Probiotics are defined as viable microorganisms, which confer potential health benefits on the host when taken in proper amounts12. They are easily available, do not require a prescription, and are administered extensively for the relief of abdominal symptoms13. According to a recent systematic review and meta-analysis study, probiotics caused a modest improvement in the overall IBS symptoms14. The rationale for using probiotics for IBS is based on the assumption that they modify the composition of the intestinal microflora15 and regulate intestinal permeability by modulating the epithelial tight junctions16. Duolac7S (DUO), a probiotic mixture, contains 7 bacterial species including Bifidobacterium, Lactobacillus, and Streptococcus. Each of these bacterial species is reportedly beneficial for IBS17. DUO has been approved by the Korean Food and Drug Administration for restoring the ecological balance of intestinal microflora and improving intestinal symptoms18. Although DUO has been widely used as an over-the-counter product for IBS-related symptoms, no clinical trials have investigated the efficacy of DUO on IBS.