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Background Cannabis use in adolescence is a known risk factor for developing schizophrenia. Decline in intellectual functioning is a well-studied phenomenon of schizophrenia. Schizophrenia in its first classification was conceptualized as a dementia - ‘Dementia Praecox’ indicating a dominance of effect of cognitive symptoms on functioning at that time. First episode psychosis patients with histories of cannabis use compared to those without cannabis use have superior cognitive functioning suggesting different disease phenotypes. Our aim was to investigate whether cannabis had an effect on full scale IQ in general population samples to further inform understanding of this disease pathway. Methods We developed a search strategy through an iterative approach with a qualified information specialist. We searched three databases: Medline, Embase and PsychInfo. We included conference abstracts and full text publications in English. We contacted authors for additional information in cases where an effect size was not calculable. We included longitudinal studies of non-help-seeking young people in the community with a pre-drug exposure standardized measure of IQ prior to the age of 18 and a comparable measure at subsequent follow up. We defined the case group as individuals with a history of heavy cannabis use (more than 25 lifetime uses, at least weekly use for 6 months and/or meeting criteria for a cannabis use disorder) and the control group as similar young people who had no or very minimal experimental exposure to cannabis (<5 lifetime uses). Two reviewers independently extracted the data and assessed for bias using the Newcastle-Ottawa risk of bias tool. We performed random effects meta-analysis in Stata version 14 given expected heterogeneity in measurement of cannabis use across studies. Results We identified 7 cohort studies detailed in 6 reports including over 650 subjects and 5600 controls. Study designs included twin cohorts, representative cluster sampled community cohorts and birth cohort studies. Range of follow up was between 2 and 23 years. All studies showed relative decline in IQ of which two were statistically significant. Our findings show that cannabis use in youth is associated with modest IQ differences that equate to approximately to a 2-point decrease in young cannabis users. Studies examining twin pairs showed that twins discordant for cannabis use did not have divergent cognitive trajectories, however these analyses were relatively underpowered to find an effect. Discussion Our findings demonstrate the harmful effects of cannabis use on brain development in young people. These findings are of public health importance and provide further evidence for the detrimental effects of early cannabis use on mental health and cognition.

schizophrenia bulletin

O9.6. SYSTEMATIC REVIEW AND META-ANALYSIS OF THE EFFECTS OF CANNABIS USE IN ADOLESCENCE ON IQ IN LONGITUDINAL STUDIES ACCOUNTING PRE-EXPOSURE BASELINE PERFORMANCE