New research suggests that adolescents who begin using cannabis may experience a slowing in their cognitive development compared to peers who do not use the drug. The findings, published in Neuropsychopharmacology, indicate that while these youth often start out with slightly better cognitive scores in late childhood, their rate of mental improvement falls behind as they age.
Adolescence is a time of extensive brain development. During these years, the brain undergoes major structural and functional changes that lay the groundwork for adult cognitive abilities. Because the brain is changing so rapidly, it is particularly sensitive to outside chemical influences during this window.
Cannabis contains more than 120 cannabinoids, but the two most studied are THC and CBD. THC, or delta-9-tetrahydrocannabinol, is the primary psychoactive component responsible for the high associated with cannabis use. CBD, or cannabidiol, is a non-intoxicating compound sometimes used for perceived medical or therapeutic benefits. Some adult studies suggest CBD might blunt THC’s effects on thinking, though results are mixed and little is known about its impact on teens.
The research, led by Natasha E. Wade of the University of California, San Diego, aimed to map how the onset of cannabis use tracks with cognitive changes across adolescence. Prior studies have often relied on teenagers self-reporting their substance use, which can lead to inaccuracies. For instance, a 2023 study of young adults indicated that self-reports often underestimate the prevalence of most illicit substances by 30 to 60 percent when compared to objective hair testing.
To get a more accurate picture, Wade and her colleagues combined self-reported data with objective biological tests. This approach allowed the researchers to capture a more complete timeline of when teenagers started using cannabis and what specific types of cannabinoids were present in their systems.
The scientists drew data from the Adolescent Brain Cognitive Development (ABCD) study, a large national project tracking the biological and behavioral development of youth across the United States. The current analysis included up to 11,036 youth between the ages of 9 and 17, depending on the test. For instance, working memory was assessed in 4,371 participants and verbal recall in 4,487.
To identify which participants had started using cannabis, the research team looked at annual interviews where the youth reported their own substance use alongside results from biological samples like hair, urine, breath, and oral fluid tests. Self-report flagged 67 percent of cannabis users, hair tests 22 percent, and urine tests 21 percent. These figures overlap because some youth were flagged by more than one method, but they show that the biological tests caught youth who did not report their use.
Youth were counted as cannabis users from the first time any use beyond a single puff or taste was reported or detected, and they stayed in that group for the rest of the study, regardless of how often they used afterward. In the full cohort, about 2,200 youth were identified as having used cannabis, compared with about 9,700 who had not.
The participants completed a battery of cognitive tests every other year to measure a variety of mental skills. For example, working memory was tested by asking participants to mentally sort a list of animals and foods by size. Inhibitory control, or the ability to suppress impulsive responses, was measured using a task where youth had to quickly identify the direction of an arrow while ignoring distracting arrows pointing the other way. Other tasks measured language skills, processing speed, visuospatial processing, and episodic memory.
The researchers accounted for a wide range of factors that might independently influence brain development or drug use. These included the highest level of education achieved by the parents, family history of substance use problems, prenatal exposure to drugs or alcohol, parent-reported emotional and behavioral problems at the start of the study (ages 9 to 10), sex, race and ethnicity, and the use of other substances like alcohol and nicotine.
The results indicated that the cognitive trajectories of youth who initiated cannabis use differed from those who did not. In late childhood between the ages of 9 and 11, the youth who would eventually use cannabis tended to score slightly higher on most of the tests than their peers.
As the youth progressed through adolescence, this pattern reversed. While the non-using youth showed steady, age-typical improvements in their cognitive scores, the youth who initiated cannabis use experienced a flattened trajectory. On most tasks they still improved, just not as quickly. By ages 15 to 17, the teens who used cannabis performed worse than their non-using peers across almost all measured domains, including processing speed, verbal recall, and visuospatial skills.
The researchers described most differences as small, with a medium-sized effect for episodic memory and a large effect for working memory. They noted that even modest effects could matter for real-world activities like schooling and driving.
The researchers also wanted to know if different chemical components of cannabis might relate to these cognitive changes in different ways. They looked at a smaller subset of 645 participants who had repeated hair toxicology tests between the ages of 12 and 16. Hair testing mainly detects regular use, roughly twice a month or more, so this analysis likely reflects heavier use than the main one. The team divided this group into those with no cannabinoids in their hair, those with THC (81 youth), and those with CBD (21 youth, 13 of whom also had THC in their hair).
The hair analysis indicated that teens with objective evidence of recent THC exposure had a reduced rate of improvement in episodic memory compared to teens with no cannabinoids in their hair. Episodic memory involves the ability to recall specific events and experiences. By ages 15 to 17, the THC group showed worse episodic memory performance than the control group, whereas the cognitive trajectories of teens with CBD in their hair did not differ from the control group.
Episodic memory was the only task on which the THC group differed from controls; they did not differ on language, processing speed, inhibitory control, or reading. This analysis also adjusted for fewer factors than the main one: sex, other substances detected in hair, and study site.
“This is an impressive and timely study using longitudinal data from the ABCD database to examine cognitive function over time in relation to cannabis use, in a large sample of youth aged 9-17,” Harriet de Wit, a professor in the Department of Psychiatry and Behavioral Neuroscience at the University of Chicago who was not involved in the research, told PsyPost. “The authors controlled for numerous potential confounding factors,” she added. “The self-reported use of the drug was validated with biological samples. These data confirm a growing public concern that cannabis use among adolescents can have serious adverse developmental consequences.”
The findings are in line with an earlier study of the same group of youth covered by PsyPost in 2024, which linked cannabis use to poorer episodic memory at ages 13 to 14. They also echo a 2021 meta-analysis that tied frequent teen cannabis use to a drop of about two IQ points, though it measured overall IQ rather than specific mental skills, and a 2017 study that linked starting cannabis around age 14 to poorer verbal and learning abilities by age 20, though it followed only disadvantaged boys and relied on self-reports. The results stand in some tension with a study covered by PsyPost in 2018, which found that adults who used cannabis as teens performed as well as or better than non-users on a spatial working memory task, though that study tested adults at age 28 rather than tracking them through adolescence.
As with all research, there are a few things to keep in mind. The study cannot show that cannabis caused the slower gains. The authors note that whatever makes some youth more likely to try cannabis, such as maturing earlier, could also shape their cognitive development. They also note that scores that start out above average tend to drift back toward the average over time, a statistical pattern called regression to the mean.
The secondary analysis focusing on specific cannabinoids had a relatively small sample size. Only 21 participants were identified as having CBD in their hair, which makes it difficult to draw broad conclusions about the impact of CBD on the developing brain.
Additionally, teenagers using CBD might be doing so to manage underlying symptoms, such as sleep issues or anxiety. These pre-existing conditions could influence cognitive test scores independently of the CBD itself. The researchers also note that some participants might be exposed to THC inadvertently, as product mislabeling is a known concern even in regulated markets.
Finally, the participants in this study are still in the middle of adolescence. Their brains will continue to mature into their mid-twenties. Continued monitoring of this group will help clarify how quitting or heavier use relates to cognition over time.
The study, “Longitudinal neurocognitive trajectories in a large cohort of youth who use cannabis: combining self-report and toxicology,” was authored by Natasha E. Wade, Ryan M. Sullivan, Alexander L. Wallace, Rachel Visontay, Veronica Szpak, Krista M. Lisdahl, Marilyn A. Huestis, Priscila Dib Gonçalves, Hollie Byrne, Louise Mewton, Joanna Jacobus, and Susan F. Tapert.
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