Review of: Cobb SJ et al, JAMA Psychiatry 2018;75(6):585–595

Our patients typically tell us that, according to the internet, weed is perfectly safe and does not affect their ability to think or function. Thirty states and the District of Columbia have laws legalizing cannabis, supporting the notion that people have begun to think of marijuana as relatively harmless. Rates of marijuana use in young adults are rising (Hasin DS, Neuropsychopharmacology 2018;43(1):195–212). Moreover, a recent study reported that cannabidiol (CBD), a “non-psychoactive” component of marijuana, may reduce psychotic symptoms (Arain M et al, Neuropsychiatr Dis Treat 2013;9:449–461).

Given that the brain continues to develop into a person’s mid-20s, how dangerous is marijuana use in adolescence and young adulthood? And what do we tell our young patients who are regular users? A new meta-analysis attempts to answer part of that question as it relates to the impact of cannabis use on cognitive function in adolescents and young adults.

The meta-analysis assessed cognitive effects in young adults and adolescents whose primary clinical problem was cannabis use. The analysis included 69 studies of 2152 regular cannabis users and 6575 people with minimal use of cannabis. After combining the results from all of these studies, the authors concluded that cannabis does have a mild negative correlation with various aspects of cognition. Specifically, studies showed that use of the drug is negatively associated with executive functioning, speed of information processing, delayed memory, working memory, and attention. But in the aggregate, effect sizes range from -0.21 to -0.33, indicating minimal impact on cognition. Verbal language, visuospatial functioning, and motor functioning were relatively spared. Studies that required at least 72 hours of cannabis abstinence before testing reported no significant effect on cognitive function.

CCPR’s take
At first glance, these results may seem reassuring. Cognition was minimally impacted, and the effects did not extend beyond active use. However, many of the studies were small, measurement of cannabis use and potency varied, and significant publication bias was noted. Also, the meta-analysis focused solely on neurocognitive effects and ignored other clinically pertinent outcomes. A new study looking at 3826 seventh graders found neurotoxic effects of cannabis on memory and inhibitory control (Morin JG et al, Am J Psychiatry 2018 Oct 3:appiajp201818020202). Moreover, we have ample evidence that marijuana use is associated with poor academic and social function, that the tetrahydrocannabinol (THC) component of marijuana is associated with an overall doubling of psychosis risk in youth, and that this increased psychosis risk is dose-dependent. So however you interpret this analysis, THC is clearly not off the hook.