Little Evidence to Show Cannabis Helps Treat ADHD, Review Finds
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Attention deficit hyperactivity disorder (ADHD) is associated with impairment in multiple aspects of cognitive function. People with ADHD commonly report having difficulty focusing for extended periods of time and struggling with executive function tasks. Teen cannabis use has also previously been linked to problems in cognitive development, including problems with verbal and short-term memory, and attention and inhibitory control.
Notably, previous research has shown that around one-in-four adolescents with a substance use disorder also have comorbid ADHD, and youth with ADHD are around six times more likely than their peers to have problems with drug or alcohol abuse. As a result of this overlap between ADHD, drug use, and cannabis-related cognition problems, it is important that scientists understand how cannabis use might affect the brains of young people with ADHD.
In a new systematic review, published last week in the Harvard Review of Psychiatry, researchers from the Department of Psychiatry and Behavioral Sciences at Stanford University found that the current evidence base is too limited to properly establish whether cannabis has any additional adverse or beneficial effects on cognition in young people with ADHD. In light of this, they are urgently calling for more research into the potential effects of cannabis on brain structure, function, and behavior in this population.
Current studies cannot determine the cause of any observed differences
The systematic review focused on research that was available on PubMed, Embase, PsycINFO, and the Cochrane CENTRAL trials register up until January 2020. Out of hundreds of initial ‘hits’ for research concerning cannabis use and youth wit ADHD, the researchers were able to identify just 11 studies that assessed any kind of neurodevelopmental outcome in teen or young adult cannabis users with ADHD, versus their peers who did not use cannabis.
Seven of the eleven studies used neuroimaging in their research, and they found some significant differences in brain structure between both groups. Cannabis users were found to have decreased thickness in some brain regions associated with motor and sensory function (right precentral and postcentral gyri) and increased thickness in areas involved in the brain's reward system (left nucleus accumbens). The cannabis users also tended to have a lower density of dopamine transporters, which would also affect this reward system.
However, while these findings seem interesting on the surface, the Stanford reviewers note that these studies were unable to determine whether these relationships were causal or not and so should be interpreted with caution.
The remaining four studies involved neuropsychological tests or questionnaires given to young people with ADHD. While the cannabis using groups generally performed more poorly on tests of sustained attention, these studied found no significant interactions between ADHD and cannabis use.
“Surprisingly, as cannabis use demonstrates clear and consistent adverse effects on cognition as measured by neuropsychological task performance, no study identified a significant differential impact of cannabis use on these measures for individuals with ADHD compared to non-users,” the study authors write. “However, this lack of interaction may just be due to the limited number of studies to date, rather than a true lack of impact.”
Upcoming research promises wider sample sizes and long-term data
The reviewers conclude that largest limitation on the current body of research is not just the relatively small number of studies, but also the very small number of total participants involved in these studies.
Some of the studies covered in the review indicated that there may be differences in effects depending on the age of the user. Additionally, these studies rarely considered the potency of cannabis being used or the frequency of use.
The reviewers point to one upcoming study – the Adolescent Brain Cognitive Development Study, or “ABCD” study – as a promising source for potential future examination. The ABCD study is an ongoing longitudinal study following over 10,000 young people for a decade between the ages of 10 to 20. Using a mixture of brain imaging and neurocognition tests, the reviewers believe that the data gathered in this study could help to provide a better understanding of developmental trajectories in young people with ADHD who also use cannabis.
“[T]his important study may begin to provide answers to some of the questions that this paper has shown to be unanswered – including understanding whether cannabis does truly alter neural circuitry in youth with ADHD, how this impacts task performance, and perhaps most critically, the longer-term functional outcomes for adolescents with ADHD who also use cannabis,” the authors conclude.
The effects of cannabis use in teens and young adults
While specific research into how cannabis affects young people with ADHD has been rather sparse, there have been several important studies looking at how cannabis impacts teen behavior and development as a whole.
Teen cannabis use has been associated with greater odds of developing problems with substance abuse later in life and with higher odds of using other illicit drugs in the future. Exposure to cannabis and stress in adolescence is thought to be linked to the development of anxiety disorders in adulthood, while teens struggling with both a mood disorder and cannabis use disorder (CUD) are at a higher risk for self-harm than their peers.
Studies on brain development and cognition in adolescents have also indicated that teen cannabis use may alter how the neurons function in brain areas related to decision-making, planning, and self-control. Teens who use cannabis may also be adversely affecting their inhibitory control, which is a risk factor for the development of other addictive behaviors.