Patients with schizophrenia frequently report cannabis use, yet its effects on neurocognitive functioning in this population are still unclear. This meta-analysis was conducted to determine the magnitude of effect of cannabis consumption on cognition in schizophrenia without the confounding effects of other co-morbid substance use disorders. Eight studies met inclusion criteria yielding a total sample of 942. Three hundred and fifty six of these participants were cannabis users with schizophrenia, and 586 were patients with no cannabis use. Neuropsychological tests were grouped into seven domains (general cognitive ability and intelligence; selective, sustained and divided attention; executive abilities; working memory and learning; retrieval and recognition; receptive and expressive language abilities and visuo-spatial and construction abilities). Effect sizes were computed for each cognitive domain between cannabis-using patients and patients with no history of cannabis use. Effect size differences in cognitive performance in the schizophrenia group as a function of cannabis use were in the small to medium range, denoting superior performance in cannabis-using patients. Explanations for these findings are discussed and suggestions for future research in this area are recommended.
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