Schaefert et al (2014) published a meta-analysis of the effectiveness of hypnosis as a treatment for IBS. Their analysis included the results of 8 randomised controlled trials. At the end of treatment hypnosis was found to be superior to control conditions in producing symptom relief, and in improving gastrointestinal symptoms. At follow-up, hypnosis was still more effective in producing symptom relief, but not on a measure of gastrointestinal symptoms. The authors conclude that hypnosis provided long-term adequate symptom relief in 54% of patients.

In a systematic review of the evidence Wilson and colleagues (2006) identified 20 studies and 2 case series with over half the studies identifying hypnotherapy as being effective in the management of IBS. However, they note that the majority of the trials are uncontrolled (i.e. with no placebo or control treatment arm) and so potentially overestimate the size of treatment effects because they do not take into account the placebo effect or regression to the mean. They recommend that a high-quality randomized placebo controlled trial be conducted.
Reviewing the same studies, and again acknowledging the limitations of the literature, Whitehead (2006) concludes that hypnosis demonstrates a consistent therapeutic impact on IBS - even for patients unresponsive to standard medical interventions.

In an early study Whorwell et al (1984) compared 15 patients randomly assigned to hypnotherapy with 15 patients assigned to psychotherapy plus placebo pills. All patients had severe unresponsive symptoms at the beginning of the study. The hypnosis group showed a greater improvement in well-being and bowel activity, and showed no relapses during the three-month follow-up.
Houghton et al (1996) compared 25 IBS patients given hypnosis with 25 patients treated with other methods and demonstrated greater benefits in the hypnosis group.
Palsson et al (2006) compared patients given a home treatment hypnosis programme with those given standard medical care. By three months 53% of the hypnosis group had improved compared to 26% of the controls.
Roberts et al (2006) conducted a randomized controlled trial of gut-directed hypnotherapy for IBS. In contrast to many other studies which have taken patients from secondary (specialist) care this study took patients from primary care, where the majority of IBS cases are seen. Patients either received standard care or standard care + gut-directed hypnotherapy. After 3 months, at the end of treatment, patients in the intervention group improved more than those in the control group on measures of pain and diarrhoea. However, the patients in standard medical care continued to improve and differences were not evident at 12 month follow-up except for reduced medication usage in the hypnosis group.