Stratification and Treatment in Early Psychosis Study - ENHANCE

The purpose of this study is: * To investigate whether the response to antipsychotic treatment can be enhanced by adding cannabidiol (CBD) to the existing treatment, compared to placebo, in participants with a first episode of psychosis, who have had a suboptimal or no response to their first antipsychotic treatment. * To confirm the safety of CBD in people with psychosis. The study is a randomized, double-blind, placebo-controlled, multi-centre, clinical trial. Individuals with a diagnosis of first-episode psychosis, who have had a suboptimal or no response to their first antipsychotic treatment will be recruited. These participants are randomised to treatment with CBD oral solution 500mg twice daily, or a matching placebo for 6 weeks, as an adjunct to their existing antipsychotic treatment. By using a battery of clinical outcome assessments, the trial will also assess several biomarkers to determine if they can be used 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 blood and stool samples, and may be asked to complete neuroimaging assessments at certain eligible sites.