Causal pathways from social media use to adolescent cannabis use: An instrumental variable mediation analysis of behavioral problems

INTRODUCTION: Cannabis use among adolescents is an increasing public health concern, particularly in the context of the digital era, where social and behavioral influences are rapidly evolving. However, the mechanisms linking social media exposure to cannabis use remain unclear. This study investigates the association between social media use and cannabis use in late adolescence, examining the mediating roles of behavioral problems using an instrumental variable (IV) approach to address endogeneity. METHODS: We analyzed data from 1766 participants in the Longitudinal Study of Australian Children (aged 18-19 years at Wave 8) using a control-function approach, with mobile phone ownership with home internet access as instrumental variables for social media use. We evaluated adolescent cannabis use using self-reported lifetime cannabis use from longitudinal surveys of children aged 18-19 years at Wave 8. Generalized structural equation modeling (GSEM) was applied in the second stage to estimate IV-based direct, indirect, and total effects through behavioral problems, with bias-corrected bootstrapped confidence intervals. RESULTS: By late adolescence, 34.6% reported cannabis use. Instrumental mediation analysis showed instrument-induced frequent social media use was associated with nearly threefold higher odds of cannabis use (OR = 2.85; 95% CI: 1.99-4.10). Externalizing behaviors significantly mediated this IV-based relationship, accounting for a 22% increase in cannabis use odds via this pathway (OR = 1.22; 95% CI: 1.10-1.37). Internalizing behaviors did not mediate the association. The total IV based indirect effect through behavioral problems was significant (OR = 1.20; 95% CI: 1.07-1.36), confirming behavioral problems as key mechanisms linking social media use and cannabis use. CONCLUSIONS: IV-based frequent social media use is robustly associated with increased cannabis use in adolescents, primarily through elevated externalizing behaviors. These findings highlighted the need for integrated digital and behavioral interventions targeting externalizing symptoms to mitigate substance use risk in the digital era. Enhanced parental engagement and digital literacy may further buffer against adverse outcomes associated with social media exposure.