BACKGROUND: Pairing e-cigarettes with healthy lifestyle contexts in influencers' promotional social media posts may increase e-cigarette use risk among young adults. This study examined the effects of e-cigarette and healthy lifestyle content on young adults' perceptions of influencer credibility, harm perceptions of, and susceptibility to use, e-cigarettes. METHODS: In this survey-based online, repeated-measures experiment (2023), California young adults (N=1494, M RESULTS: Participants in the experimental group were more likely to report lower harm perceptions (adjusted OR (AOR) 1.12
95% CI 1.03
1.21) and higher susceptibility to e-cigarette use (AOR 1.22, 95% CI 1.11
1.34), than participants in the control condition. Similar results were found when influencers were perceived as credible, but not when they were perceived as non-credible. CONCLUSIONS: Social media influencers' posts promoting e-cigarette use along with healthy activities may contribute to young adult e-cigarette use. Decreasing perceived credibility of influencers could decrease the negative effects of their promotional posts. TRIAL REGISTRATION NUMBER: NCT06433466.