Researchers from the University of Connecticut at Storrs ran an analysis of the effect of adding hypnosis to a behavioral program for weight reduction.
Averaged across post treatment and follow-up assessment periods from all the studies, the mean weight loss was 6.00 lbs. (2.72 kg) without hypnosis and 11.83 lbs. (5.37 kg) with hypnosis.
The mean effect size of this difference was 0.66 SD (standard deviations) Correlational analyses indicated that the benefits of hypnosis increased substantially over time (r = .74).
Citation: Kirsch I. Hypnotic enhancement of cognitive-behavioral weight loss treatments--another meta-reanalysis. Journal of Consulting Clinical Psychology. 1996 Jun; 64(3): pages 517-519.