, oral contraceptive, contraceptive implant, or injectable hormonal contraceptive).
  • Double-barrier method (eg, condom plus intrauterine device, diaphragm plus spermicide).
• Patient has a diagnosis of IBS-D as defined by the modified Rome IV child/adolescent criteria*: Must include all of the following:
  •  Abdominal pain at least 4 days per month over at least 2 months associated with one or more of the following:
    •  Related to defecation
    •  A change in frequency of stool
    •  A change in form (appearance) of stool
  • After appropriate evaluation, the symptoms cannot be fully explained by another medical condition
  • Patient has predominantly diarrheal stool symptoms defined as Bristol stool types 6 or 7 for more than 25% of bowel movements and Bristol stool types 1 or 2 for less than 25% of bowel movements that occur in the absence of laxatives
• All criteria fulfilled for at least 2 months prior to Visit 1 (screening). 
• Patient has been compliant with the eDiary by completing both the morning and evening assessments for at least 8 out of the 14 days immediately preceding Visit 3 (randomization).
• Patient has an average daytime abdominal pain scoreless than or equal to 2.0 over the 2 weeks prior to randomization. 
• Patient has at least 1 daytime bowel movement with a consistency of Type 6 or Type 7 on the pediatric Bristol Stool Form Scale (p-BSFS) on at least 2 days per week during the 2 weeks prior to randomization that occurs in the absence of laxatives. 
• Patient has no clinically significant findings on a physical examination, vital sign assessment, electrocardiogram (ECG), and clinical laboratory tests (clinical chemistry panel, complete blood count, urine drug screen, urinalysis) after providing informed assent and after written consent is obtained, but before receiving the first dose of study treatment. (A central laboratory will be used to evaluate all urine [except urine pregnancy tests] and blood samples and will utilize reference ranges specific to a patient's age and gender. ECGs will be performed and electronically transmitted to a central ECG laboratory for analysis by a pediatric cardiologist in accordance with the instructions provided by the central ECG laboratory. The Investigator will determine if a particular finding is clinically significant. [In making this determination, the Investigator will consider whether the particular finding could represent a condition that would exclude the patient from the study, could represent a safety concern if the patient participates in the study, or could confound the study-specific assessments of safety or efficacy.])

Exclusion Criteria:

• Patient has no gallbladder, (ie, agenesis of the gallbladder or cholecystectomy). 
• Patient has had any of the following surgeries:
  • Any abdominal surgery within the 3 months prior to Screening; or
  • A history of major gastric, hepatic, pancreatic, or intestinal surgery.  (Note: appendectomy, hemorrhoidectomy, or polypectomy greater than 3 months post-surgery are allowed. For the purposes of this study, laparoscopic surgeries without complication are considered minor and non-exclusionary, provided the condition for which the surgery was performed was not exclusionary.)
• Patient has a history of chronic or severe constipation or sequelae from constipation, or known or suspected mechanical GI obstruction or pseudo obstruction. 
• Patient has a history or current diagnosis of constipation with encopresis. 
• Patient meets the child/adolescent Rome IV criteria of IBS with constipation, IBS with constipation and diarrhea (mixed), unspecified IBS, or Patient has a history of intestinal obstruction, stricture, toxic megacolon, GI perforation, fecal impaction, gastric banding, bariatric surgery, adhesions, ischemic colitis, or impaired intestinal circulation. 
• Patient has a documented history of hepatic impairment as defined by Child-Pugh Classification Grade A, B or C. 
• Patient has a history or current diagnosis of inflammatory or immune-mediated GI disorders including inflammatory bowel disease (ie, Crohn's disease, ulcerative colitis, microscopic colitis). 
• Patient has celiac disease, or a positive serological test for celiac disease and the condition has not been ruled out by endoscopic biopsy.
• Patient has any congenital and/or acquired malabsorption syndrome (eg, Shwachman-Diamond syndrome). 
• Patient has a history of a microbiologically documented (ie, stool culture or medical history) GI infection within 3 months prior to Screening. 
• Patient has a known lactose or fructose intolerance that is associated with diarrhea, abdominal pain or discomfort, and that could confound assessments in the study. 
• Patient has a history of diverticulitis within 3 months prior to Screening.