Associations of adolescent cannabis use with academic performance and mental health: A longitudinal study of upper middle class youth

Background: There is a hypothesis that low socioeconomic status (SES) may explain the link between cannabis use and poorer academic performance and mental health. A key question, therefore, is whether adolescent cannabis use is associated with poorer academic performance and mental health in high SES communities where there is reduced potential for confounding. Methods: Youth (n= 254) from an upper middle class community were followed prospectively through the four years of high school (from age 14/15 to age 17/18). Past-year frequency of cannabis use was assessed annually. Official school records of academic performance and self-reported mental health symptoms (externalizing and internalizing symptoms) were assessed in grades 9 and 12. Results: Persistent cannabis use across the four years of high school was associated with lower grade-point average (β= -0.18, p= 006), lower Scholastic Aptitude Test (SAT) score (β= -0.13, p= 038), and greater externalizing symptoms (β= 0.29, p< 001) in 12th grade, but not with greater internalizing symptoms (β= 0.04, p= 53). Moreover, persistent cannabis use was associated with lower grade-point average (β= -0.13, p= 014) and greater externalizing symptoms (β= 0.24, p= 002) in 12th grade, even after controlling for 9th grade levels of these outcomes. Similar associations were observed for persistent alcohol and tobacco use. Effects for persistent cannabis use became non-significant after controlling for persistent alcohol and tobacco use, reflecting the difficulties of disentangling effects of cannabis from effects of alcohol and tobacco. Conclusions: Low SES cannot fully explain associations between cannabis use and poorer academic performance and mental health.