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dependence ratings compared to the users of nicotine-free e-cigarettes [23]. When comparing nicotine dependence among exclusive e-cigarette and cigarette users, Liu et al. showed that among smokers, time to the first cigarette after waking is shorter (20 min) compared to people using e-cigarettes (29.2 min) [29]. Also, people who smoke cigarettes more often consider themselves addicted, with more reporting a strong compulsion to smoke. On this basis, the researchers stated that e-cigarettes are less addictive [29]. Int. J. Environ. Res. Public Health 2019, 16, 2279 7 of 10 Several hypotheses should be considered to explain why we found higher levels of addiction among e-cigarettes users compared to smokers in contrast to findings in studies by González et al. (2017) [28], Etter et al. (2015) [23], and Liu et al. (2017) [29]. First, our study was performed in early 2018, and it is possible that our participants were using more effective e-cigarettes with higher addictive potential. The e-cigarette market is quickly changing with newer models and generations of e-cigarettes available. Advanced e-cigarettes, especially those with a high-capacity battery or accumulator, are characterized by the production of a larger volume of aerosol and delivery of significantly higher doses of nicotine compared to older models (first or second generation) of e-cigarettes [30]. In our study, participants, on average, used four different e-cigarette devices, and more than half of the participants used technically advanced e-cigarettes. Second, our study was performed among young adult e-cigarette users, aged 22.4 ± 2.2 years old, a younger study population compared to those of earlier studies performed by González et al. [28], Etter et al. [23], and Liu et al. [29]. The patterns of e-cigarette use by young adults differ from those of older populations of e-cigarettes users [24,31]. It should also be noted that patterns of smoking may differ and older smokers tend to smoke more cigarettes per day compared to adolescents, and may therefore have higher FTND scores. However, adolescents and young adults (under 25 years) are also at the greatest risk of nicotine addiction [2]. These factors could have contributed to higher FTND scores being found among e-cigarette users than among smokers in our study compared to previously published studies [23,28,29]. Third, studies by González et al. (2017) [28], Etter et al. (2015) [23], and Liu et al. (2017) [29], had different designs and larger sample sizes compared to our study, which could also have affected the results. In our study, e-cigarette users consumed an average of 4.2 mL of e-liquid per day. Slightly lower results were obtained by Hajek et al. at 3.3 mL per day [31] and Dawkins et al. at 3.4 mL [24] of e-liquid per day. In the current study, 70% of all e-cigarette users had used self-prepared e-liquids. This percentage is more than twice as high as that described in the literature (30.8% by Wong et al. [32] and 32.9% by Farsalinos et al. [33]). Such a high percentage of people preparing e-liquids themselves may contribute to the high consumption of e-liquids as self-preparation of e-liquids is a way to reduce costs of e-cigarette use, especially in such a young population of e-cigarette users. Strengths of this study include the detailed description of patterns of use of young, experienced e-cigarette users and the investigation of the degree of addiction of e-cigarettes and cigarettes using an established measure of addiction among a key population group—young adults. Further separation of this group into groups of exclusive e-cigarette users and dual users allowed for a fuller understanding of the differences in behaviors and addiction presences between these groups. There are several potential limitations of the current study. First, our study was limited to a small sample of young e-cigarette users from one region of Poland. This is a specific group of e-cigarette users which was identified among the 3002 young adults who participated in an across-sectional survey (YUPESS—Katowice branch), with the sample being representative of young adults with higher education (university students) in Poland [34] but not of non-university-attending young adults. Secondly, nicotine dependence levels were not confirmed