An intramuscular injectable formulation of risperidone long-acting formulation is under development with the aim to provide a sustained and stable level of risperidone during each treatment cycle given every 4 weeks. As this formulation is a long acting formulation, patients will be treated with a 1 mg immediate release (IR) formulation in the first week to confirm that the patient does not develop an allergic reaction to the drug before receiving the long acting injectable (LAI) formulation. Each patient receives a total of two injections. The first one is the immediate release formulation and the second one, which is given at least a week later, is the new long acting formulation. Eligible patients can stay on their previous oral medication, no tapering of medication is needed. Blood samples will be collected to determine the levels of risperidone and its major metabolite in plasma. Blood samples will be obtained by venipuncture (needle stick) immediately before (pre-dose) and at 5, 10, 20, 30, 45 min, 1, 1.5, 2, 4, 6, 8,12, 16, 24, 36, 48, 72, 96 hours after the 1st injection and at 2, 6, 12, 24, 48, 96 hrs, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 39, 43, 50, 57, 64, 71, 78, 85 days after the second injection of risperidone. Safety will assessed at visits throughout the study by monitoring scores of the Clinical Global Impression - Severity (CGI-S), scores of Extrapyramidal Symptom Rating Scale (ESRS), adverse events (AE), changes in clinical laboratory results, changes in physical examinations (PE), changes in electrocardiograms (ECGs), as well as patient and investigator evaluation of the injection site. The study has two periods. In the first period all patients will receive one injection of 1 mg risperidone IR (immediate release) solution (liquid). The follow-up duration is 96 hours. In the second period, patients will receive one injection of risperidone 4-week formulation of 75 mg. The follow-up duration is 85 days. The injections, administered in the muscle of the buttocks, will be given 7 to 14 days apart.