What's the science?

Recent policies in North America have shifted towards increased acceptance of cannabis use, and perceived harm has decreased. Chronic cannabis use could have effects on cognitive function in developing adolescents, however, this risk is not well understood. This week in JAMA Psychiatry, Scott and colleagues perform a meta-analysis of studies on cannabis use to determine whether there is an association between cannabis use and cognitive function in youth and young adults.

How did they do it?

They performed a systematic review of the literature. They included all cross-sectional studies where participants were adolescents or young adults, frequent or heavy cannabis use was assessed as a variable of interest (not just acute cannabis use) and cognitive performance on at least one neuropsychological test was measured. They also ensured that each of these studies had sufficient power to calculate effect sizes. 69 studies met their criteria (2152 cannabis users and 6575 control participants with minimal cannabis use). They performed the statistical analysis using a mixed effects multivariate model, which accounts for correlated within-study effects and multiple different effect sizes between studies. Cognitive performance on neuropsychological tests was the measure of interest.

What did they find?

The overall effect size in a model without explanatory variables (i.e. variables that could potentially affect the association between cannabis and cognitive function such as age) was small but significant, showing a lower cognitive performance in cannabis users compared to the comparison group. Cognitive performance in heavy cannabis users was lower in the domains of learning, executive function, information processing speed, delayed memory, inhibition, working memory and attention. When explanatory variables were included, the effects were not explained by age category, age at first cannabis use or alcohol use. Hours of abstinence from cannabis (self-reported) was associated with a lower effect size, suggesting that a longer period of abstinence from cannabis reduced the negative effects on cognitive performance. When looking at studies where a period of greater than 72 hours of abstinence prior to study participation was required, effect size was no longer significant.