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Study of Over 11,000 Adolescents Links Cannabis Use to Slower Cognitive Development

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Large Study Links Adolescent Cannabis Use to Slower Cognitive Development

A long-term study tracking more than 11,000 children from ages 9 to 17 found that adolescents who used cannabis showed slower development in cognitive skills such as memory, attention, and processing speed compared to their non-using peers. The research, published in the journal Neuropsychopharmacology, analyzed data from the Adolescent Brain Cognitive Development study, combining self-reports with biological testing. While the study establishes an association, the researchers note it does not prove causation.

"Adolescence is a critical time for brain development, and what we're seeing is that teens who start using cannabis aren't improving at the same rate as their peers."
— Neuropsychologist Natasha Wade, the study's first author

Study Design and Methodology

This research represents the largest analysis of its kind conducted in the United States. It followed 11,036 participants from approximately age 9-10 through age 16-17.

  • Data Source: The study utilized data from the ongoing Adolescent Brain Cognitive Development (ABCD) study.
  • Substance Use Measurement: Researchers combined annual self-reported substance use information with biological samples, including hair, urine, breath, and saliva, to detect drug exposure.
  • Cognitive Assessment: Participants underwent annual cognitive testing measuring skills including immediate and delayed recall, processing speed, inhibitory control, visuospatial processing, language, and working memory.

Key Findings

The analysis compared the cognitive development trajectories of adolescents who used cannabis with those who did not.

  • Slower Cognitive Growth: Adolescents who used cannabis showed restricted growth over time across all measured cognitive domains compared to peers who did not use cannabis.
  • Pre-Use Performance: Prior to initiating cannabis use, the cohort that later used cannabis performed similarly to or slightly better than their peers on cognitive assessments.
  • Effect of Specific Cannabinoids:
    • Adolescents with evidence of tetrahydrocannabinol (THC) exposure showed greater memory problems over time than non-users.
    • A smaller group of adolescents with evidence of only cannabidiol (CBD) exposure did not show the same pattern of slower development.
  • Accounting for Other Factors: Researchers reported that the pattern of slower cognitive growth in cannabis users remained after accounting for factors including family background, mental health, and the use of other substances.

Researcher Statements and Context

The researchers noted that while the measured differences between users and non-users were small, they could translate to clinically significant impacts in competitive academic environments.

Wade added that the results point to THC as a likely driver of the observed changes and noted the complexity of cannabis products, as some marketed as CBD may still contain THC.

The study was conducted against a backdrop of legalization in many U.S. states, increased THC content in available products, and decreased public perception of risk associated with cannabis use. Previous research suggests recreational drugs may impact adolescent brains differently than adult brains.