Teenagers who began using cannabis improved more slowly in memory, attention, language, and processing speed than peers who did not, according to an analysis of 11,036 children followed from ages 9 and 10 through ages 16 and 17.
The pattern the researchers describe is specific. In some cases, these teens performed as well as, or slightly better than, other children when they were younger. As they got older and started using cannabis, their progress leveled off while their peers continued improving.
That distinction between a slowed trajectory and a decline is the whole finding, and it is routinely lost in headlines describing cannabis as damaging teenage brains. The study did not find that anyone got worse.
The Difference Between Slower Growth and Decline
Adolescent cognition is a moving target. Memory, attention, language and processing speed all improve substantially between ages 10 and 17 as the brain matures, and any measurement taken once captures a point on a rising line.
Trajectory analysis measures the slope of that line rather than its position. Researchers test the same children repeatedly over years and compare rates of change between groups.
A slower slope means a person is still improving, just less rapidly than they otherwise would. Over seven years, a modest difference in slope compounds into a measurable gap even though no participant lost ability at any point.
This matters for how families interpret the result. It does not describe a teenager who could remember things last year and cannot now. It describes a teenager who gained less than they would have, a difference visible in group averages rather than obvious in any individual.
Natasha Wade, an assistant professor of psychiatry at the University of California San Diego School of Medicine and lead author, framed the accumulation directly. "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," she said. "These differences may seem small at first, but they can add up in ways that affect learning, memory and everyday functioning."
The Study's Design and Its Methodological Strength
The analysis drew on the Adolescent Brain Cognitive Development Study, the largest long-term investigation of brain development in US youth, and was published in Neuropsychopharmacology in April.
Its main methodological advantage is combining self-report with toxicology, reflected in the paper's title. Adolescent self-report of substance use is unreliable in both directions, and biological confirmation addresses that. Participants underwent hair, urine, breath and saliva testing each year alongside questionnaires about cannabis and other substances.
Within a smaller subgroup with toxicological evidence of exposure, teens showing tetrahydrocannabinol exposure had worse memory over time than nonusers. Teens with evidence of cannabidiol did not show the same pattern, though that group was small and the comparison correspondingly weak. "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 labeled as CBD may still contain THC."
The researchers adjusted for family background, mental health and use of other substances.
The Causal Question the Design Cannot Settle
The researchers state plainly that the study does not prove cannabis use directly causes these changes, and that other factors including environment or personality may play a role.
Two alternative explanations remain live. Adolescents who begin using cannabis may differ from those who do not in ways that independently affect cognitive development, including family instability, school engagement, sleep patterns and peer environment. Adjustment reduces confounding but never eliminates it.
Reverse direction is also possible. Teens whose cognitive growth was already slowing for other reasons may be more likely to start using cannabis, since academic struggle and disengagement precede substance use in many trajectories.
The study cannot establish dose, frequency or timing thresholds either. It does not identify how much use, at what age, produces what magnitude of effect, which is the question parents most want answered. Whether the gap closes once use stops is arguably the most consequential unanswered question, and the current data cannot address it.
The Guidance This Research Actually Supports
The practical guidance the researchers offer is delaying use. "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."
For parents, the useful framing is not a warning about permanent damage, which this research does not support, but about a developmental window. Adolescence is a period of rapid cognitive gain, and anything that slows accumulation during it has more consequence than the same exposure would in adulthood.
Conversations grounded in specifics tend to work better than general warnings. Roughly one in five high schoolers uses cannabis, product potency has risen substantially in recent years, edibles produce delayed and unpredictable effects that lead to overconsumption, and vaping devices deliver high concentrations with little sensory feedback about dose.
Teens using cannabis daily, using it to manage anxiety or sleep, or finding it difficult to cut back should be seen by a clinician. Those patterns predict problems more reliably than occasional use does, and they are also the ones most likely to escape a parent's notice, because they tend to look like a teenager managing stress rather than a teenager in trouble. Pediatricians can screen and refer, and treatment for adolescent cannabis use disorder exists and works. MedicalDaily has covered how virtual mental health care has expanded and where community clinics are growing.
Parents noticing declining grades, withdrawal from activities, or changes in sleep should raise it with a pediatrician rather than assuming cannabis is the cause, since those signs have many explanations, including depression and anxiety.
Key Questions Answered
What did the study find? Teens who began using cannabis showed slower gains in memory, attention, language and processing speed than peers who did not, across 11,036 children followed from ages 9 and 10 to ages 16 and 17.
Did the teens get worse at these skills? No. They continued improving, just more slowly than peers. Some performed as well as or slightly better than others when younger before their progress leveled off.
How is slower growth measured? By testing the same children repeatedly over years and comparing the rate of change between groups, rather than comparing scores at a single point.
Does this prove cannabis caused the difference? No. The researchers state it does not. Environment, personality, sleep and school engagement could contribute, and teens whose growth was already slowing may be more likely to start using.
What did toxicology testing add? Biological confirmation of exposure through annual hair, urine, breath and saliva testing, which addresses unreliable self-report. In a smaller subgroup, teens with THC exposure showed worse memory over time. Those with evidence of CBD did not, but that group was small.
How much use is too much? The study cannot answer that. It did not establish thresholds for dose, frequency or age of onset.
When should parents seek help? If a teen uses daily, uses cannabis to manage anxiety or sleep, or struggles to cut back. Declining grades, withdrawal or sleep changes warrant a pediatrician visit, since those signs have many possible causes.
