Teens who use marijuana may be at greater risk of permanent brain damage later in life, a study has revealed.
About 18 million Americans report using marijuana daily or nearly every day, and use is on the rise, largely due to recent decriminalisation across the US. From 1992 to 2022, for example, daily and near-daily use saw a 15-fold increase.
About one in seven of those users are teenagers, whose brains are still undergoing rapid changes in areas responsible for judgement, decision-making and memory.
Now, the largest-ever study of American teenagers has found regular cannabis use restricted vital growth in memory, attention, language and processing speeds over time, despite normal development when they were younger.
However, 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.
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.
A study from the University of California San Diego found cannabis use in adolescence may slow developmental improvements in memory and attention (stock image)
‘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.’
In the US, marijuana is fully legal – for recreational and medicinal use – in 29 states. It remains fully illegal in four states.
Laws in the remaining states are mixed, meaning the drug may be permitted for medicinal use, allowed only in the form of CBD oil, decriminalised, or subject to a combination of these rules.
While a growing body of research has examined marijuana’s effect on the heart, lungs and brain, few studies have focused on teenagers, whose brains are still developing.
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.
The above graph shows the improvements in working memory (the brain’s ability to hold information for tasks like learning and reasoning) in cannabis users and non-users. Cannabis users are represented by the blue line while non-users are the red line. Progress in this area slowed for cannabis users
‘Delaying cannabis use supports healthy brain development,’ Dr Natasha Wade, lead study author and assistant professor of psychiatry at the University of California San Diego, said (stock image)
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.
‘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.’
