The term 'alternative' or 'complementary' diet refers to any modification to a normal diet that claims to cure or treat cancer. These diets may be advocated for use in conjuction with, or instead of, conventional anti‐cancer treatment. Although treated with scepticism by the dietetic and medical professions, they are popular with patients. Most of the dietary therapies share the same common philosphy and strict dietary principles. In general, these diets appears to be more harmful than beneficial, in nutritional terms at least, Some cancer patients do seem to drive some benefit from these therapies. This article proposes the theory that this benefit is psychological rather than nutritional, a placebo effect. The psychological theories of expectancy effects, cognitive dissonance, anxiety reduction and the therapist effect are explored in order to support this theory. The author proposes that sound nutritional advice, given with attention to the psychological theories stated, could achieve the same if not greater benefit, and proposes further research in this field.