Researchers at the University of California San Diego have confirmed what many a stoner's report card already hinted at: teenagers who start using cannabis may develop memory and thinking abilities more slowly than their abstaining peers. The study, published in Neuropsychopharmacology, crunched data from more than 11,000 participants in the Adolescent Brain Cognitive Development (ABCD) Study, the largest long-term investigation of brain development among young people in the United States. Because nothing says 'critical period for brain development' like a little THC.

"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 Natasha Wade, PhD, assistant professor in the Department of Psychiatry at UC San Diego School of Medicine and lead author of the study. "These differences may seem small at first, but they can add up in ways that affect learning, memory and everyday functioning." In other words, that 'it's just a little weed' attitude might not age as well as you'd hope.

The research team followed 11,036 children from ages 9 to 10 through ages 16 and 17, monitoring changes in cognitive performance and substance use. To measure cannabis exposure more accurately, they used both participant reports and biological tests - hair, urine, and saliva samples - because, as we all know, teenagers are paragons of truthfulness. Teens who used cannabis showed slower improvement across several cognitive areas: memory, attention, language, and processing speed. Some initially performed as well as or better than peers at younger ages, but after they started using, their cognitive gains leveled off while non-users kept climbing the brain-power ladder.

In a smaller subgroup, teens with evidence of tetrahydrocannabinol (THC) exposure - the primary intoxicating compound in cannabis - experienced greater declines in memory performance over time than non-users. The same pattern didn't appear for cannabidiol (CBD) exposure, though that group was limited. "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." Surprise, surprise: the stuff that gets you high is the stuff that messes with your memory.

Now, before you panic, the cognitive differences were relatively modest. But the brain develops rapidly during adolescence, so even minor changes in memory, attention, or thinking speed could influence classroom performance and everyday activities. The study doesn't prove cannabis directly caused the slower growth - environmental factors, personality traits, and other substances could contribute. The researchers adjusted for family background, mental health, other substance use, and pre-existing cognitive abilities, but they can't rule out every confounder. Still, the message is clear: maybe don't let your teenager hit the bong every day.

The team plans to follow participants into young adulthood to see if the effects persist and whether the age of onset or frequency of use matters. "Delaying cannabis use supports healthy brain development," Wade said. As cannabis becomes more widely available, it's worth remembering that the developing brain might prefer a Netflix marathon over a smoke session.

The study included co-authors from UC San Diego, the University of Sydney, University of Wisconsin-Milwaukee, Huestis & Smith Toxicology, and Columbia University, and was funded by the National Institute on Drug Abuse, among others. So, if you're a teen, take note: your brain is still under construction, and cannabis might be the wrecking ball.