Systematic review of structural and functional neuroimaging studies of cannabis use in adolescence and emerging adulthood: evidence from 90 studies and 9441 participants

Abstract

Cannabis use peaks in adolescence, and adolescents may be more vulnerable to the neural effects of cannabis and cannabis-related harms due to ongoing brain development during this period. In light of ongoing cannabis policy changes, increased availability, reduced perceptions of harm, heightened interest in medicinal applications of cannabis, and drastic increases in cannabis potency, it is essential to establish an understanding of cannabis effects on the developing adolescent brain. This systematic review aims to: (1) synthesize extant literature on functional and structural neural alterations associated with cannabis use during adolescence and emerging adulthood; (2) identify gaps in the literature that critically impede our ability to accurately assess the effect of cannabis on adolescent brain function and development; and (3) provide recommendations for future research to bridge these gaps and elucidate the mechanisms underlying cannabis-related harms in adolescence and emerging adulthood, with the long-term goal of facilitating the development of improved prevention, early intervention, and treatment approaches targeting adolescent cannabis users (CU). Based on a systematic search of Medline and PsycInfo and other non-systematic sources, we identified 90 studies including 9441 adolescents and emerging adults (n = 3924 CU, n = 5517 non-CU), which provide preliminary evidence for functional and structural alterations in frontoparietal, frontolimbic, frontostriatal, and cerebellar regions among adolescent cannabis users. Larger, more rigorous studies are essential to reconcile divergent results, assess potential moderators of cannabis effects on the developing brain, disentangle risk factors for use from consequences of exposure, and elucidate the extent to which cannabis effects are reversible with abstinence. Guidelines for conducting this work are provided.

Fig. 1. PRISMA flowchart.

A systematic literature search of Medline and Psychinfo was conducted on June 10th, 2019, with 25 search terms, encompassing the following parameters: adolescen* and cannabis and mri. This produced 510 studies. 2 additional studies were identified during peer review. Studies were screened based on inclusion criteria: (1) participants with personal cannabis use histories (THC administration studies in healthy volunteers and studies of prenatal cannabis exposure were removed), (2) mean age ≤ 25, and (3) minimum cell size of 15 participants, and 90 studies were identified for inclusion in the current review. Note that studies that include multiple fMRI tasks or assess multiple structural characteristics are included in multiple sections, as appropriate.

Fig. 2. Prevalence of male and female participants across reviewed study domains.

Across all studies, only 35.9% of participants were female. Figure 2 displays the number of male and female participants included in each category reviewed.