Adolescent brain surface area changes after starting alcohol and cannabis differed from alcohol-only users | Research

The takeaway

A longitudinal study of 69 youth scanned before and after substance use initiation found that alcohol-only users showed greater surface area decreases in the medial orbitofrontal cortex than those who used both alcohol and cannabis.

Neuroscience researchers studying adolescent brain development; substance use prevention researchers; clinicians working with adolescent populations.

Alcohol-only users showed greater orbitofrontal surface area loss than alcohol-plus-cannabis users

What the researchers found

Researchers obtained brain scans from 69 adolescents at baseline (ages 12-14, before any substance use) and again at follow-up (ages 17-21).

Participants were split into three groups: alcohol-only initiators, alcohol-plus-cannabis initiators, and minimal-use controls.

All groups showed surface area decreases over time (consistent with normal brain maturation), but the pattern differed by substance use.

A significant group-by-time interaction appeared in three regions: bilateral medial orbitofrontal cortices and right insula. In these areas, the alcohol-only group showed more substantial surface area decreases than the alcohol-plus-cannabis group.

The right medial orbitofrontal cortex finding survived conservative vertex-wise analysis, with the alcohol-only group showing significantly greater surface area reduction compared to the combined alcohol-plus-cannabis group.

Why it matters

This is one of few studies with pre-substance-use baseline brain scans, which helps distinguish pre-existing brain differences from substance-induced changes. The unexpected finding that alcohol-only users showed greater surface area loss than cannabis-plus-alcohol users challenges assumptions about additive brain effects.

The numbers in context

69 youth total (23 per group). Baseline ages 12-14, follow-up ages 17-21. Significant group-by-time interactions in bilateral medial orbitofrontal cortices and right insula. Right medial orbitofrontal cortex finding survived vertex-wise analysis at p < .001.

How the study worked

Longitudinal neuroimaging study with baseline scans before substance use initiation and 6-year follow-up. Groups matched demographically. Used FreeSurfer parcellation of 34 cortical regions per hemisphere. Repeated measures ANCOVA for group comparisons with vertex-wise analysis for confirmation.

What this study cannot tell us

Small sample size (23 per group) limits statistical power. Groups differed in substance use patterns beyond just the alcohol/cannabis distinction. Surface area is one metric of brain structure and may not capture functional changes. The alcohol-plus-cannabis group may have had different overall exposure levels.

How to read the evidence

Moderate. Longitudinal design with pre-use baselines is strong, but small sample size and inability to control all confounders limit conclusions.

When this study was published

Published in 2018. Longitudinal adolescent neuroimaging studies have continued to refine understanding of substance effects on brain development.

The bigger picture

Most adolescent brain-substance studies are cross-sectional, making it hard to know whether observed differences existed before substance use began. This longitudinal design with pre-use baselines represents a stronger approach, though the sample size limits firm conclusions.

Questions still open

Why did alcohol-only users show greater surface area loss than those who also used cannabis? Could cannabis have a neuroprotective interaction with alcohol in certain brain regions, or is this a statistical artifact of small samples?

Common questions

Does this mean cannabis protects the brain from alcohol damage?

Not necessarily. The finding that alcohol-only users showed greater surface area decreases could reflect differences in drinking patterns, genetics, or other unmeasured factors between the groups. The sample was small and this finding needs replication before drawing any protective conclusions.

Why is surface area important?

Cortical surface area is a measure of brain structure that reflects the amount of folded cortical tissue. Changes in surface area during adolescence are part of normal development, but unusual patterns may indicate substance-related effects on brain maturation.

Read the original research

Adolescent Brain Surface Area Pre- and Post-Cannabis and Alcohol Initiation.

Journal of studies on alcohol and drugs, 79(6), 835-843

View on PubMed ↗

Citation

Infante, M Alejandra; Courtney, Kelly E; Castro, Norma; Squeglia, Lindsay M; Jacobus, Joanna. (2018). Adolescent Brain Surface Area Pre- and Post-Cannabis and Alcohol Initiation.. Journal of studies on alcohol and drugs, 79(6), 835-843.