This clinical trial will prove whether a large number of people with intellectual disability and treatment-resistant psychosis could benefit from the use of clozapine. Benefit will mean a measurable significant improvement in subjects' clinical response and quality of life.

Randomized, open-label, multicenter phase II clinical trial that seeks to evaluate the safety and efficacy of clozapine versus standard clinical treatment in patients between the ages of 16 and 55 with intellectual disability and treatment-resistant psychosis.

Clozapine is the most effective antipsychotic for patients with non-affective psychosis who do not respond to other first and second generation antipsychotic treatments. In addition, it has been shown to be very effective in another series of clinical situations such as hostility and aggressiveness, polydipsia and in behavioral disorders and psychosis, frequent situations in people with intellectual disabilities.

The primary objective is to assess the efficacy and safety of clozapine versus standard clinical practice treatment in patients with intellectual disability and resistant psychotic disorder, as measured by change in Clinical Global Impression: Clinical Global Impression-Schizophrenia scale (ICG-SCH) global score over trial visits.

The study determines to reach a sample size of 114 patients distributed among the 25 active centers. Randomization is 1:1 and consists of 6 visits to the center spread over 12 months. At each visit, the patient will undergo a physical examination and sample collection, along with a clinical and cognitive evaluation using the scales provided in accordance with the clinical guidelines.

In addition, if the patient falls into the experimental arm (Clozapine), it is necessary to collect a blood sample weekly during the first 18 weeks and biweekly until completing the 12 months of the study, so that the medical team has special control in the analytical parameters.

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