Patients will receive either lubiprostone 24 micrograms (mcg) orally twice a day for 8 weeks or placebo. Primary and secondary endpoints will be the number of SBMs/week and colonic transit time as measured by the Smartpill capsule, respectively. The number of SBMs/week will be evaluated at 0, 2, 4 and 8 weeks after initiation of therapy. The investigators will over-sample African American patients to achieve approximately 50% enrollment of this group. In a subanalysis, the investigators will assess response to treatment between the general population and African Americans.