Male or female subjects aged ≥ 18 years suffering from a primary psychotic disorder treated with two antipsychotic drugs for at least 30 days (excluding "prn" or "as needed" antipsychotic prescriptions) will be eligible to participate in this study. Subjects with a history of treatment with a depot antipsychotic within 6-months of enrollment will be excluded form the study unless the depot antipsychotic is considered the "main" antipsychotic drug in this study. The "main" antipsychotic drug will be determined by the treating physician, except for the case of clozapine which will be considered the 'main' antipsychotic. Eligible subjects will be randomly allocated to two groups: Group 1 will continue taking the concomitant antipsychotic drug; Group 2 will be allocated to placebo in place of the concomitant antipsychotic drug. The dose of the "main" antipsychotic drug will remain open and flexible at the discretion of the attending physician. The dose of the "second" antipsychotic drug will remain fixed throughout the study period.