This study has been designed as a multicenter, randomized, double-blind, placebo controlled parallel dose group trial. The study will be open to both women and men. At screening the patient will be assessed to see if they meet the Rome III criteria for IBS-D. After screening, patients will undergo a 14-day run-in period evaluation, during which eligibility to be randomized to drug or placebo will be determined. Patients will complete daily telephone diaries during the run-in and double-blind phases of the study. Patients will be evaluated at regularly scheduled clinic visits during double-blind phase of the study.