Scott JC, et al. - Researchers sought to provide the first quantitative assessment of the literature looking at frequent or heavy cannabis use and whether it is associated with cognitive dysfunction in adolescents and young adults (with a mean age of 26 years and younger). Cross-sectional studies of adolescents and young adults showed that links between cannabis use and cognitive functioning are small and may be of dubious clinical importance for most individuals. In addition, abstinence of longer than 72 hours reduced cognitive deficits accompanying cannabis use. Despite no assessment of other outcomes (eg, psychosis) in the included studies, these results indicate that prior studies of cannabis in youth may have inflated the magnitude and persistence of cognitive deficits associated with use. Reported deficits might be residual effects from acute use or withdrawal.

Methods

• Researchers searched PubMed, PsycInfo, Academic Search Premier, Scopus, and bibliographies of relevant reviews for peer-reviewed, English-language studies from the date the databases began through May 2017.
• They used consensus criteria to determine which studies were included through abstract and manuscript review.
• This study followed Meta-analysis of Observational Studies in Epidemiology guidelines.
• They determined effect size estimates using multivariate mixed-effects models for cognitive functioning outcomes classified into 10 domains.
• Primary outcomes included results from neurocognitive tests administered in cross-sectional studies; they examined the influence of a priori explanatory variables on variability in effect size.

Results

• Researchers included 69 studies of 2,152 cannabis users (mean [SD] age, 20.6 [2.8] years; 1,472 [68.4%] male) and 6,575 comparison participants with minimal cannabis exposure (mean [SD] age, 20.8 [3.4]; 3,669 [55.8%] male).
• They noted a small overall effect size (presented as mean d) for reduced cognitive functioning associated with frequent or heavy cannabis use (d, 0.25; 95% CI, 0.32 to 0.17; P < .001).
• Sample age or age at cannabis use onset did not influence the magnitude of effect sizes.
• Nonetheless, studies which required an abstinence period longer than 72 hours (15 studies; n = 928) showed an overall effect size (d, -0.08; 95% CI, -0.22 to 0.07) that was not markedly different from 0 and smaller than studies with less stringent abstinence criteria (54 studies; n = 7,799; d, -0.30; 95% CI, -0.37 to -0.22; P=.01).