Assessing vulnerability to psychotic illness amongst cannabis users: correlates, discriminating factors and scale development

Background: Schizophrenia is a pervasive and often debilitating disorder, although vulnerability is not easily assessed. Cannabis has a positive relationship with schizophrenia. To date, it is unknown whether or not this is a causal relationship. Nonetheless, those with vulnerability to psychosis have displayed a differential sensitivity to cannabis. Aims: There were two main aims to this programme of research: 1) Contribute to discussions relating to ‘causal inference’ in the relationship between cannabis and psychosis. 2) Assess the reliability and validity of the Cannabis Experiences Questionnaire (CEQ) as a measure of psychotic vulnerability based on a differential sensitivity to cannabis. Methods: Two studies were conducted. The first was a Cross-sectional investigation in which two groups of cannabis users were recruited, participants with self-reported depression (n = 85) and participants with self-reported psychotic disorder (n = 48). This investigation also considered data from a community sample recruited as part of other research studies. These consisted of cannabis users (n = 861) and non-users (n = 306). These groups were compared on measures of schizotypy and cannabis induced experience. The second study was an experience sampling investigation, in which regular cannabis users (n = 36), submitted 7 responses per day via a mobilephone, for a period of 14 days. Participants completed measures of: psychotic-like states, stressed states, calm states, drug consumption, stressful and pleasurable events, and aversive cannabis induced experience. Results: Cross-sectional investigation: There was no significant difference between cannabis users with reported depression and reported psychotic disorder in the disorganised or interpersonal domains of schizotypy. The cannabis-using groups of participants displayed a differential sensitivity to cannabis, with those who reported psychotic illness having significantly more aversive cannabis experiences than the community sample (U = 15106.5, z = 3.142, p = .002, r = 0.10) and participants with reported depression (U = 1241.0, z = 3.746, p < .001, r = 0.32) . The most effective means of identifying psychotic vulnerability consisted of a two-step process, firstly utilising assessments of schizotypy and secondly assessments of aversive cannabis induced experience. Experience sampling investigation: In a dose dependent fashion cannabis predicted increases in interpersonal (b = 0.24 95% CI 0.07 to 0.42, p = .006) and disorganised psychotic like experience (PLE) (b = 0.16 95% CI 0.04 to 0.27, p = .006). However, disorganised PLE significantly increased the odds of cannabis consumption (OR = 1.245 95% CI 1.045 to 1.247, p = .003). Cannabis positively and significantly predicted ‘calm’ states in a dose dependent fashion (b = 0.23 95% CI 0.07 to 0.39, p = .006). Cannabis and stressed states interacted to significantly predict PLEs (b = 0.33, 95% CI 0.17 to 0.49, p < .001). Aversive cannabis induced experience significantly predicted PLEs both within (b = 0.22, 95% CI 0.10 to 0.33, p < .001) and between participants (b = 0.66, 95% CI 0.06 to 1.27, p = .033). Previously documented aversive cannabis experiences significantly predicted propensity to experience stressed states (b = 0.15, 95% CI 0.05 to 0.24, p= .002). Conclusion: Aim 1): Within a continuum model of psychosis the results of these studies support three mechanisms of a cannabis-schizophrenia interaction; cannabis use causes schizophrenia; schizophrenia causes cannabis use; schizophrenia and cannabis use maintain one another. There is evidence to suggest psycho-social stressors interact with cannabis to induce PLEs. This may indicate that cannabis causes schizophrenia via a cross-sensitisation mechanism. At-risk groups should be warned against using cannabis as a stress coping mechanism. Aim 2): These results confirm a differential sensitivity to cannabis in those vulnerable to psychotic disorder. This investigation has demonstrated that psychosis vulnerability can be assessed by aversive cannabis induced experience. This investigation has demonstrated concurrent, convergent, and predictive validity of the CEQ as an assessment of psychotic vulnerability. This scale could be useful for drug education programmes and risk assessment in recreational cannabis users; screening for medicinal cannabis prescription; screening for research trials with cannabinoids or other known psychotomimetics; and in the allocation of psychological intervention for cannabis dependence, and (possibly) stress-reduction in those with disorder or at ultra-high risk