Study: Too Soon to Tell Whether Cannabis Affects Brain Development in Young People With ADHD


Cawkwell and his colleagues emphasized the need for further research to clarify the possible effects of cannabis on brain structure, function, and behavior in young people with ADHD.

A review in the July/August issue of the Harvard Review of Psychiatry found that there is currently limited research showing whether cannabis has additional adverse effects on brain development or functioning in adolescents or young adults with attention-deficit hyperactivity disorder (ADHD).

"[T]he evidence to date does not clearly support either an addictive effect or an interaction – whether protective or harmful with cannabis use," said study author Philip B. Cawkwell, MD, of Stanford University School of Medicine, in a press release.

Cawkwell and his colleagues emphasized the need for further research to clarify the possible effects of cannabis on brain structure, function, and behavior in young people with ADHD.

The team performed a systematic review of research on the combined effects of cannabis use and ADHD in adolescence. The search identified 11 studies assessing any type of neurodevelopmental outcome in adolescents or young adults with ADHD who used cannabis compared with those who did not use cannabis.

There were some significant differences shown in the 7 studies assessing neuroimaging findings for young patients with ADHD in brain structure among cannabis users, according to the researchers. The findings included decreased thickness in areas involved with motor and sensory function and increased thickness in areas involved in the brain’s “reward” system. There were limitations of the study, which the authors noted because they said it was impossible to determine whether these findings reflect causal relationships.

Further, functional imaging studies reported differences in cannabis users with ADHD and in performance on standardized tasks and reduced density of dopamine transporters. This affects the availability of dopamine, which plays a critical role in the reward system, according to the study authors.

Four of the studies examined the results of neuropsychological tests or questionnaires in young people with ADHD who did and did not use cannabis, since its use was associated with impaired performance on tests of sustained attention. However, neither of these studies found a significant interaction 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," Cawkwell said in the press release. “However, this lack of interaction may just be due to the limited number of studies to date, rather than a true lack of impact.”

A key limitation to the research were the number of studies and the overall number of participants, in addition to the findings that suggest differences in the effects of cannabis use at earlier ages, which is a major gap for researchers. Additional factors needing further analysis include potency of cannabis and frequency of use.

"[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,” Cawkwell said in the press release.


Does cannabis affect brain development in young people with ADHD? Too soon to tell, reports Harvard Review of Psychiatry. Wolters Kluwer. July 2, 2021. Accessed July 19, 2021.

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