Stratification and Treatment in Early Psychosis Study -ASSIST

The purpose of this trial is: * To investigate whether the response to clozapine treatment can be enhanced by adding cannabidiol (CBD), compared to placebo, in treatment resistant psychosis patients. * To confirm the safety of CBD in people with psychosis. The trial is a randomised, double-blind, placebo-controlled, multi-centre, international, clinical trial. Individuals with a diagnosis of treatment resistant psychosis in their illness who have had a suboptimal or no response to clozapine treatment will be recruited. These patients are randomised to treatment with oral CBD 500mg twice daily, or a matching placebo for 12 weeks, in addition to clozapine, which is standard care treatment for this population. By using a battery of clinical outcome assessments, the trial will assess several optional biomarkers to predict clinical outcomes and response to treatment with CBD. Biomarkers are being assessed as an exploratory outcome measure. Participants will be invited to provide additional blood samples, stool samples, and complete neuroimaging assessments.

Inclusion Criteria: Participation in the trial requires meeting all of the following inclusion criteria: 1. 16 to 50 years of age (inclusive) who are willing and able to provide written informed consent/assent (country specific requirements apply). 2. The patient has been treated with clozapine for at least 8 weeks with a clozapine plasma concentration of at least 0.35 mg/L at screening. If a patient has a lower plasma concentration at screening, the patient can enter the trial at a later date once their plasma concentration is above the threshold\*. 3. Within 10 years of first antipsychotic treatment prescribed for psychosis. 4. The patient meets DSM-5 criteria for schizophrenia, schizoaffective disorder or schizophreniform disorder, as confirmed through the SCID-5-RV. The patient does not meet modified Andreasen et al (2005) criteria for symptomatic remission cross-sectionally (time requirement does not apply), i.e., a severity rating score of no more than mild (score of \