A couple of research articles about the effectiveness of hypnotherapy for weight loss...
Hypnosis is more than 30x more effective for weight loss
Cochrane et al conducted a study into hypnosis and weight loss with 60 female participants who were more than 20% overweight. He divided the participants into 2 groups, one who received hypnosis treatment, and a control group who did not receive any hypnosis.
Results showed that the group who received the hypnosis lost more weight. Each participant on average losing 17lbs over the course of the study, compared to women in the control group losing only 0.5lbs
Cochrane concluded that hypnosis can make your weight loss efforts more than 30x more effective.
Hypnosis enhances weight loss substantially over time
Kirsch and Irving compared 100s of studies where either cognitive-behavioral treatment, or cognitive-behavioral treatment with hypnosis too was used (with weight loss being the goal) in a meta analysis.
Their analysis showed that on average after the final follow-ups, the average weight loss was 6.03 lbs. (2.74 kg) without hypnosis (with just cognitive-behavioral treatment) and 14.88 lbs. (6.75 kg) when hypnosis was used.
They concluded that when hypnosis is used it is likely to lead to more than double the weight loss than if just cognitive-behavioral treatment alone was employed. They also summarized that the benefits of hypnosis increased substantially over time.