Although declining, e-cigarettes remain the commonest tobacco products among youth in the United States (US). In 2022, about 2.5 million youth (9.4 %) reported past 30-day (P30D) e-cigarette use (Park-Lee, 2022). Youth susceptibility to e-cigarette use is more pronounced among persons with mental/psychological conditions (Green et al., 2018, Riehm et al., 2019). In 2021, compared to youth with no psychological distress, current use of any tobacco product was 2.6 times, 2 times, and 1.7 times more prevalent in youth with severe, moderate, and mild psychological distress, respectively (Gentzke, 2022). Notably, many tobacco naïve youth are likely to begin e-cigarette use to cope with mental health conditions (e.g., depression) (Erskine et al., 2015, Gentzke, 2022). Distress from mental health conditions that are often expressed inwards are termed internalizing symptoms (Cosgrove et al., 2011). In 2019, about 18 %–38 % of adolescents in the US reported at least one internalizing mental health condition (e.g., depression, anxiety) in their lifetime (Audrain-McGovern et al., 2021, Bitsko, 2022, Conway et al., 2018, Pérez et al., 2022, Riehm et al., 2019). Depression is the most prevalent self-reported internalizing condition, with approximately 37 % of US youth experiencing symptoms of depression (i.e., sadness and hopelessness for ≥2 weeks) at least once in 2019 (Bitsko, 2022).

While some studies found no association between certain internalizing conditions (e.g., anxiety, obsessive compulsion) and youth e-cigarette use (Audrain-McGovern et al., 2021, Leventhal et al., 2016), others suggest a bidirectional relationship between some internalizing conditions (e.g., depression, panic) and youth e-cigarette use (Audrain-McGovern et al., 2021, Conway et al., 2018, Green et al., 2018, Lechner et al., 2017, Leventhal et al., 2016, Riehm et al., 2019). For example, among youth who have never used tobacco, two prospective studies found that compared to no/low/moderate severity, the presence of high-severity internalizing symptoms at baseline was associated with a 40 %–60 % increase in the odds of e-cigarette initiation at one-year follow-up (Green et al., 2018, Riehm et al., 2019). Youth with internalizing symptoms have an increased risk of initiating e-cigarette use (Green et al., 2018, Riehm et al., 2019); using e-cigarettes at an earlier age (Pérez et al., 2022); future tobacco use (Conway et al., 2018, Lechner et al., 2017); vaping more frequently (Audrain-McGovern et al., 2021); using multiple tobacco products (Conway et al., 2018, Sawdey et al., 2019) and experiencing multiple substance use disorders (Conway et al., 2018, Hautala et al., 2019) than those with no/low internalizing symptoms. Also, youth who use e-cigarettes are more likely to report internalizing symptoms (e.g., depression, anxiety) and more severe symptoms with increased e-cigarette use frequency than non-users, especially with increased e-cigarette use frequency (Becker et al., 2021, Conway et al., 2018, Jacobs et al., 2022, Leventhal et al., 2016). Previous studies also show that youth with e-cigarette use have similar internalizing symptom scores as those who smoke cigarettes (Becker et al., 2021, Sawdey et al., 2019). Therefore, compared to youth without internalizing symptoms, there is an increase in the odds of nicotine dependence among youth cigarette users with internalizing symptoms (Bainter et al., 2020, Dierker and Donny, 2008, Dierker et al., 2015, Hautala et al., 2019).

Exposure to nicotine in adolescent mice is found to alter neuronal pathways in the brain, resulting in amplified neuronal activity with episodes of depressive and anxiety-like behavioral malfunctions that persist into adulthood (Hudson et al., 2021, Jobson et al., 2019). Additionally, nicotine dependence often co-exists with psychiatric conditions, including depression (Becker et al., 2021, Conway et al., 2018, Leventhal et al., 2016) escalating the progression from tobacco initiation to nicotine dependence over time among those who smoke cigarettes (Dierker & Donny, 2008). Moreover, studies on cigarette smoking indicate that both depression and lifetime smoking levels are independently associated with nicotine dependence (Dierker et al., 2015). Youth and young people with higher lifetime smoking levels report greater nicotine dependence. Conversely, those with higher depression scores have greater nicotine dependence scores even after adjusting for participants within and between lifetime smoking levels (Dierker et al., 2015).

This body of research suggests that the presence of one or more internalizing symptoms among young people who use tobacco is associated with increased odds of and progression to nicotine dependence compared to the general youth population without these symptoms (Bainter et al., 2020, Dierker and Donny, 2008, Dierker et al., 2015, Hautala et al., 2019). However, for young people who use e-cigarettes, no study has looked at the relationship between time to nicotine dependence and internalizing symptoms. Therefore, this study aims to fill this knowledge gap using nationally representative data from Waves 1–5 (2013–2019) of the Population Assessment of Tobacco (PATH) study. The study objective is to determine the relationship between past year internalizing symptoms (any vs. no) and time to nicotine dependence symptoms among young people following their first P30D e-cigarette use. Nicotine dependence was assessed with three validated measures that reflect early (cravings and really needing to use) and severe (use within 30 min of waking) signs of nicotine dependence (DiFranza and Wellman, 2005, DiFranza et al., 2002). The hypothesis is that the time to nicotine dependence among P30D e-cigarette users will be shorter for those with any compared to those with no past year internalizing symptoms across all nicotine dependence indicators.