I love the Cochrane Collaboration. They provide meta-analyses of health-related studies. This involves critically examining all of the relevant studies on a topic, and grouping the data together to give more statistical power, and making the results freely available.
They have looked into hypnotherapy, and found the evidence to be rather limited when it comes to smoking and obesity.
Looking at smoking first:
- Barnes J, Dong CY, McRobbie H, Walker N, Mehta M, Stead LF. Hypnotherapy for smoking cessation. Cochrane Database of Systematic Reviews 2010, Issue 10. Art. No.: CD001008. DOI: 10.1002/14651858.CD001008.pub2
Eleven studies compared hypnotherapy with 18 different control interventions. There was significant heterogeneity between the results of the individual studies, with conflicting results for the effectiveness of hypnotherapy compared to no treatment, or to advice, or psychological treatment. We did not attempt to calculate pooled risk ratios for the overall effect of hypnotherapy. There was no evidence of a greater effect of hypnotherapy when compared to rapid smoking or psychological treatment. Direct comparisons of hypnotherapy with cessation treatments considered to be effective had confidence intervals that were too wide to infer equivalence.
We have not shown that hypnotherapy has a greater effect on six-month quit rates than other interventions or no treatment. There is not enough evidence to show whether hypnotherapy could be as effective as counselling treatment. The effects of hypnotherapy on smoking cessation claimed by uncontrolled studies were not confirmed by analysis of randomized controlled trials.
So, despite looking at 11 different studies, they were unable to show that hypnotherapy worked.
Unfortunately, they were unable to pool all the data together to figure out if there was a statistical difference. (The full article explains that
Studies were very diverse so could not be combined in a single meta-analysis. Pooling small groups of relatively comparable studies did not provide any additional evidence for a specific effect of hypnotherapy.
None of this is strong enough to say "hypnotherapy doesn't work" or "hypnotherapy is worse than [whichever control treatment is chosen]", but it is suggestive that there's not enough difference to be worth spending your money.
A different study looks at weightloss (for the overweight and obese, who, presumably are the target audience of the adverts.)
They looked at a number of different techniques psychological intervention techniques, some of which were significantly improved outcomes.
When it came to hypnotherapy, however, the evidence was not clear. They only found one suitable study:
In this study of 54 participants in the hypnotherapy group lost 7.9 kg compared with participants in the placebo group who lost 0.2 kg by six months follow-up.
That wasn't enough to convince them, although it was suggestive (especially in the context of the other techniques):
There was not enough evidence to reach a conclusion about other psychological forms of therapy, such as relaxation therapy and hypnotherapy, however the evidence that is available suggests that these therapies may also be successful in improving weight loss.
Hypnotherapy might be helpful for smokers and obesity. There isn't enough quality evidence for obesity, and there isn't enough consistency between different studies for smoking, to be entirely sure.
In the meantime, there are more trustworthy intervention techniques for obesity, and plenty of competing techniques for stopping smoking.
There is little reason to trust adverts from hypnotherapists promising good results.