Marijuana use and academic performance: A longitudinal study

Background: Marijuana is a popular illicit drug among adolescents and its impact on academic performance has been widely debated. The objectives of this study are to: 1) describe distinct trajectories of marijuana use in a cohort of adolescents evaluated annually from Grade 6-12 (2003-2009), and 2) examine the association among trajectories, study skills, and high school completion. Methods: The sample consisted of a cohort of 619 randomly-selected students (52% males; 48% White, 36% Black, 12% Latino). Every spring, students reported their 30-day frequency of marijuana use, and teachers rated study skills. Schools provided dropout data. We used semi-parametric group based modeling to identify trajectories and mixed effects models to examine the association of marijuana trajectories and study skills. Results: From Grade 6-12, marijuana prevalence increased from 2.3% to 18.6%. Adolescents followed three distinct marijuana use trajectories: Abstainer (78%), Increasing-Low (12%), and Increasing-High (10%). The Increasing-High and Increasing-Low groups had significantly higher dropout rates (28% and 33%) than Abstainers (14%). Teachers rated the study skills of the two marijuana-using groups significantly lower than abstainers, even in sixth grade. Conclusion: The majority of adolescents consistently abstained from using marijuana throughout middle and high school. However, a significant proportion of youth followed increasing trajectories of marijuana use and had higher dropout rates and worse study skills. Interestingly, low study skills preceded marijuana use and did not worsen as marijuana use increased. Thus, we cannot conclude that marijuana use led to the poor academic outcomes, but they may have shared risk factors.