A Fine-Grained Longitudinal Study of Adolescent Cannabis Use and Its Relation to Cognitive Performance in Young Adulthood.

Janousch C.; Grob U.; Eggenberger L.; Johnson-Ferguson L.; Ribeaud D.; Eisner M.; Shanahan L.; Quednow B. · International journal of mental health and addiction · 2026

**Background & Methods** This longitudinal study analyzed data from 1,522 participants in the Zurich Project on Social Development from Childhood to Adulthood (z-proso), tracking cannabis use frequency via self-report at six time points between ages 11 and 24, with cognitive assessments (sustained attention, spatial working memory, visuospatial declarative memory) conducted at age 24. Analyses employed regressions, repeated-measures ANOVAs with Helmert contrasts, and latent class growth analyses, adjusted for prior cannabis use and sociodemographic/behavioral confounders. **Key Findings** • Cannabis use at ages 13, 17, and 20 was significantly associated with reduced sustained attention and declarative memory performance at age 24, with the strongest associations observed among almost daily/daily users, suggesting a dose-response relationship between frequency and cognitive impairment. • No statistically significant associations were identified between adolescent cannabis use and spatial working memory at age 24, indicating differential cognitive vulnerability across domains. • Four distinct cannabis usage trajectories were identified through latent class growth analysis; however, these trajectory patterns did not significantly differ in their cognitive outcomes, suggesting that frequency and timing of use may be more predictive than overall usage patterns. **Dosage & Administration** Not reported. The study assessed frequency of use (categorized to include almost daily/daily users as the highest frequency group) rather than specific dose quantities or routes of administration. **Safety & Adverse Effects** Not reported. The study focused exclusively on cognitive performance outcomes and did not assess adverse effects, psychiatric symptoms, physical health consequences, or other safety parameters related to cannabis use. **Evidence Quality** This longitudinal study provides moderate-to-good evidence quality with prospective data collection across adolescence and young adulthood, reducing recall bias compared to retrospective designs. Strengths include adjustment for multiple confounders and use of validated cognitive assessments. Limitations include reliance on self-reported cannabis use (subject to underreporting), lack of information on cannabis potency/THC concentration, inability to establish causation despite temporal sequencing, and potential selection bias from attrition over the 13-year follow-up period. The study lacks data on concurrent substance use, mental health diagnoses, or socioeconomic factors that could mediate associations. Classification as a "Review" type in the database is unclear given the empirical longitudinal design described.

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