There are two Cochrane Reviews on hypnotherapy.
The review about hypnotherapy for treatment of irritable bowel syndrome states
The quality of the included trials was
inadequate to allow any conclusion
about the efficacy of hypnotherapy for
irritable bowel syndrome. More
research with high quality trials is
The review about hypnotherapy for smoking cessation states
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
It is not really possible to use a placebo treatment for hypnotherapy, as it is pretty obvious whether you are being hypnotized or not. Most studies compare hypnotherapy to other methods used to treat the studied conditions, some compare to no intervention.
The studies for smoking cessation were very heterogenuous which complicated the analysis. The most convincing of the studies selected for the review1 consisted of 281 participants and found hypnotherapy comparable in effectiveness to behavioural counselling, which is a known effective treatment.
About the studies included in the review about treatment of irritable bowel syndrom the authors state
The results of the included studies
need to be interpreted with caution
due to the small size and
methodological ﬂaws of the included
I would summarize those results as, "At the moment, we don't know if hypnotherapy works."
 Carmody TP, Duncan C, Simon JA, Solkowitz S, Huggins J, Lee
Set al. Hypnosis for smoking cessation: a randomized trial.
Nicotine & Tobacco Research 2008;10(5):811–8.