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Cannabis Use Thins Adolescent Brains, Study Finds

By Leo Bear-McGuinness

Published: Jun 25, 2021   
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It’s long been known that the earlier in life one starts to use cannabis, the more likely it is that the drug will have long-lasting effects on the brain.

And now a new study has shown that cannabis use in a person’s teenage years can reduce the thickness of certain brain regions.

THC in the membrane

Published in JAMA Psychiatry, the study examined the brain scans of 799 teenagers, all of whom were enrolled in the IMAGEN study, a European research project that aims to study how biological, psychological, and environmental factors during adolescence affect brain development and mental health.

On average, the participants were 14.4 years old at the time of their first magnetic resonance imaging (MRI) scan and 19 years old at their second scan. The frequency of the teens’ cannabis use was determined by their responses to a questionnaire. Experience with the drug varied, but at least 161 participants reported using cannabis between 10 to 40 times.

After studying the scans, the researchers found a significant association between cannabis consumption and cortical thickness. On average, the left and right prefrontal lobes of the teens who had consumed cannabis were thinner than those of the teens who hadn’t used the drug.

As these brain regions help control motor function, problem solving, memory, and many other functions, the researchers say their findings “underscore the importance of further longitudinal studies of adolescent cannabis use, particularly given increasing trends in the legalization of recreational cannabis use.”

The researchers even hypothesize that the drug may be affecting the prefrontal lobes in particular as these regions of the brain are rich in CB1 receptors, which cannabis compounds like THC bind to.

However, while the researchers accounted for the impact of alcohol and nicotine in their study, they cannot rule out that some other factor or preexisting cognitive differences may have contributed to the thinning of the prefrontal lobes.

Although, given that the study is thought to be the largest longitudinal neuroimaging study of cannabis use to date, the authors’ conclusion – that cannabis use negatively affects cortical thickness in adolescents – still carries some weight.

Teen cannabis use

Previous studies have linked teenage cannabis use to changes in the developing adolescent brain and an increased risk of depression and suicidal ideation

But how cannabis legalization affects young people’s marijuana habits is more unclear.

A recent survey study published in the Journal of Studies on Alcohol and Drugs found significant increases in lifetime and past-30-day cannabis use among nearly all demographic groups post-legalization in California.

Yet other recent studies have found that legalization promotes the opposite effect. Published in the American Medical Association in 2019, one study found that US states that legalized recreational cannabis were associated with an 8 percent fall in the number of high school-age teenagers who claimed they used cannabis in the last 30 days.

“I think the big takeaway is that we find no evidence that teen marijuana use goes up after legalization for medicinal or recreational purposes,” Mark Anderson, an associate professor at Montana State University and lead author of the study, told Analytical Cannabis at the time.

“We view this as a very important result from a policy perspective because opponents often claim that teen use will skyrocket after these laws are passed. Based on our analysis, this has simply not been the case.”


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