Schizotypy and psychosis-like experiences from recreational cannabis in a non-clinical sample, Emma Barkus & Shon Lewis, 2008

Schizotypy and psychosis-like experiences from recreational cannabis in a non-clinical sample

Emma Barkus, Shon Lewis

Psychological Medicine, 2008, 38, 1267-1276.


Research On Line, University of Wollongong, 2008

Copyright Cambridge University Press.


Background : The relationship between cannabis use and psychosis is still a matter for debate. Accounting for the individual differences in subjective experiences to recreational cannabis use in the general population may hold some clues to the aetiological relationship between cannabis and psychotic symptoms. We hypothesized that schizotypy would account for the individual differences in subjective experiences after cannabis use but not in patterns of use.

Method : In a sample of 532 young people who had used cannabis at least once, we examined the relationship between the Cannabis Experiences Questionnaire (CEQ) and the Schizotypal Personality Questionnaire (SPQ). Additionally, we examined the psychometric properties of the CEQ.

Results : We replicated our previously reported findings that schizotypy was associated with increased psychosis-like experiences and after-effects, but also found that high-scoring schizotypes reported more pleasurable experiences when smoking cannabis. Using new subscales derived from principal components analysis (PCA), we found that the psychosis-like items were most related to varying rates of schizotypy both during the immediate use of cannabis and in the after-effects of cannabis use. High scoring schizotypes who used cannabis experienced more psychosis-like symptoms during and after use.

Conclusions : Our results suggest that cannabis use may reveal an underlying vulnerability to psychosis in those with high schizotypal traits.

Keywords : experiences, recreational, cannabis, non, clinical, like, sample, schizotypy, psychosis


In patients with established schizophrenia, recreational cannabis use has been reported to increase
relapse and symptom severity (Linszen et al. 1994; Baigent et al. 1995). In addition, administration of
the principal psychoactive substance in cannabis, D9- tetrahydrocannabinol (D9-THC), transiently exacerbates the positive, negative and cognitive symptoms in stabilized patients with schizophrenia (D’Souza et al. 2005).

There is also evidence that cannabis use is a risk factor for the initial onset of psychosis. In a longitudinal
community study, van Os et al. (2002) demonstrated that baseline cannabis use predicted the
emergence of psychotic symptoms and need for care due to psychotic symptoms at follow-up. A recent review of the longitudinal studies to date reported that regular cannabis seems to increase the risk of developing schizophrenia (Degenhardt & Hall, 2006). However, these studies do not determine the nature of the relationship between cannabis and psychosis: are those who are psychosis prone attracted to using cannabis (an association model), does cannabis use directly increase proneness to psychosis (a causal model), or is there another factor that links psychosis proneness and cannabis use (an indicator-variable model; Dumas et al. 2002)? A number of reviews have tried to address the evidence for causal and association models (e.g. Hall et al. 2004; Verdoux et al. 2005; Degenhardt & Hall, 2006; Fergusson et al. 2006). The conclusion reached by authors on the basis of current data is that, in individuals with an underlying predisposition to psychosis, cannabis use may precipitate a psychotic episode, but it is difficult to argue for a direct and large causal role for cannabis use in psychosis. However, Ferdinand et al. (2005) also highlight the possibility that the nature of the relationship between cannabis use and psychotic symptoms may be bidirectional. This is a conclusion that could be reached by most association studies, particularly those that do not attempt to control for baseline levels of psychotic symptoms or psychosis proneness.

One way to explore the relationship between psychotic symptoms and cannabis use is to examine the
impact of cannabis use in healthy individuals with psychometrically defined psychosis proneness, or schizotypy. Schizotypal trait has been reported to be higher in relatives of patients with schizophrenia (e.g. Appels et al. 2004), may share some of the same risk genetic loci as schizophrenia (Fanous et al. 2007) and may also lead to increased cognitive deficits in relatives of patients with schizophrenia (Diwadkar et al. 2006). Schizotypy is characterized by attenuated psychotic symptoms that comprise both positive (unusual beliefs and perceptual experiences) and negative (social anxiety and withdrawal) features. Pre-existing schizotypy has been reported to increase the risk of psychotic states from cannabis use (Henquet et al. 2005) and also modulate sensitivity to the effects of D9-THC (Henquet et al. 2006). Although cannabis use per se has been reported to increase schizotypy scores (Kwapil et al. 1996; Williams et al. 1996; Moss et al. 2001; Skosnik et al. 2001; Dumas et al. 2002), these results have not been consistent (Schiffman et al. 2005; Earleywine, 2006).

An alternative and perhaps more ecologically valid approach is to examine the experiences that individuals report after using cannabis rather than placing any emphasis on full psychotic syndromes. Henquet et al. (2006) and D’Souza et al. (2004) tested the effects of D9-THC in healthy individuals ; however, D9-THC is only one component of cannabis, and other ingredients may be involved in the recreational effects of cannabis. In addition, the effects of cannabis may be environmentally modulated and administration of the D9- THC in a controlled and artificial environment may not produce the same effects as when it is used naturalistically. This naturalistic approach has been taken in two previous studies. First, Verdoux et al. (2003) used experience sampling, a method of charting subjective experience at random points during the day to demonstrate that those with high psychosis vulnerability (defined by a structured interview) were more likely than those with low psychosis proneness to report unusual perceptual experiences and thoughts following recreational cannabis use. Second, we
have previously reported an association between high schizotypy score, a measure of psychosis proneness, and recreational cannabis-induced psychosis-like experiences and subsequent ‘after-effects’, using the newly developed Cannabis Experiences Questionnaire (CEQ; Barkus et al. 2006). Given that there are individual differences in people’s self-reported responses to cannabis, it is important to try to determine the possible mechanisms that may underpin these differences in experience; particularly as it is becoming clear that individuals differ in their risk for experiencing psychotic symptoms following cannabis.

The current study aimed to replicate the findings of Barkus et al. (2006) in a larger sample and also to refine the psychometric properties of the CEQ. Specifically, we were interested in comparing the effects of extreme schizotypy scores on experiences from cannabis use. We hypothesized (i) that schizotypy score would not be related to patterns of cannabis use in terms of whether used or not, age at first use, or frequency of use, but that (ii) individuals with high schizotypy scores would report increased levels of psychosis-like symptoms and subsequent after-effects with cannabis use compared to mean- or low scoring schizotypes.