rationalskepticism.org

The NHS wrote:Hypnotherapy is a type of complementary therapy that uses hypnosis, an altered state of consciousness.
Hypnosis is widely promoted as a treatment for various long-term conditions and for breaking certain habits. This is despite the fact there is no strong evidence to support these uses.
However, hypnosis does seem to have an effect, though scientists disagree about how it works. Some experts see it as a relaxation technique that uses the power of suggestion and relies on the placebo effect.
When might it be helpful?

Irritable bowel syndrome

Some research studies have suggested that hypnotherapy may help with the symptoms of irritable bowel syndrome (IBS), such as abdominal pain.
These studies do not provide any strong evidence for its effectiveness, but the National Institute for Health and Clinical Excellence (NICE) has nevertheless recognised hypnotherapy as a possible treatment for IBS (PDF, 191.4kb) in those who haven't responded to other treatments.

The APA wrote:
Is there evidence that hypnosis works?
Yes. While there are plenty of examples in the scientific literature attesting to the usefulness of clinical hypnosis, a study published in the journal Gut is noteworthy. The study involved 204 people suffering from Irritable Bowel Syndrome (IBS). Treatment consisted of 12 weekly sessions of hypnosis (lasting about one hour each). Fifty-eight percent of the men and 75 percent of the women reported significant symptom relief immediately after finishing treatment. More than 80 percent of those who reported initial relief were still improved up to six years later. Fewer than 10 percent of the participants tried other treatments after hypnotherapy. (Gut, November 2003).