Background and objective: Clozapine and olanzapine are some of the most effective antipsychotic drugs, but unfortunately, both drugs induce weight gain and conveys a high degree of metabolic disturbances. The antipsychotic-induced side-effects cause a major clinical problem among patients diagnosed with schizophrenia receiving antipsychotic treatment. Limited effects have been demonstrated for counteracting the side-effects by the switch of antipsychotic therapy, non-pharmacological/behavioural interventions or adjunct pharmacological treatments. Semaglutide, a glucagon-like peptide-1 receptor agonist (GLP-1RA,) is approved for the treatment of type 2 diabetes worldwide. The objective of the study is to investigate effects of semaglutide once-weekly vs. semaglutide placebo once-weekly on the metabolic state in prediabetic or diabetic patients with schizophrenia, who have initiated treatment with clozapine or olanzapine. Methods and analysis: Trial design, intervention and participants: The study is a 26-week, double-blinded, randomized, parallel-group, placebo-controlled, good clinical practice (GCP)-monitored, clinical trial. 104 prediabetic or diabetic patients diagnosed with a schizophrenia, age 18 years and 65 years, who have initiated of clozapine- or olanzapine-treatment within 5 years will be included in the study. The patients will be randomized to receive blinded treatment in one of the two study arms; semaglutide once-weekly vs. semaglutide placebo. All participants who complete the 26 weeks of intervention, will be invited for a follow up visit 1.5 yeras after study completion. The primary endpoint is the change from baseline in glycated haemoglobin A1c (HbA1c). Secondary endpoints include change in body weight, hip and waist circumference, vitals, and plasma levels of insulin, glucose, C-peptid, insulin sensitivity, beta cell function, glucagon, liver function, lipid profile, incretin hormones, lipid profile, bone makers, body composition, bone density and proteomic analyses. Additional endpoints include alcohol, tobacco and drug use, food preferences, psychopathology, activity and quality of life.