A long-term study of more than 11,000 children and teens has linked recreational cannabis use to slightly slower development of memory, attention, and thinking skills.
The findings add to public health advice that teens should avoid cannabis use, as it may harm the developing brain.
"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," says neuropsychologist Natasha Wade from UC San Diego, first author of the study.
"These differences may seem small at first, but they can add up in ways that affect learning, memory, and everyday functioning."
This is the largest analysis of its kind conducted in the US. It's based on data from the Adolescent Brain Cognitive Development study, which tracked the brain health and function of 11,036 young people from age 9 or 10 until they reached age 16 or 17.
Each year, participants underwent hair, urine, breath, and saliva tests, and they answered questions about their use of cannabis and other substances.
Most cannabis use was recreational, although a small percentage were using medicinal CBD (cannabidiol) with permission from their parents or doctor.
The combined approach of self-reporting and biosamples likely made the study's measurement of the teens' cannabis use more reliable than self-report alone.
The researchers assessed participants' cognitive development over time using a series of tests that gave scores on immediate recall, delayed memory, processing speed, inhibitory control, visuospatial processing, language, and working memory.
They found that teens who used cannabis tended to have slower growth in all areas of cognitive development. This cohort actually tended to score just as well as or even better than their peers prior to cannabis use. But once cannabis had become part of their lives, their progress waned.
While this study only provides a link – meaning it's unclear whether cannabis is actually the cause of slower cognitive development – it did account for many of the other factors that might explain the pattern, like family background, mental health, and the use of other substances.
The pattern also became clearer when cannabis use was broken down further by the cannabinoids teens were exposed to, which adds further credence to the possibility that these substances are playing a part.
Teens with exposure to tetrahydrocannabinol, or THC, had far greater problems with memory than those who didn't use cannabis, while those whose cannabis use was limited to CBD (a small subgroup) had relatively normal scores.
"These results point to THC as a likely driver of the changes we're seeing," Wade says.
"It also highlights how complicated cannabis products can be, especially since some products labeled as CBD may still contain THC."
Rates of cannabis use among young adults (13–24 years old) have actually remained relatively steady in recent decades. But legalization in many US states, along with huge increases in THC content, and decreased public risk perception, have changed the broader landscape.
Research suggests that recreational drugs, like alcohol or cannabis, can impact the brains of adolescents differently from adults, and in ways that may disrupt typical development.
Though the differences between cannabis users and non-users were small, the researchers note that in the highly competitive environment young people are immersed in, these could translate to clinically significant changes to learning and memory.
Even slight changes to such skills can affect standardized testing, grade advancement, and higher-education opportunities, the authors point out. Changes to visuospatial skills could also, theoretically, impact teen driving skills.
Related: Teen Cannabis Use Dramatically Raises Risk of Psychotic Disorders
Of course, the pressures young people face that push some toward the use of substances like cannabis do not disappear because of findings like these. But these results suggest the short-term fix could have long-term impacts.
"Delaying cannabis use supports healthy brain development," Wade says. "As cannabis becomes more widely available, it's important for families and teens to understand how it may affect the developing brain."
The research was published in Neuropsychopharmacology.