Major Study Links Teen Cannabis Use to Delayed Thinking and Memory Skills

When this Gen Xer was growing up, the accepted, anti-Reefer Madness narrative about smoking weed was that a) it wasn’t addictive, and b) it was totally safe.

Fast forward several decades, and we’ve now got a growing body of evidence that not only can it be addictive, with Cannabis Use Disorder a real possibility for some heavy or long-term users, but that its use, especially in adolescence, can lead to real-world harms from increased anxiety to psychiatric disorders.

Now a major study — published today, just in time for 4/20 — links cannabis use among teens to slower development of thinking and memory skills as they 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, PhD, assistant professor in the Department of Psychiatry at UC San Diego School of Medicine and lead author of the study, in a press release. “These differences may seem small at first, but they can add up in ways that affect learning, memory and everyday functioning.”

The study, published in Neuropsychopharmacology, analyzed data from more than 11,000 participants in the Adolescent Brain Cognitive Development (ABCD) Study, the largest long-term study of brain development in U.S. youth. It followed children starting at age 9 or 10 and tracked them through 16 or 17, along with substance use and cognitive performance.

Teens who used cannabis showed restricted growth over time across many skills — memory, attention, language and processing speed — compared to those who did not. In some cases, skills were comparable when the adolescents were younger, but progress leveled off as they got older and started using cannabis, while skills of their peers continued to improve.

A smaller subset of the study found that teens with evidence of exposure to tetrahydrocannabinol (THC), the main psychoactive ingredient in cannabis, showed worse memory over time than those who did not use the drug. Teens with evidence of cannabidiol (CBD), or the non-psychoactive component, did not show the same pattern.

“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 differences between teens seen in the study were relatively modest, but researchers say they could still matter. During adolescence, the brain is rapidly developing, and even small changes to memory, attention, or thinking speed can affect school performance and daily life.

The researchers do note that the study does not prove cannabis use directly causes these changes, noting that other factors — such as environment or personality — may play a role, although they did account for many of these influences in the study.

Bottom line, said Wade, “Delaying cannabis use supports healthy brain development.” And as the drug becomes more widely available, “it’s important for families and teens to understand how it may affect the developing brain.”