Objective: Systematically review the scientific literature to characterize the effects of cannabis use on brain structure, function, and neurodevelopmental outcomes in adolescents and young adults with ADHD.
Method: Systematic review following PRISMA guidelines utilizing PubMed, Embase, PsycINFO, and Cochrane CENTRAL trials register from inception until 1 January 2020. Articles that examined the impact of cannabis use on youth with ADHD were included.
Results: Eleven studies were identified that compared outcomes for individuals with ADHD who used cannabis or synthetic cannabinoids against those with ADHD who did not. Seven of these studies used neuroimaging techniques, including fMRI, structural MRI, and SPECT. Differential regions of activation were identified, including the right hippocampus and cerebellar vermis, and bilateral temporal lobes. Morphological differences were identified in the right precentral and postcentral gyri, left nucleus accumbens, right superior frontal and postcentral gyri. No study identified any additive or ADHD × cannabis use interaction on neuropsychological tasks of executive function. Two studies found adverse differential impacts of early-onset cannabis use in this population.
Conclusion: A dearth of evidence is available on the impact of cannabis use on the developing brain and functioning for individuals with ADHD, despite the elevated risk for substance use in this population. The limited, potentially underpowered evidence does not support the hypothesis that cannabis use has a deleterious impact on neuropsychological tasks in transitional age youth with ADHD. Larger and longer-term studies are needed, however, to better inform clinicians and patients as to the impacts of cannabis use in youth with ADHD.
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