A new study has revealed that teenagers who use marijuana may be at greater risk of permanent brain damage later in life. Approximately 18 million Americans report daily or near-daily marijuana use, a figure that has surged 15-fold from 1992 to 2022, largely due to recent decriminalisation across the United States. About one in seven of these users are adolescents, whose brains are still undergoing rapid changes in areas responsible for judgement, decision-making, and memory.
Largest-Ever Study of American Teens
In the largest-ever study of American teenagers, researchers found that regular cannabis use restricted vital growth in memory, attention, language, and processing speeds over time, despite normal development when participants were younger. As they moved into later adolescence, their development in these areas slowed compared with their peers.
Experts from the University of California San Diego believe the culprit is tetrahydrocannabinol (THC), the psychoactive ingredient in marijuana, which was linked to worse memory in teens compared with cannabidiol (CBD), which is not intoxicating. It is thought THC may lead to shrinkage of the hippocampus – the brain’s memory centre – and changes in white matter, which controls how different parts of the brain communicate with each other.
Subtle Shifts with Major Impact
The researchers said while the deficits were small, the brain develops so quickly during adolescence that even subtle shifts could affect school performance and daily life. ‘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,’ said Dr Natasha Wade, lead study author and assistant professor of psychiatry at the University of California San Diego. ‘These differences may seem small at first, but they can add up in ways that affect learning, memory and everyday functioning.’
Study Methodology
The new study, published in the journal Neuropsychopharmacology, followed 11,036 children from ages nine and 10 through to ages 16 and 17 to track their cognitive performance and substance use. Each year, the children and their parents completed surveys about their health and substance use, while researchers collected hair, urine, and saliva samples to measure drug exposure. While saliva and urine can detect drug exposure over a matter of days, hair can show signs for up to 90 days if collected close to the scalp. Longer strands may reveal use stretching back as far as a year.
Participants also regularly completed tests measuring memory, processing speed, attention, language, and visuospatial skills, such as interpreting maps or solving puzzles. Those who regularly consumed THC showed reduced improvements over time in memory, processing speed, attention, and language, even though they had performed similarly to those who did not use cannabis at baseline when they were younger. In particular, THC exposure was linked to slower gains in episodic memory, which involves recalling specific personal experiences, events, or emotions from the past. CBD, which does not have psychoactive effects, showed no meaningful difference compared with non-users.
Implications for Brain Development
‘These results point to THC as a likely driver of the changes we’re seeing,’ Wade said. ‘It also highlights how complicated cannabis products can be, especially since some products labelled as CBD may still contain THC.’ While the teens did not show immediate cognitive deficits, slower development during adolescence may disrupt the brain’s pruning process, which helps shape how it handles emotions and impulses. Interference with that process could raise the risk of mental health disorders or cognitive problems later in life.
It is unclear exactly how THC may slow brain development in teens, but it has been linked to reduced volume of the hippocampus and white matter, as well as poorer neuroplasticity – the brain’s ability to reorganise its structures and functions in response to learning, experience, or injury. The researchers cautioned that the new study does not prove cannabis directly caused these changes in teenagers’ brains, but said they will continue tracking participants into young adulthood to better understand the long-term effects of cannabis.
‘Delaying cannabis use supports healthy brain development,’ Wade said. ‘As cannabis becomes more widely available, it’s important for families and teens to understand how it may affect the developing brain.’
