This is an open-label, fixed sequence, multiple dose design. Following screening evaluations, subjects will check-in to the clinical research unit. After confirmation of continuation criteria, subjects will undergo an oral glucose tolerance test (oGTT), mixed meal tolerance test (MMTT) and spirulina breath test (GEBT). After these assessments are completed, subjects will have their prior antipsychotic (PA) or any other medication with psychotropic propensity washed out (dependent on their antipsychotic elimination half-life).~Subjects will undergo SEP-363856 titration schedule, followed by the oGTT, MMTT and GEBT tests during the SEP-363856 Stable Dose Period.~Subjects will be stabilized on their prior antipsychotic, discharged from the clinical research unit, and return to the unit for the follow-up visit 7 + 2 days after discharge.
A Clinical Study that will look at an investigational medication, SEP-363856 (called study medication) in patients with schizophrenia and assess whether it changes:~how the body processes (uses) glucose (blood sugar)~how much insulin the pancreas can make. Insulin is a hormone that lowers blood sugar levels in the body.~The information from this study will help to understand any effect the study medication may have on how the body uses and stores glucose.~This study is accepting both male and female subjects. It will be held in approximately 6 locations in the United States. Participation could last up to 12 weeks.