Abstract
BACKGROUND: Cannabis use is generally associated with an unfavourable course of illness in first-episode schizophrenia, including non-remission of psychopathology symptoms, higher rates of relapse and re-hospitalization, and poorer functioning. The aim of this study was to explore the influence of cannabis use on clinical and treatment outcomes in first-episode schizophrenia spectrum disorder patients over 24 months of assured antipsychotic treatment. METHODS: The present longitudinal study included 123 minimally treated or antipsychotic-naive first-episode patients assessed over 24 months of treatment with flupenthixol decanoate according to a standardized regimen. Time to relapse, rates of symptomatic and functional remission, as well as recovery were compared between cannabis users (n=41) and non-users (n=82) stratified based on a combination of self-report and urine toxicology results over the course of treatment. In addition, visit-wise changes in psychopathology severity and overall functioning were compared between these two study groups. We hypothesized that 1) ongoing cannabis use would present with more severe psychopathology and poorer overall level of functioning, and 2) rates of remission and recovery would be lower in cannabis users compared to their non-using counterparts. RESULTS: At study entry, cannabis-using patients were younger, more likely to be male and to use methamphetamine, and scored lower in social and occupational functioning. Moreover, while cannabis users were more likely to relapse at any point over 24 months of treatment, cannabis non-users were more likely to achieve remission within the first six months, although this effect was not statistically significant. However, our most important finding was the interaction between cannabis use and time for total psychopathology as well as for the PANSS positive factor. While differences were not evident at either study entry or endpoint, cannabis users recovered at a slower rate than non-users. DISCUSSION: These results suggest a poorer treatment response in cannabis users compared to non-users in the context of assured adherence to antipsychotic medication. Therefore, regardless of the neurobiological impact of cannabis use in schizophrenia, the behaviour of substance use itself needs to be targeted as part of first-line treatment in order to improve the treatment outcomes of substance-using patients.