There is an unmet treatment need for people at clinical high risk (CHR) for psychosis 1 . As only a minority of them go on to develop a psychotic disorder, interventions need to be particularly safe and well tolerated.

Cannabidiol (CBD), a non‐intoxicating constituent of cannabis, has potential anxiolytic and antipsychotic properties 2 and a good safety profile. In two out of three clinical trials in patients with established psychosis, evidence of its antipsychotic efficacy has been reported 3 , 4 , 5 . However, there have not been trials of a period of treatment with CBD in CHR individuals. We assessed the clinical effects of a course of CBD treatment in people with a CHR state following a protocol approved by the National Research Ethics Service Committee London (Camberwell, St. Giles) (ISRCTN46322781).

The study was conducted on antipsychotic‐naïve subjects attending early detection services in the UK who met one or more criteria for CHR state for psychosis: a) attenuated psychotic symptoms; b) brief limited intermittent psychosis (i.e., a psychotic episode lasting <1 week which remitted without treatment); c) recent functional decline and either schizotypal personality disorder or first‐degree relative with psychosis. Key exclusion criteria were history of previous psychotic disorder or manic episode, neurological disorder, or current DSM‐IV diagnosis of substance dependence.

Thirty‐three subjects were recruited after they provided written informed consent. They were advised to refrain from using cannabis for 96 hours, alcohol for a minimum of 24 hours, nicotine for 6 hours, and any other recreational drugs for 2 weeks before entering the study, and to continue to refrain from using cannabis or other recreational drugs during the course of the study. Baseline assessments included the Comprehensive Assessment of At‐Risk Mental States (CAARMS) 6 ; the Spielberger State‐Trait Anxiety Inventory, State Subscale (STAI‐S) 7 ; and the Positive and Negative Syndrome Scale (PANSS) 8 .