PROJECT SUMMARY/ABSTRACT Adolescents and young adults (AYA) are exposed to, share, and create content about non-cigarette other tobacco products (OTP) on social media, which has been demonstrated to shape tobacco-related perceptions and behaviors. New research has documented content that promotes negative health behaviors including OTP use on TikTok, a social media app on which users create and share short-form videos and that is regularly used by more than 60% of US teens. However, TikTok content changes quickly, and it is unknown what OTP message and delivery features are used on TikTok, especially over time, and how they affect message reach and engagement. The long-term goals of this research program are to elucidate social media’s influence on AYA OTP use and to inform unique up-to-date regulatory and public health interventions appropriate for different tobacco products, social media platforms, and generations of young people. The overall objective of this application is to identify what and how OTP messages (i.e., perceptions, opinions, advice, information) are conveyed in short-form social media videos created by TikTok users using an adaptive community-engaged approach. The rationale is that adaptive youth-engaged investigation will illuminate how social media might perpetuate pro-tobacco sentiments in a way that is transferable to other platforms and risk behaviors over time. The objective will be addressed with three aims: (1) Identify and characterize underlying messages (e.g., perceptions, information) in OTP content on the short-form video-based social media platform TikTok, (2) Elucidate how delivery features (i.e., app features, creator choices) convey OTP messages on TikTok; (3) Estimate the relationships between key OTP messages and delivery features and video reach (i.e., views) and engagement (e.g., likes, comments) on TikTok. These aims will be accomplished in a longitudinal observational study that identifies OTP TikTok content by training the TikTok algorithm to push relevant content to TikTok’s For You Page (i.e., “feed”) for multiple accounts. The study leverages both qualitative and quantitative analytic techniques and engages YA researchers at two sites in all aspects of study design, execution, and dissemination. The research proposed is innovative, in the applicants’ opinion, because its immersive, youth-engaged approach can simulate the user experience and provide rich, rigorous, interpretation of the OTP messages embedded in AYA’s social media environment. The proposed research is significant because it will help scholars and practitioners contextualize AYA OTP use in the information environment on TikTok and, by identifying video features that stay popular and engaging over time, it can enhance the authenticity of health communication that counters pro-tobacco sentiments on TikTok. Ultimately, such knowledge has the potential to enhance primary prevention efforts that reduce AYA nicotine addiction and tobacco use.