ObjectiveUsing social network analysis, we assessed the mechanisms of social influence that promote e-cigarette use in adolescent networks.MethodsData on health behaviours and friendship networks from a cohort of 10 high schools in Southern California (N=1599) were collected in grade 9 Spring 2021 (W1), grade 10 Fall 2021 (W2) and Spring 2022 (W3). Two mixed effects logistic regression models were estimated (full sample and subsample of non-vapers only) to evaluate the associations of W1 and W2 pro-vaping norms, peer e-cigarette use exposure and prior e-cigarette use (full sample) on past 6-month vaping at W3, adjusting for demographic covariates and school clustering.ResultsPrevious vaping was the strongest predictor of past 6-month vaping at W3 among the full sample. Greater exposure to friend e-cigarette use at W2 (adjusted OR (AOR)=12.2, 95% CI 4.04 to 36.5) and greater pro-vaping norms at W2 (AOR=2.63, 95% CI 1.24 to 5.55) were significantly and positively associated with increased odds of initiating e-cigarette use at W3 among students with no lifetime e-cigarette use.ConclusionPeer network exposure and pro-vaping norms are significant predictors of vaping initiation even when network vaping prevalence is low.