Study Of An FDA-approved Drug As Additional Therapy In Patients With Schizophrenia

Inclusion criteria: * Diagnosed with schizophrenia. * Exhibits persistent positive symptoms that have persisted for a minimum of 3 months prior to screening. * Continuously taking 1-2 specific atypical antipsychotics for a minimum of 3 months and must be on a stable dose for at least 1 month prior to entering the study. * Participant or a legal guardian is able to understand and sign the consent form. Exclusion criteria: * PANSS (Positive and Negative Syndrome Score) total score increases or decreases by more than 20% between the Screening and Baseline visits. * Predominant Axis I disorder other than schizophrenia within 6 months prior to screening. * History of clinically significant or unstable medical disorder or treatment that would interfere with the study. * History of autistic disorder or another pervasive developmental disorder, organic brain disease, dementia, stroke, epilepsy or a history of seizures requiring treatment (this does not include febrile seizures as a child), or those who have suffered a traumatic head injury. * Taking psychotropic or primarily centrally active medication at screening. * Use of antidepressant medications or mood stabilizers within 1 month of screening.