Marijuana Might Not Affect Young Brains as Much as Scientists Thought

But the results, crucially, show only a mild effect.

As cannabis legalization gains momentum in the United States, concerns about the drug’s effects on cognition are back on everyone’s minds. Does weed make you dumb? The science isn’t settled: Some research shows chronic smoking can cause cognitive and emotional deficits, whereas other studies show positive effects on cognitive health. New research published in JAMA Psychiatry on Wednesday, however, is helping clear the debate.

In the paper, researchers led by neuropsychiatrist J. Cobb Scott, Ph.D., of the University of Pennsylvania, show that marijuana does have effects on cognition, but they are small. Public concerns about the cognitive effects of marijuana on teens and young adults motivated the researchers to conduct a systematic review and meta-analysis of existing studies: “One possible risk of increased cannabis use is poorer cognitive functioning, especially in youth.” But the results, crucially, show only a mild effect, and certainly not a permanent one. Their examination of 69 different studies of 2,152 chronic or heavy cannabis users with a median age of 26 or younger showed that the cognitive effects of smoking marijuana are fairly small and that these effects usually even out after 72 hours or so.

“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,” they write. In this paper, “cognitive functioning” is an umbrella term for many aspects of cognition examined in the individual studies, such as learning, abstraction, speed of information processing, delayed memory, inhibition, working memory, and attention. The team’s analysis showed that marijuana has “nonsignificant” effects on language, visuospatial ability, and motor functioning. In most studies, the authors point out, any effects subsided with abstinence.

“Results indicate that previous studies of cannabis in youth may have overstated the magnitude and persistence of cognitive deficits associated with use,” they write. “Reported deficits may reflect residual effects from acute use or withdrawal.” That being said, previous research has shown some negative effects of chronic smoking, such as decreased gray matter volume. Coupled with the fact that this study did show some negative short-term effects of marijuana, the research suggests that determining whether marijuana is good or bad for cognitive function depends on the aspect of cognitive function and patterns of marijuana use being examined.

We still have a lot to learn about the dynamic effects of marijuana on the brain, especially the developing brains of young people. Researchers say that large-scale longitudinal studies — following marijuana users over many years — will be needed to clear up the picture.

In the meantime, the takeaway point from this research is that the cognitive effects of marijuana are fairly small, and they’re mostly limited to short time frames. But previous studies say long-term effects could occur over time, and future research will tell us exactly how these two dynamics interact.

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