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The liberalisation of cannabis laws, the increasing availability and potency of cannabis has renewed concern about the risk of psychosis with cannabis. The objective of the WFSBP task force was to review the literature about this relationship. Converging lines of evidence suggest that exposure to cannabis increases the risk for psychoses ranging from transient psychotic states to chronic recurrent psychosis. The greater the dose, and the earlier the age of exposure, the greater the risk. For some psychosis outcomes, the evidence supports some of the criteria of causality. However, alternate explanations including reverse causality and confounders cannot be conclusively excluded. Furthermore, cannabis is neither necessary nor sufficient to cause psychosis. More likely it is one of the multiple causal components. In those with established psychosis, cannabis has a negative impact on the course and expression of the illness. Emerging evidence also suggests alterations in the endocannabinoid system in psychotic disorders. Given that exposure to cannabis and cannabinoids is modifiable, delaying or eliminating exposure to cannabis or cannabinoids, could potentially impact the rates of psychosis related to cannabis, especially in those who are at high risk for developing the disorder. [ABSTRACT FROM AUTHOR]

The authors reflect on the epidemiological studies on the connection between cannabis use and psychosis. Also cited are a study showing an increased prevalence of self-reported psychotic disorders among adolescents and young adults in the U.S., the relationship between proxy measures of dose and self-reported psychosis, and the genome-wide association studies (GWAS) of schizophrenia and lifetime cannabis use.

Cannabis has been implicated as both a potential cause and adverse prognostic factor in psychotic disorders. Investigating the contributory role of cannabis toward the overall burden of psychotic illnesses may represent an important step toward psychosis prevention and treatment. The current study samples consecutive admissions (N = 246) to two community based first-episode psychosis services to characterize timing of cannabis use relative to psychosis and attenuated symptom onset, differences between those with and without cannabis exposure, and the association of age at first cannabis exposure with clinical and demographic variables. Both cannabis exposure (78%) and cannabis use disorders (47%) were highly prevalent at admission. In 94% of participants, cannabis use preceded the onset of both attenuated and full-threshold psychosis symptoms by several years. Earlier age at first exposure to cannabis was associated with younger age at prodrome and psychosis onset, worse premorbid functioning, and greater severity of cannabis use disorder at admission. The timing of first exposure to cannabis may have individual prognostic as well as public health significance. Documenting the prevalence and impact of cannabis use in early psychosis samples, as well as the overall incidence of psychotic disorders, will be of vital public health significance as the United States enacts cannabis legalization and cannabis products become more widely available. [ABSTRACT FROM AUTHOR]