Tan, G., Hammond, D. C., & Gurrala, J. (2005). Hypnosis and Irritable Bowel Syndrome: A Review of Efficacy and Mechanism of Action. American Journal of Clinical Hypnosis, 47(3), 161–178.
doi:10.1080/00029157.2005.1040
“Irritable bowel syndrome (IBS) is a functional gastrointestinal disorder characterized by abdominal pain, distension, and an altered bowel habit for which no cause can be found. Despite its prevalence, there remains a 
significant lack of efficacious medical treatments for IBS to date. In this paper we reviewed a total of 14 published studies (N = 644) on the efficacy of hypnosis in treating IBS (8 with no control group and 6 with a control group). We concluded that hypnosis consistently produces significant results and improves the cardinal symptoms of IBS in the majority of patients, as well as positively affecting non-colonic symptoms. When evaluated according to the efficacy guidelines of the Clinical Psychology Division of American Psychological Association, the use of hypnosis with IBS qualifies for the highest level of acceptance as being both efficacious and specific. In reviewing the research on the mechanism of action as to how hypnosis works to reduce symptoms of IBS, some evidence was found to support both physiological and psychological mechanisms of action.” Link here.