Does frequent cannabis use affect thinking skills in heavy-drinking young adults? | Research

The takeaway

In a study of 598 heavy-drinking young adults aged 21-25, daily cannabis use was associated with poorer working memory, more impulsive decision-making, and more ADHD symptoms, but occasional use showed no deficits and earlier age of initiation did not worsen outcomes.

Young adults curious about how cannabis frequency relates to cognitive performance, and clinicians assessing substance use in emerging adults.

Daily use: deficits; occasional use: none

What the researchers found

Daily cannabis users showed significantly poorer working memory, more impulsive delay discounting, and greater ADHD symptom endorsement compared to non-users. Occasional users (monthly or weekly) did not differ from non-users on any measure. Effect sizes were small. Earlier age of cannabis initiation was not independently or interactively associated with worse performance.

Why it matters

This study separates daily from occasional use, revealing that the cognitive concerns about cannabis may be specific to heavy use patterns. The lack of deficits in occasional users challenges blanket warnings about any cannabis use damaging cognition.

The numbers in context

598 participants; 276 non-users, 201 occasional users, 121 daily users; ages 21-25; daily use linked to poorer working memory, impulsive discounting, and ADHD symptoms; effect sizes small; no effect of age of initiation

How the study worked

Cross-sectional study of 598 high-risk drinking emerging adults (ages 21-25) categorized as non-users (n=276), occasional users (n=201), or daily users (n=121). Assessed working memory, attention, behavioral inhibition, delay and probability discounting, verbal intelligence, and ADHD symptoms. Controlled for age, race, sex, income, education, tobacco, and alcohol use.

What this study cannot tell us

Cross-sectional design cannot determine causation. All participants were heavy drinkers, limiting generalizability. Self-reported cannabis use categories. Small effect sizes suggest modest practical significance.

How to read the evidence

Large sample with well-controlled analyses, but cross-sectional design and heavy-drinking sample limit causal inference and generalizability.

When this study was published

Published in 2021; longitudinal studies would better clarify whether daily use causes or follows cognitive differences.

The bigger picture

In a sample already using alcohol heavily, cannabis effects on cognition appear dose-dependent. The absence of age-of-initiation effects challenges the common assumption that earlier exposure is categorically more harmful. However, cross-sectional data cannot determine whether daily cannabis use caused these deficits or whether people with pre-existing cognitive differences gravitate toward daily use.

Questions still open

Would the same pattern hold in non-drinking cannabis users? Do the cognitive differences associated with daily use reverse with abstinence? Are pre-existing differences in working memory or impulsivity driving both daily cannabis use and poorer test performance?

Common questions

Does occasional cannabis use affect cognition?

In this study, occasional (monthly or weekly) cannabis users performed no differently from non-users on any cognitive measure, including working memory, attention, and inhibition.

Does starting cannabis younger cause more cognitive damage?

This study found no evidence that earlier age of initiation was associated with worse cognitive performance, either independently or in combination with frequency of use.

How big were the cognitive effects of daily use?

Effect sizes were small, meaning that while daily cannabis users scored lower on average, the differences were modest and many daily users performed within normal ranges.

Read the original research

Cannabis Use, Age of Initiation, and Neurocognitive Performance: Findings from a Large Sample of Heavy Drinking Emerging Adults.

Journal of the International Neuropsychological Society : JINS, 27(6), 533-545

View on PubMed ↗Publisher’s paper ↗

Citation

Parlar, Melissa; MacKillop, Emily; Petker, Tashia; Murphy, James; MacKillop, James. (2021). Cannabis Use, Age of Initiation, and Neurocognitive Performance: Findings from a Large Sample of Heavy Drinking Emerging Adults.. Journal of the International Neuropsychological Society : JINS, 27(6), 533-545. https://doi.org/10.1017/S1355617721000618