Why cannabis-related psychosis can be difficult to diagnose early

Despite the high prevalence of comorbid cannabis use at first onset of psychotic symptoms, high transition rates from cannabis-induced psychosis to schizophreniform psychoses and bipolar disorder, and evidence of cannabis use as a risk factor in developing primary psychosis, only a few studies have addressed the difficulties in differential diagnosis focusing on the early stages of the disease. The present study is the first to date to examine clinical and sociodemographic characteristics of individuals with early psychosis, comparing cannabis-induced psychotic disorder (CIPD), non-substance-induced psychotic disorder with concurrent cannabis use (NSIPD+C), and non-substance-induced psychotic disorder (NSIPD). A retrospective chart review of patients aged 18-35 with early psychosis who received inpatient treatment at FRITZ Early Intervention Center between December 2016 and September 2021 was conducted. Sociodemographic characteristics included age, biological sex, family history of psychiatric illness, migration background, living status, education/work status. Clinical characteristics included insight into illness, duration of untreated psychosis (DUP), social functioning, pathway to treatment, treatment motivation, treatment recommendation, and type of discharge. Of 509 participants, 42.6% had a diagnosis of NSIPD, 33.3% of CIPD, 7% of NSIPD+C. NSIPD had a significantly longer treatment duration compared to CIPD, a higher treatment motivation, and a higher discharge in accordance with clinicians' recommendation compared to both CIPD and NSIPD+C. No significant group differences were found in sociodemographic characteristics, DUP, and functioning scores. The examined sociodemographic parameters as well as level of functioning and DUP do not aid in the differential diagnostic classification when cannabis use and psychotic symptoms occur simultaneously. The absence of differences in DUP across diagnoses may reflect the efficacy of low-threshold access provided by FRITZ and further supports the need for comprehensive implementation of early intervention services.