This is a multicenter study including Montefiore Medical Center, Concorde Medical Group PLLC and the Medical Research Center of Connecticut/Yale-New Haven Hospital Langone Medical Center. Patients with IBS-D will be recruited from outpatient gastroenterology clinics at these institutions and referrals from the medical community.

FMT capsules and placebo capsules, provided by OpenBiome, Medford, MA, will be used for this study. Patients will be randomized to undergo FMT using fecal capsules (experimental group) or placebo capsules (control group) via a computer-generated program. All patients will cross-over into the alternate arm of the study at 12 weeks. Therefore, all patients enrolled will receive the experimental drug during the course of the study. Each patient will be enrolled in the study for a total of 6 months.

Intestinal microbiome analyses using DNA sequencing and non-cultivation-based approaches (16S DNA technology) will be performed in all patients in the experimental and control groups to assess stability of the microbiome over time.