Data from the National Health Interview Survey indicate that current use of electronic nicotine delivery systems (ENDS) among 18–24-year old young adults in the United States (US) increased significantly from 2017 to 2018, after a period of decline from 2014 to 2017 (Creamer et al., 2019). Data from the young adult cohort of the Population Assessment of Tobacco and Health (PATH) study also showed an increase in past 30-day ENDS use among 18–24-year-olds from 2016/2017–2018/2019, with the rate almost doubling from 15.1 % to 26.2 % in the one year period (United States Department of Health and Human Services et al., 2021). The increase in ENDS use has been attributed to the surge in popularity of vape pods that contain nicotine salts, particularly JUUL, a sleek USB-shaped device that dominated retail sales of ENDS products from late 2017–2019 in the US (Gaber et al., 2022, Huang et al., 2019, King et al., 2018). Sales and popularity of JUUL began to fall in late 2019 due to voluntary actions and regulatory requirements that resulted in the removal of flavors, except menthol and tobacco, from their products (Gaber et al., 2022). However, other vape pod “knock-offs,” like Puff Bar, gained popularity, and vape pods remain among the most popular ENDS devices used by young people (Gaber et al., 2022, Hammond et al., 2022).

Electronic nicotine delivery systems are often marketed as alternatives to cigarettes (Collins et al., 2019) and adults report using ENDS to try to quit smoking cigarettes (Osibogun et al., 2022). Although ENDS and cigarette use co-occur (Osibogun et al., 2017), some young adults who use ENDS have never smoked cigarettes (Bandi et al., 2021) and most college student young adults report using ENDS for reasons other than quitting or cutting down on smoking (Leavens et al., 2019). Young adults report initiating and using ENDS for various other purposes including socialization, belonging, stress reduction, and appetite suppression (Clendennen et al., 2021, Robertson et al., 2022). Use of ENDS by young adults is concerning because these products contain nicotine, the substance that contributes to addiction/dependence and to long-term, regular use (Benowitz, 2010).

Vape pods, like JUUL, contain particularly high concentrations of nicotine (Goniewicz et al., 2018). High concentrations are possible because these vape pods contain nicotine salt liquids rather than the freebase nicotine liquid used in earlier generation ENDS products. Nicotine salts facilitate the inhalation of high concentrations of nicotine that are otherwise too harsh to inhale when in freebase form. As such, earlier generation ENDS products contain nicotine liquids with concentrations that range from 3 to 24 mg/mL in comparison to a 5 % JUUL pod that contains 59 mg/mL (Benowitz et al., 2021). Moreover, one 5 % JUUL pod delivers approximately as much nicotine as one pack of cigarettes (Prochaska et al., 2022). In part because of the high nicotine concentrations, more young adults report a greater nicotine hit as a reason for using vape pods with nicotine salts than they do for earlier generation ENDS (Case et al., 2020). Similarly, there is evidence that youth and young adults who use vape pods report more symptoms of nicotine dependence than those who use earlier generation ENDS products (Boykan et al., 2019, Tackett et al., 2021) though some studies find mixed evidence for differences in dependence symptoms across device type (Case et al., 2020, Lin et al., 2022). Moreover, data from repeated cross-sectional surveys conducted in Canada, England, and the US indicate that the prevalence of ENDS dependence symptoms among 16–19-year-olds increased from 2017 to 2019 (Hammond et al., 2021), when vape pods with nicotine salts were gaining popularity.

One symptom of dependence reported by adults who use ENDS is use of their device upon waking (Sidani et al., 2019, Simpson et al., 2021, Soule et al., 2020). Qualitative research indicates that 18–25-year old young adults keep their device close while sleeping, such as under their pillow, so that they can use it when they wake (Simpson et al., 2021). Time to first cigarette is a validated and widely used measure of nicotine dependence (Heatherton et al., 1991), and there is evidence that time to first use of ENDS is correlated with adolescent and young adults’ reported frequency of ENDS use and the concentration of nicotine in their ENDS devices (Pienkowski et al., 2021). Some research indicates that adults who use ENDS daily report a longer time to first use upon waking than adults who use cigarettes daily (Liu et al., 2017). However, more recent research with young adults who are using later generation devices indicates that a greater proportion of those who use ENDS exclusively report ENDS use within the first 30 min of waking (54 %) compared with those who use cigarettes exclusively who report cigarette use within the first 30 min (18 %) (Jankowski et al., 2019). These findings underscore the disruptive nature of later generation ENDS devices and suggest that there may be a new generation of nicotine dependent young adults who are using ENDS rather than cigarettes. Despite evidence that those who use later generation devices may be more likely than those who smoke cigarettes and those who use other ENDS products to report symptoms of nicotine dependence, relatively little is known about changes in nicotine dependence after the 2017 surge in popularity of vape pods, like JUUL. Moreover, longitudinal studies tracking ENDS dependence before and after the surge are particularly lacking.

The purpose of the present study was to examine the longitudinal trajectory of ENDS dependence across a 4.5-year period from 2014 to 2019 among young adults. Dependence was assessed with one item, using ENDS within 30 min of waking, and we assessed if the proportion of young adults who reported ENDS dependence increased after the 2017 surge in popularity of vape pods containing nicotine salts. Participants were 18- to 25-year-old college students involved in a longitudinal study who reported past 30-day ENDS use in at least one of eight study waves. Vape pods are popular among college students (Roberts et al., 2022) and national data indicate that college students were more likely than their same-aged peers to report ENDS use from 2018 to 2020 (Schulenberg et al., 2020); thus, this population may be at elevated risk for developing symptoms of dependence. We hypothesized that the proportion of young adults reporting ENDS use within 30 min of waking would increase only after 2017, from 2018 to 2019, consistent with limited evidence showing increased prevalence of ENDS use (Creamer et al., 2019, Loukas et al., 2021, United States Department of Health and Human Services, 2021) and dependence (Hammond et al., 2022) among youth and young adults after the 2017 surge in popularity of vape pods containing nicotine salts. Given evidence that ENDS and cigarette use co-occur among young adults (Osibogun et al., 2017), we controlled for cigarette dependence (using the analogous measure of smoking within 30 min of waking), when examining the ENDS dependence trajectory. We also controlled for five socio-demographic variables of participant sex, type of college attended, race/ethnicity, sexual and gender minority identity, and age.