Intelligence Quotient Decline Following Frequent or Dependent Cannabis Use in Adolescence: A Systematic Review and Meta-Analysis of Longitudinal Studies

Background: Previous systematic reviews and meta-analyses of cross-sectional data assessing the effect of cannabis on cognition show inconsistent results. We hypothesized that frequent and dependent cannabis use in the neurodevelopmental period would be associated with Intelligence Quotient (IQ) decline. Methods: This is a systematic review and meta-analysis. We searched Embase, PubMed and PsychInfo from inception to 24/01/2020. We included studies with non-treatment seeking samples of subjects as cases if they reported frequent or dependent cannabis use. We excluded studies if they did not have a pre-exposure measure of IQ and if the period of cannabis use did not start before age 26. We requested data from authors if summary data was not available from published work. We used the Newcastle Ottawa scale to assess study quality. We used random effects meta-analysis to calculate a Cohen’s D statistic. We preregistered our review with PROSPERO (ID no. CRD42019125624). Findings: We found seven cohort studies including 956 cases 5946 controls. There was a modest effect for the association between frequent or dependent cannabis use in adolescence and IQ decline, Cohen’s D = -0.122, (95% CI -0.207- -0.037) P=0.005. Study quality was moderate to high. Statistical heterogeneity between studies was also low at I2 = 15.2%. Interpretation: There are modest decreases in IQ over time in frequent and dependent adolescent cannabis users equating to loss of 1.83 IQ points (95% CI 0.56- 3.11). Funding Statement: EP was supported by Health Research Board of Ireland ‘YouLead’ Collaborative Doctoral Award (grant code 18210). MC, CH & AON were supported by European Research Council Consolidator Award to MC (Grant code 724809 iHEAR). SS was supported by Health Research Board PHR 2015-1293 awarded to DC. Declaration of Interests: The authors declare no competing interests.