It’s 4/20 – but U.S. teens should avoid cannabis unless they want to stunt their brain development, according to a new study from California researchers.

Teenagers who use cannabis in any way have a harder time developing critical brain skills like thinking and memory, according to Monday’s findings from the largest long-term study of brain development in U.S. youth.

And an analysis of more than 11,000 teens – now in their early 20s – has revealed the ingredient that gets people high may be to blame.

Teens with exposure to the psychoactive chemical compound THC, also known as tetrahydrocannabinol, had worse memory over time than those who did not use cannabis. But those with evidence of the second most prevalent active ingredient, cannabidiol, or CBD, did not show the same pattern.

“These results point to THC as a likely driver of the changes we’re seeing,” lead author Natasha Wade, an assistant professor at UC San Diego School of Medicine, explained in a statement. “It also highlights how complicated cannabis products can be, especially since some products labeled as CBD may still contain THC.”

Starting in kids between 2016-2018, the study followed 11,036 children from ages 9-10 through ages 16-17 who were a part of the Adolescent Brain Cognitive Development Study.

The federally funded study spans 21 research sites and dozens of states, helping researchers discover the impacts of sleep on adolescent mental health and how loneliness makes teens lash out.

The researchers used self-reported use data as well as hair, urine and saliva testing to determine what the teens had used.

They also used the results of cognitive tests, including identifying whether two pictures were identical, being asked to sort images in order of how they were shown, recalling words that were read to them and selecting whether a “little man’s” briefcase was in his right or left hand when he was rotated and moved.

The results showed that memory, attention, language and processing speed were all affected, with teens who had outperformed others in early years showing progress plateauing in their older years while using cannabis.

“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 Wade. “These differences may seem small at first, but they can add up in ways that affect learning, memory and everyday functioning.”

While the study does not prove that cannabis is the direct cause of these changes and other factors may play a role, the researchers say they accounted for many of those potential influences, including past cognitive performance, family history, any other substance use and family background.

The study builds on 2025 research showing any cannabis use in teens can worsen mental health and academic performance and a February study linking use to an increased risk of mental health disorders.

The study also supports data from the Centers for Disease Control and Prevention, showing that problems with memory and reduced problem-solving and attention span are linked to teen cannabis use.

More than 30 percent of high school seniors reported using cannabis in the past year, according to the agency’s most recent data.

The researchers will continue to track the participants into young adulthood to better understand the effects of cannabis use, but note that brain development during the teenage years is critically important.

As a teenager, the prefrontal cortex, which helps with decision-making and impulse control, is still growing and changing.

Harvard Medical School says studies have shown that the adolescent brain is only about 80 percent developed, and the prefrontal cortex does not fully mature until the mid-to-late 20s, according to Northwestern Medicine.

“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.”