Patient's perceptions of the cannabis-psychosis link--a systematic review

Over the past years a growing research effort has investigated the relation between cannabis use and schizophrenia at a neurobiological, epidemiological and clinical level. A number of systematic reviews and meta analyses have summarized the available evidence in the field. Conversely the patient's perception of the link between cannabis use and psychosis has been under investigation. Since patient's beliefs and attitudes strongly correlate with adherence to all forms of treatment, we conducted a systematic PUBMED database search for any English and German-language articles published until January 2012 that addressed patient's perception of a cannabis psychosis link. Six studies including psychotic subjects met inclusion criteria yielding a total sample of 97. The vast majority of patients with either schizophrenia or a recent psychosis disagreed with a causal link between cannabis use and their mental illness. We qualitatively reviewed the explanatory models underlying their views, which were multi-factorial, psychological, social, biological, esoteric and irrational factors. Most patient's believed that the temporal sequence of events did not clearly indicate a causal relationship for them. They thus discarded the hypothesis of a causal link between cannabis use and psychosis. Despite the heterogeneity of the included studies, findings are comparable and support the robustness of this review. Limitations and implications for clinicians and psychosis research are discussed.