Few supplements have been studied for IBS-D specifically, but enteric-coated peppermint oil can reduce abdominal pain and certain probiotic strains are effective for multi-symptom relief.[1][2]
In a 16-week placebo controlled trial (including an 8-week double-blind phase followed by an 8-week open label phase), polymethylsiloxane polyhydrate — an over-the-counter intestinal absorbent — improved abdominal pain, stool consistency, and the frequency and urgency of bowel movements in IBS-D.[3]