Youth and cannabis use
UC San Diego researchers found childhood cannabis use restricts cognitive development in multiple areas, including verbal memory and learning. Though other researchers appreciate the study’s scale, they raised concerns.
Experts have warned against childhood cannabis use since before America first said “Just Say No,” but its effects on brain development are not fully understood.
That’s changing, as new research from UC San Diego sheds light on how cannabis impacts youth and their ability to maintain attention, react and retain information.
Across the nation, more than 11,000 children are participating in the Adolescent Brain Cognitive Development Study, a long-term project that began in 2016. It is the largest study of child health and brain development in the country.
The findings come as cannabis use remains common among teens: about one-third of 12th graders reported using it in 2022, according to the Centers for Disease Control and Prevention. The agency warns that using cannabis during adolescence can have permanent effects on the developing brain, especially when use is frequent.
Dr. Natasha Wade, assistant professor of psychiatry at UCSD School of Medicine, led a research team to analyze the first seven years of data from the ABCD Study. Their findings were published in April – cheekily, released on 4/20, the high holiday of herb.
“We were unique in our approach in that we use both toxicological testing and self-report to do our cannabis-use grouping,” Wade said. “Across the board, those who were using cannabis didn’t show the same level of cognitive growth. They were improving, but not at the same rate as their non-using peers.”
Stronger effect
Cannabis users’ verbal memory was the most affected, with test scores showing no improvement over time.
The drug appeared to inhibit the growth of learning, memory and visual-spatial skills while also limiting attention span and processing speed. Wade said these changes could affect skills like test-taking and driving.
“Some of the results have effect sizes that are considered large by typical statistical standards,” Wade said. “This is more consistent and a little bit stronger of an effect than we anticipated when we launched this.”
However, researchers caution that differences in how cannabis is used – such as dose, frequency and product type – make it difficult to draw firm conclusions.
Participants are interviewed privately to avoid pressure from parents, who may or may not know if their child is using cannabis. Despite this, participants may not feel comfortable discussing their use, so researchers use drug tests to improve accuracy.
Instead of standard urine testing, this study takes a different approach.
Hair-based testing can detect frequent cannabis use within three months, compared to the 30-day window for urine tests. Since participant responses are collected every two years, this longer time frame better shows who is using and who is not.
These tests find traces of THC, the drug’s psychoactive compound that gives a “high.” A single use won’t be detected in hair samples; it must be used a few times per month, indicating more frequent use, according to Wade.
“Everything you consume in your body gets into the bloodstream. Every molecule of hair is connected to your bloodstream, so it’s not external contamination from smoke, but what gets deposited as your hair is growing out,” Wade said. “We’ve focused more on the metabolite of THC, called carboxy-THC, that is only from the personal ingestion of cannabis.”
This identifies cannabis use no matter its form, including edibles and vapes.
CBD is difficult to detect because it does not have a clear metabolite – any substance produced after food is converted into energy. Since they only found it in 21 participants, the researchers’ findings are based on levels of THC.
Cannabis users identified by hair testing showed the strongest relationship between level of use and reduced long-term memory.
“THC is probably a driving mechanism of what’s going on; that fits with pre-clinical literature, where they do rodent tests and can actually administer THC,” said Wade.
Real risk
Because this study is done using observation, not controlled experimentation, researchers cannot say that cannabis is directly responsible for restricting cognitive development. Family history of substance use disorders, prenatal exposure and mental health are other possible causes.
“All of those things are controlled for within our modeling, so that we can say that cannabis does have a real relationship here. What that tells me is that there is real risk for teenagers. The brain is changing so dramatically during this time period that use is not without risk, so it’s better for them to wait,” Wade said. “Now, anyone can make their own decisions, but I think it’s important for them to know that there is some risk with this.”
UC San Diego has been at the forefront of medical cannabis research, operating a research center since the turn of the millennium. Previous research from the center has found that high doses of THC may not improve pain relief while the drug’s non-intoxicating components may hold more medical promise.
The center is a member of the Association of Cannabis Research Centers, a national collective furthering research on the drug and identifying gaps in health-related research.
Other researchers recognized the value of the study, but raised concerns about blind spots in the analysis.
Fellow West Coast cannabis researcher Devan Kansagara – a professor and physician at Oregon Health and Science University – is interested in the study’s approach, but sees issues that limit what can be concluded from it.
Kansagara explains that analyzing the cognitive differences between users and non-users does not show how ongoing and different levels of use affect developing minds.
“Youth with positive hair toxicology were presumably using more frequently and in higher amounts on average than the group with just any lifetime use,” he said.
Kansagara noted that while one might expect more significant cognitive impacts in the group identified by hair testing, the most notable findings – such as issues with learning, attention and processing speed – actually weren’t replicated in that specific analysis.
He added, however, that the study’s link between active cannabis use and declining memory is consistent with existing research.
Complex reality
Robert Welch is the director of the National Center for Cannabis Research and Education at the University of Mississippi. He says the study’s large sample size makes it strong, but the “user vs. non-user” approach simplifies a more complex reality.
“It really needs to be highlighted more in terms of the potency of the cannabis and the frequency of use,” he said. “It’s very difficult to pinpoint exactly what kind of products these consumers are using. It’s such a complex plant and the products that are produced from the plant are all so different.”
In addition, national comparisons are complicated by differences in state laws – especially for cannabis.
“In states that have recreational laws like California, it’s less of an issue because they have access to higher quality products,” Welch said. “If you went to a gas station in Mississippi… those products are very different and not regulated.”
As researchers continue tracking participants into adulthood, future data may clarify how lasting these cognitive differences are – and whether they persist after cannabis use stops.
