While previous studies have demonstrated there are acute cognitive defects associated with marijuana use, little is known about the long-term cognitive effects of marijuana among adolescents and young adults. A recent systematic review and meta-analysis attempted to fill this gap in the literature. The authors quantified potential long-term associations between frequent marijuana use and cognitive functioning among adolescents and young adults within the peer-reviewed literature; Meta-analysis of Observational Studies in Epidemiology (MOOSE) guidelines were followed. Inclusion criteria included (1) being an observational, cross-sectional study,  (2) having a sample with mean age of 26 years or under, (3) identifying heavy, frequent, and/or problematic marijuana use as the primary independent variable, (4) not focusing on acute cognitive effects of marijuana use, and (5) utilizing an appropriate comparison group. The final analytic sample consisted of 69 studies. Across studies, there were 8,727 participants, of whom 25% (n = 2,152) were cannabis users and 75% (n = 6,575) were comparison participants. The former group was 68% male and had a mean age of 20.6 years, while the latter group was 56% male and had a mean age of 20.8 years. The mean time of abstinence required by the studies was 152.7 hours (SD = 335.2 hours), yet 32% of the sample (n = 22) reported either zero hours of abstinence or did not specify the required number of hours. The standardized mean difference statistic (d) was used as the measure of effect size and statistical analyses were conducted using a mixed-effects multivariate model. Results indicated the overall mean neurocognitive effect size was −0.247 (SE, 0.038; 95% CI, −0.32 to −0.17), and the between-study variance estimate was 0.070 (SE, 0.018; P < .001), meaning variance between studies was significantly greater than variance due to sampling error alone. Significant negative effect sizes were found in the cognitive domains of learning, executive functioning, speed of information processing, delayed memory, and attention. Results of follow-up analyses indicated studies with an abstinence period greater than 72 hours had an overall effect size that was not significantly different from zero (d = −0.08, 95% CI, −0.22 to 0.07; P = .29). The authors concluded there was a weak negative association between marijuana use and non-acute cognitive functioning among adolescents and young adults, but these effects may be of little clinical significance, especially after sustained abstinence.

Take away: Results of this meta-analysis indicated frequent, heavy, or problematic marijuana use is weakly associated with declines in long-term cognitive functioning among young adults and adolescents. However, this relationship may not be clinically significant, especially after sustained abstinence from marijuana.

Cobb Scott, J., Slomiak, S.T., Jones, J.D., Rosen, A.F.G., Moore, T.M. & Gur, R.C. (2018). Association of cannabis with cognitive functioning in adolescents and young adults: A systematic review and meta-analysis. JAMA Psychiatry [published online ahead of print April 18, 2018] doi: 10.1001/jamapsychiatry.2018.0335