A large new study has raised a quiet but unsettling question for families and educators: what if some teenagers are not losing their thinking skills but are simply not gaining them as fast as they should?

Researchers from the University of California San Diego have reported that adolescents who begin using cannabis show slower progress in memory, attention and overall thinking ability compared to their peers. The findings, published on April 20, 2026, in Neuropsychopharmacology, draw on data from more than 11,000 participants in the long-running Adolescent Brain Cognitive Development (ABCD) Study—the largest effort of its kind tracking how young brains grow.

“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, lead author of the study and assistant professor of psychiatry. “These differences may seem small at first, but they can add up in ways that affect learning, memory and everyday functioning.”

The research followed children from ages 9–10 into their late teens, checking their cognitive performance year after year. In lab settings, participants completed structured memory and attention tasks while researchers carefully logged changes. In addition to these efforts, scientists did more than just fill out questionnaires. Hair strands were cut, urine samples were taken, and saliva swabs were taken. Each of these was quietly tested in labs to see if there were any signs of drug use over the course of weeks or even months.

That detail is important. Teenagers often exaggerate their drug or alcohol use. The biological testing helped uncover patterns that might otherwise stay hidden.

Across multiple areas—language skills, focus, and processing speed—the pattern was consistent. Some teens who later used cannabis started off performing just as well, or even slightly better, than others. But over time, their progress flattened. Meanwhile, their peers continued to improve, sharpening their skills year after year.

The difference is subtle. Not a dramatic drop. More like a slowdown.

Scientists then looked closer at what part of cannabis might be responsible. In a smaller subgroup, they found that exposure to tetrahydrocannabinol, or THC—the chemical that produces the “high”—was linked to worsening memory over time. Those exposed only to cannabidiol, or CBD, did not show the same pattern, though the sample size 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 labelled as CBD may still contain THC.”

The study stops short of proving direct cause. Human behaviour is rarely that simple. Factors like family environment, mental health, and personality traits can all shape outcomes. Still, the researchers accounted for many of these variables, including earlier cognitive ability and use of other substances. The signal remained.

What makes the findings more pressing is timing. Adolescence is a period of rapid brain “upgrade”, when neural connections are refined and thinking becomes faster and more efficient. Even a small delay during this phase can ripple outward—lower grades, slower learning, and difficulty picking up new skills.

Short-term, it may look like nothing. Long-term, it may not.

“Delaying cannabis use supports healthy brain development,” Wade said, noting that the team will continue to track participants into adulthood to better understand lasting effects.

As cannabis becomes more available and socially accepted in many parts of the world, the study adds complexity to the conversation. For teenagers, the risk may not be obvious or immediate. It may feel like everything is normal.

But under the surface, the brain may simply not be keeping up.