After eligibility is confirmed, the patient will return for baseline measurement of colonic transit to ensure that the transit profile is not greater than the mean transit profile of healthy controls in order to avoid a ceiling effect. A patient must have a geometric center of ≤2.65 at 24 hours, or ≤3.0 at 24 hours and ≤3.9 at 48 hours in order to be randomized to a 5-day Treatment Period of study medication. Eligible patients will receive oral study medication for 5 days during which colonic transit will be measured.