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Cannabis effect on cognitive function small and short-lived

Associations are small and may be of questionable clinical importance for most individuals, study suggests

Louise Prime

Friday, 20 April 2018

The effect on cognitive function of frequent cannabis use by adolescents and young adults is small and temporary, according to the findings of a large study* published in JAMA Psychiatry. The systematic review and meta-analysis found that although continued cannabis use might be associated with small reductions in cognitive functioning, cognitive deficits appear to be substantially diminished with abstinence.

The team of researchers were prompted by recent substantial shifts in perception and policy regarding cannabis in the US, and concerns that increased use might risk poorer cognitive functioning, especially in young people, to conduct what they believed to be the first quantitative synthesis of the literature examining cannabis and cognitive functioning in adolescents and young adults.

They analysed the outcomes of 69 cross-sectional studies that included a total of 2,152 cannabis users (mean age 20.6 years) and 6,575 comparison participants (mean age 20.8 years). Their results indicated a small overall effect size for reduced cognitive functioning associated with even frequent or heavy cannabis use; and the magnitude of effect sizes did not vary by sample age or age at cannabis use onset.

They also reported that their analysis of just those 15 studies that had required an abstinence period of longer than 72 hours found no statistically significant effect of cannabis use on cognitive function.

They acknowledged that functional outcomes might be more important than measures of cognitive function, and that their study cannot make causal conclusions about marijuana and cognitive functioning.

But they concluded: “Associations between cannabis use and cognitive functioning in cross-sectional studies of adolescents and young adults are small and may be of questionable clinical importance for most individuals. Furthermore, abstinence of longer than 72 hours diminishes cognitive deficits associated with cannabis use.

“Although other outcomes (e.g. psychosis) were not examined in the included studies, results indicate that previous studies of cannabis in youth may have overstated the magnitude and persistence of cognitive deficits associated with use. Reported deficits may reflect residual effects from acute use or withdrawal.”

They added that future studies should examine individual differences in susceptibility to cannabis-associated cognitive dysfunction.


* Scott JC, Slomiak ST, Jones JD, et al. Association of cannabis with cognitive functioning in adolescents and young adults. JAMA Psychiatry. Published online April 18, 2018. doi:10.1001/jamapsychiatry.2018.0335.

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