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one-fifth of the participants initiated nicotine use bye-cigarette. Most subjects started e-cigarette use due to health-related (harm reduction or for smoking cessation) or financial reasons. The participants consumed relatively large amounts of nicotine-containing e-liquid (almost half a bottle per day). More than half of the participants chose technically advanced e-cigarette devices. Our key finding was that nicotine dependence levels measured with FTND were over two times higher among exclusive e-cigarette users (mean 3.5) compared to traditional tobacco smokers (mean 1.6; p < 0.001). Similarly, among dual users, nicotine dependence levels were higher when using an e-cigarette (mean 4.7) compared to a traditional cigarette (mean 3.2; p = 0.03). The high levels of nicotine dependence observed among young adult e-cigarette users compared to smokers in our study suggest this group may be at high risk of addiction. We also found that dual users used e-liquids with higher concentrations of nicotine and consumed more milliliters of e-liquid per day compared to exclusive e-cigarette users. These findings together with the higher mean FTND score suggests a higher nicotine dependence level in this group of e-cigarette users. However, by contrast exclusive e-cigarette users presented with a longer duration of e-cigarette use (on average 1.3 months longer) compared to dual users, although this was not statistically significant. Our findings contrast with results obtained by González et al. [28], Etter et al. [23], and Liu et al. [29]. In a study conducted by González et al., e-cigarette users scored lower in FTND (mean 4.38) than cigarette smokers (mean 5.57; p = 0.03) [28]. Etter et al. concluded that e-cigarettes are less addictive than tobacco cigarettes and their addictive potential is comparable to nicotine gums [23], although they also found that users of nicotine-containing e-cigarettes presented higher 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