Effect of illegal substance use on cognitive function in individuals with a psychotic disorder: a review and meta-analysis.

OBJECTIVE: Impairment in cognitive function is a core feature of schizophrenia and other psychoses. Substance misuse is associated with impairment in cognitive function in the healthy population. Due to the high prevalence of substance misuse in those with a psychotic disorder, there is concern that a "double deficit" may result in this population. The aim of the current systematic review was to give an overview of the literature, to date, that has investigated the effect of illegal substance abuse or dependence on cognitive function in those with a psychotic disorder and to evaluate the differences and limitations in the methodologies used. METHOD: A systematic review and evaluation of the current literature comparing those with a diagnosis of a psychotic disorder with and without a history of substance abuse or dependence was conducted. A meta-analysis was also conducted to quantitatively assess the association between substance use and cognitive function in those with a psychotic disorder. RESULTS: The current literature investigating the impact of substance misuse on cognitive function in those with a psychotic disorder suffers from many methodological limitations. The results of the meta-analysis show that substance users performed significantly better than nonusers in the cognitive domains of attention and psychomotor speed and verbal memory, with no "double deficit" apparent. CONCLUSIONS: The results of this meta-analysis should be interpreted with caution, in light of the methodological difficulties reviewed.