Atypical antipsychotics have become widely used since the introduction of clozapine in the U.S. in 1990. It is generally accepted that atypical antipsychotics have less side effects than older drugs but they can cause weight gain, insulin resistance and lipid abnormalities, all of which can have serious effects on health. One theory is that the atypical antipsychotics stimulate the production of a hormone called ghrelin which stimulates food intake and fat accumulation. A 2-week trial on healthy participants in the U.S. divided them up into three groups taking olanzapine, risperidone and a placebo respectively. The olanzapine group put on the most weight, followed by the risperidone group while the group taking the placebo stayed more or less the same. However, there was no significant difference in ghrelin levels between the groups and the participants' ghrelin levels did not change over the course of the study suggesting that increased levels of the hormone were not to blame for the weight gain.
Roerig, James L. ... [et al] - A comparison of the effects of olanzapine and risperidone versus placebo on ghrelin plasma levels Journal of Clinical Psychopharmacology February 2008, 28(1), 21-26