Previous studies of the relationship between adolescent marijuana use and adult intelligence have yielded mixed results. A new study attempted to shed light on this association by examining data from the first three waves of the National Longitudinal Study of Adolescent to Adult Health (Add Health), which followed a nationally representative sample of U.S. adolescents for six years. The first wave began when participants were aged 12 – 21 years and the second and third waves were administered one and six years later, respectively. To control for potential confounding from the effects of marijuana use prior to wave I, only participants who reported never using marijuana were selected into the study (99.2% of the sample at wave I, 88.0% at wave II, and 30.3% at wave III). Constructs measured were verbal intelligence, lifetime marijuana use, and past-month marijuana use. Verbal intelligence change scores between waves and ordinary least squares (OLS) regression models were used to determine if either lifetime or past-month marijuana use were associated with changes in intelligence scores, adjusting for age, sex, race, socioeconomic status, and personality traits. Findings included a significant association between lifetime marijuana use at wave II and a decrease in intelligence scores between waves I and III; this decrease was 2.1 points, relative to abstainers. Lifetime marijuana use at wave III was associated with a 1.1-point decrease in intelligence scores between waves I and III, relative to abstainers. There was no evidence past-month marijuana use at waves II or III was significantly associated with changes in intelligence scores.
Take away: Results of this study show ever trying marijuana during adolescence is associated with small, but significant, declines in intelligence scores over time; however, there was no evidence of a dose-dependent relationship between adolescent marijuana use and adult intelligence scores.