Study to Compare Clozapine vs Treatment as Usual in People With Intellectual Disability & Treatment-resistant Psychosis

Inclusion Criteria: * Subjects aged between 16 and 55 years * Diagnosis of intellectual disability according to the Diagnostic and Statistical Manual of Mental Disorders Diagnostic and Statistical Manual of Mental Disorders (DSM-5) (confirmed by a Intelligence Quotient (IQ) Score between 35 and 70 in the Kaufman test) * Diagnosis of psychosis according to the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) (confirmed by clinical interview). * Treatment Resistant to antipsychotic drugs except clozapine. * Behavioural disturbances and self-injurious behaviour over the last 6 months. * Written informed consent of patients or legal representative. * Negative pregnancy test (if apply) Exclusion Criteria: * Leukocytes \< 3500/mm3 and neutrophils \< 2000/mm3. * Hypersensitivity to clozapine or excipients. * Myeloproliferative disorders * Uncontrolled epilepsy in the last 2 years. * Paralytic ileus in the last 3 months. * Diagnosis of an autism spectrum disorder * Pregnancy and breastfeeding * Any diseases with clozapine contraindicated. * Any uncontrolled serious condition * Need of treatment with more than one antipsychotic drug or electroconvulsive therapy * Treatment with quinolones, drugs that cause agranulocytosis or drugs that affect the cytochrome P-450 enzymes. * Risk of suicide based on the Columbia-Suicide Severity Rating Scale