Chronic marijuana use is associated with persistent impairment of attention, verbal memory, working memory, decision making, and executive function (46, 74). Although early evidence (using traditional neuropsychological assessments) showed that cognitive deficits associated with marijuana use resolved by day 28 of abstinence (77), more recent data show subtle, persistent cognitive deficits despite prolonged abstinence (78, 79). In support of these data is the finding of a dose-dependent effect on cognition, such that early and greater quantity of marijuana use results in greater cognitive deficits (80). This is particularly true for adolescents who begin smoking marijuana in their early teens. In one study, adolescents who began smoking cannabis early (14–22 years of age) and stopped by age 22 had significantly greater cognitive deficits at age 27 than nonusing peers (81). About 10% of cannabis-dependent adolescents report experiencing a “serious problem” with memory loss (82). In a longitudinal birth cohort comprising 1,037 individuals followed through 38 years (the Dunedin study), persistent marijuana use was associated with a six-point decline in intelligence quotient; these deficits were greater (eight points) when use began in adolescence, and, importantly, these declines did not reverse after the cessation of marijuana use (47).
