 Large scale clinical trials have demonstrated that a substantial proportion of patients with psychotic disorders, such as schizophrenia and bipolar disorder, discontinue their antipsychotic medications due to either lack of efficacy or intolerable side effects, such as extrapyramidal symptoms EPS and weight gain. In clinical practice, it is essentially a trial and error process in deciding the best antipsychotic drug to start or switch to after a failed trial as there is little empirical data available to guide clinicians in drug selection. One promising tool, which can potentially provide valuable information to help guide medication management, is pharmacogenetic testing of certain genetic variants that are associated with psychiatric drug response. However, most pharmacogenetic studies to date have been retrospective, and there is no prospective clinical trial evaluating the clinical utility of pharmacogenetic testing in guiding clinical practice. Furthermore, it is unknown whether pharmacogenetic testing is cost effective. 