Cannabis use and treatment resistance in first episode psychosis: a natural language processing study.

BACKGROUND

Cannabis is frequently used among individuals with first episode psychosis and is associated with poor clinical outcomes. However, little is known about the effect of cannabis use on the response to antipsychotic medications and how use could affect outcomes. Using natural language processing on clinical data from a large electronic case register, we sought to investigate whether resistance to antipsychotic treatment mediated poor clinical outcomes associated with cannabis use.

METHODS

Data were obtained from 2026 people with first episode psychosis in south London, UK. Cannabis use documented in free text clinical records was identified with natural language processing. Data for age, sex, ethnicity, marital status, psychotic disorder diagnosis, subsequent hospital admission, and number of unique antipsychotic medications prescribed were obtained using the Clinical Record Interactive Search instrument. The association of these variables with cannabis use was analysed with multivariable regression and mediation analysis.

FINDINGS

939 people (46·3%) with first episode psychosis were using cannabis at first presentation. Cannabis use was most strongly associated with being 16-25 years old, male, and single, and was also associated with an increase in number of hospital admissions (incidence rate ratio 1·50, 95% CI 1·25-1·80), compulsory hospital admission (odds ratio 1·55, 1·16-2·08), and number of days spent in hospital (β coefficient 35·1 days, 12·1-58·1) over 5 years' follow-up. An increase in number of unique antipsychotic medications mediated an increase in number of hospital admissions (natural indirect effect 1·11, 1·04-1·17; total effect 1·41, 1·22-1·64), compulsory hospital admission (1·27, 1·10-1·45; 1·71, 1·05-2·78), and number of days spent in hospital (16·1, 6·7-25·5; 19·9, 2·5-37·3).

INTERPRETATION

We showed that a substantial number of people with first episode psychosis used cannabis and that its use was associated with increased likelihood of hospital admission and number of days spent in hospital. These associations were partly mediated by an increase in number of unique antipsychotic medications prescribed. These findings suggest that cannabis might reduce response to conventional antipsychotic treatment and highlight the importance of strategies to reduce its use.

FUNDING

National Institute for Health Research, UK Medical Research Council.