Examining the influence of adolescent marijuana use on adult intelligence: Further evidence in the causation versus spuriousness debate

•We examined the influence of adolescent marijuana use on intelligence levels.•“Ever use” of marijuana in adolescence was associated with intelligence declines.•Cumulative marijuana use was not associated with changes in intelligence scores.

ObjectivesThere is conflicting evidence regarding the association between adolescent marijuana use and adult intelligence, with some studies suggesting adolescent marijuana use can lead to declines in intelligence. The purpose of this study is to shed additional light on the potential link between marijuana use and changes in intelligence.MethodsWe employed change scores and ordinary least squares (OLS) analysis to test for associations between marijuana use and changes in intelligence scores from adolescence (ages 12-21) to adulthood (ages 18-26) using data drawn from the National Longitudinal Study of Adolescent to Adult Health.ResultsThe findings revealed that while a binary measure of marijuana use (ever/never) maintains a statistically significant association with changes in intelligence scores, the effect sizes are relatively small (β = 0.043-0.051). Additionally, our findings did not reveal a significant association between cumulative marijuana use and changes in intelligence scores.ConclusionsTaken together, the results suggest that while the binary measure of marijuana use (ever/never) has a statistically significant association with changes in intelligence scores, the binary measure accounts for at most a 1-2 point change in intelligence scores.