Researchers at UC San Diego followed more than 11,000 young people from ages 9–10 through 16–17 to understand how cannabis use affects brain development during the teenage years. They measured cognitive abilities (memory, attention, language, processing speed) and tracked cannabis exposure using both participant reports and biological samples like hair, urine and saliva.
The study, published in Neuropsychopharmacology, is part of the largest long-term brain development research in the United States.
Cannabis users fell behind in brain growth while peers moved ahead
Teenagers who used cannabis showed slower improvement in memory, attention, language and processing speed over time. Notably, some cannabis users started at the same level as non-users in younger years. But as they got older and continued using cannabis, their cognitive gains plateaued—while their peers without cannabis use kept improving. This widening gap suggests the drug may interfere with normal brain development during a critical growth period.
THC appears to be the culprit, not all cannabis products
When researchers looked at specific cannabis compounds, teenagers with evidence of THC (the intoxicating compound) exposure showed greater memory declines than non-users. A smaller group with CBD exposure (a non-intoxicating compound) did not show the same pattern, though sample sizes for this group were limited. The researchers note that many products labelled as CBD still contain THC, making it difficult for consumers to know what they're actually using.
Small differences now may become bigger problems later
Although the cognitive differences measured were modest, researchers emphasised they could have real consequences. During adolescence, the brain develops rapidly, so even minor slowdowns in memory, attention or thinking speed can affect classroom performance and everyday functioning. The study shows correlation, not direct cause-and-effect, and was conducted in the United States.
What parents can do
- Know that THC exposure during teen years is linked to measurable delays in memory and attention development.
- Be aware that cannabis products may contain THC even if labelled otherwise—the contents aren't always what they claim.
- Recognise that adolescence is a critical window for brain development, when substance use carries particular risk.