Olanzapine is a potent antagonist of serotonin and histamine 1 receptors. Significant and potentially additive weight gain is associated with blockade of serotonin 2C and Histamine 1 receptors. In addition, serotonin and its receptors are significantly involved in the regulation of gastrointestinal (GI) - related hormone secretion. Animal studies suggest significant involvement of other serotonin receptors in the regulation of appetite, satiety, and GI-related hormones. However, the interplay of selective activation or inhibition of these receptor subtypes is complex, and difficult to distinguish from effects on activity and anxiety. Suffice it to say that activation or blockade of these receptors have differential effects on appetite, satiety, metabolic activity, as well as leptin, secretin, insulin, glucagon, ghrelin, neuropeptide Y, and cholecystokinin.