Irritable bowel syndrome (IBS), a common chronic gastrointestinal disorder characterized by abdominal pain and alteration of bowel habits in the absence of structural abnormality, has a prevalence of approximately 15% in western populations1,2. Patients with IBS can be classified by their predominant bowel habits: diarrhea-predominant IBS (D-IBS), constipation-predominant IBS, or IBS with alternating bowel movements3. Although smooth-muscle relaxants, bulking agents, and anti-diarrheal agents are commonly used as conventional IBS treatments, many IBS patients turn to alternative treatments because of the lack of therapeutic advantages of these treatments4. Therefore, the development of a new therapy is necessary for IBS patients.

Gwakhyangjeonggisan (GJS; Kkako-shoki-san in Kampo Medicine; Huoxiang-zhengqi-san in Traditional Chinese Medicine), which was recorded originally in the famous ancient herbal formula literature "Formularies of the Bureau of people's Welfare Pharmacies", consists of 13 common crude herbs. GJS contains the chemical ingredients naringin, hesperidin, thymol, honokiol and magnolol5. GJS has been shown to protect intestinal barrier function6, contract the colonic muscle7, and regulate infectious diarrhea8 in vivo. In traditional Korean medicine, this herbal formula has long been used for relieving abdominal pain, diarrhea, and vomiting as an over-the-counter or prescribed medicine9-11. However, there have been no clinical trials to investigate the efficacy of GJS in IBS.

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.

Recently, several herbal formulas and probiotics have been simultaneously administered to IBS patients for relieving abdominal symptoms19. In addition to lack of clinical evidence for the use of GJS or DUO, the efficacy of their combined treatment for IBS has not been elucidated. Thus, there is a need to evaluate the safety and efficacy of administration of GJS, DUO, or a combination therapy as a frequently used treatment for IBS. In the current trial, we identified the safety and efficacy of GJS, DUO, or a combination therapy on D-IBS by evaluating IBS symptoms and quality of life. To investigate their mechanisms in humans, intestinal permeability and composition of intestinal microbiota were assessed.