Study to Examine the Effect of Betahistine on Body Weight Gain Due to Olanzapine Treatment

This is a randomized, double-blind, placebo-controlled, multicenter, multinational study. Approximately 78 subjects (39 per treatment group) will be randomized into this 16 week study. A screening visit will be used to determine subject suitability for inclusion in the trial. Within 7 days of the screening visit, subjects who meet all inclusion criteria and none of the exclusion criteria will be randomly assigned to 1 of the following 2 treatment groups: * Olanzapine OD plus betahistine 24 mg BID (48 mg/day total), * Olanzapine OD plus matching placebo BID. Double-blind treatment will continue for 16 weeks. During this period, olanzapine dosage will be determined according to the discretion of the treating physician. In addition, 5 study visits (at 2, 4, 8, 12, and 16 weeks) will take place. Study medication (betahistine or matching placebo) will be administered BID (in the morning and together with olanzapine in the evening). The primary statistical hypothesis to be tested is that the mean change from Baseline to Week 16 will be different between the treatment and placebo groups