Understanding cannabis use in schizophrenia

Abstract

Large-scale international surveys have found consistently high rates of cannabis use in schizophrenia and other psychotic populations (Merikangas et al., 1998; Degenhardt et al., 2001; Farrell et al., 2001). Despite this we still have little understanding of the aetiology of the increased rates of cannabis use in psychosis. Three models for understanding the relationship between cannabis and psychosis have been proposed (Thornicroft, 1990; Khantzian, 1997; Mueser et al., 1998). The vulnerability model posits that cannabis use contributes to the onset, symptom severity and relapse of psychosis. The symptom alleviation model proposes that individuals with psychosis use cannabis in an attempt to alleviate positive and negative symptoms, as well as depression and anxiety. The common factor model proposes that some of an individual’s vulnerability to cannabis use and psychosis is caused by shared underlying factors, such as genetic vulnerability or conduct disorder/antisocial personality disorder. Consistent with the vulnerability model, cannabis use has been strongly associated with the onset of psychotic symptoms and disorders in a growing number of large-scale longitudinal studies (Moore et al., 2007 and see Chapter 15). Little evidence for the symptom alleviation model has emerged from these studies (Arseneault et al., 2002; Fergusson et al., 2003), although individuals do report improvements in positive affect (Henquet et al., 2010). Despite self-reports of improved affect, a growing number of prospective studies have found cannabis use is predictive of a more severe psychotic symptom course and increased risk of psychotic relapse (Verdoux et al., 2003; Ferdinand et al., 2005; Hides et al., 2006; Degenhardt, 2007; Zammit et al., 2008; and see Chapter 19). However, there is also evidence of increased cannabis use during periods when psychotic symptoms are worse, suggesting that relationships between cannabis and psychosis may be bidirectional (Ferdinand et al., 2005; Hides et al., 2006; Degenhardt, 2007). Conduct disorder and antisocial personality disorder are related to an increased risk of schizophrenia (Hodgins et al., 1996; Robins, 1966) and drug use disorders, in both the general population (Anthony and Helzer, 1991) and in people with psychotic disorders (Mueser et al., 2000), suggesting this may account for some of the increased comorbidity between cannabis-use disorder and psychosis.