Pain and the disease that causes it are two different targets, and hypnosis reaches only one of them. It does nothing to the cancer itself. What it can do, with reasonable support from clinical research, is change how much pain a person feels and how hard the body braces against it.
This matters because cancer pain has more than one channel. There is the raw sensory signal, and there is the layer built on top of it, the fear, the tension, the full attention a person gives to a pain they cannot ignore. Hypnosis works mostly on that second layer and on how the brain processes the signal, which is why it can take the edge off without touching the underlying cause.
The evidence is strongest for procedural pain, the biopsies, injections, and scans that punctuate cancer treatment, and there is supportive evidence for chronic cancer pain as well. The Society for Integrative Oncology and the American Society of Clinical Oncology, in their joint guideline on pain management, include hypnosis among the approaches that may be offered for procedural pain in cancer care. Beyond pain, it has been studied for nausea, fatigue, hot flashes, and sleep.
The limits need stating as plainly as the benefits. Hypnosis supplements pain medication and oncology care, it does not replace either. The quality of evidence is moderate rather than conclusive, many studies are small, and the support thins out in advanced disease. Not everyone responds, and no one should trade a prescribed plan for a relaxation session.
For someone living with cancer, that is a narrow but genuine kind of help. Not fewer tumors, but sometimes less suffering around the treatment, and a little more control over a body that can feel like it has stopped listening.