Background. Epidemiological findings suggest that cannabis use is a risk factor for the emergence of psychosis, and that the induction of psychotic symptoms in the context of cannabis use may be associated with a pre-existing vulnerability for psychosis. This study investigated in a non-clinical population the interaction between cannabis use and psychosis vulnerability in their effects on psychotic experiences in daily life.

Method. Subjects (N = 79) with high or low levels of cannabis use were selected among a sample of 685 undergraduate university students. Experience sampling method (ESM) was used to collect information on substance use and psychotic experiences in daily life. Vulnerability to develop psychosis was measured using a clinical interview assessing the level of psychotic symptoms. Statistical analyses were performed using multilevel linear random regression models.

Results. The acute effects of cannabis are modified by the subject's level of vulnerability for psychosis. Subjects with high vulnerability for psychosis are more likely to report unusual perceptions as well as feelings of thought influence than subjects with low vulnerability for psychosis, and they are less likely to experience enhanced feelings of pleasure associated with cannabis. There is no evidence that use of cannabis is increased following occurrence of psychotic experiences as would be expected by the self-medication model.

Conclusion. Cannabis use interacts with psychosis vulnerability in their effects on experience of psychosis in daily life. The public health impact of the widespread use of cannabis may be considerable.