This article reviews the effectiveness of Kampo (traditional Japanese herbal medicine) in the treatment of functional gastrointestinal disorders, especially functional dyspepsia (FD) and irritable bowel syndrome (IBS). The results of four randomized, controlled trials (RCTs) suggested the usefulness of rikkunshito in relieving the subjective symptoms of patients with FD. Rikkunshito significantly improved not only gastric symptoms, such as epigastiric discomfort, but also extra-gastric symptoms, such as general fatigue, when compared with control drugs. The therapeutic effects of rikkunshito were more evident when it was prescribed to patients with “kyosho”, i.e., low energy. Two RCTs suggested the efficacy of keishikashakuyakuto for IBS.

Basic research studies have demonstrated that these Kampo medicines have multiple sites of action to improve subjective symptoms. For example, rikkunshito improves gastric motility dysfunction, including impaired adaptive relaxation and delayed gastric emptying, gastric hypersensitivity, and anorexia via facilitation of ghrelin secretion. It also exhibits anti-stress effects, i.e., it attenuates stress-induced exacerbation of gastric sensation and anorexia, as well as the hypothalamic-pituitary-adrenocortical axis and sympathetic activation. Keishikashakuyakuto exhibited not only an antispasmodic effect on intestinal smooth muscle, but also antidepressant-like effects. Case series suggest that other Kampo prescriptions are also effective for FD and IBS. However, further studies are necessary to evaluate their efficacy.