https://orcid.org/0000-0001-9551-1599
Abstract
Background
Electronic nicotine delivery systems (ENDS) use one of two formulations of nicotine—freebase or nicotine salt. This study examines whether maintenance or switching between nicotine formulations is associated with ENDS dependence using longitudinal survey data.
Methods
543 U.S. adults (21+) using ENDS frequently (5+ days/week) self-reported and uploaded photos of their most used ENDS liquids in wave 3–5 online surveys from September 2021 to April 2023. Nicotine formulation from photo data was used if available; otherwise, self-reported data were used. ENDS dependence was measured in each wave by a 4-item E-cigarette Dependence Scale (EDS: range 0–4, 4 being most dependent). Data were analyzed using ANCOVA.
Results
Participants using nicotine salt liquids in three waves reported the highest EDS in wave 5 (49.3%, EDS = 2.59), followed by participants switching from salt to freebase (3.2%, EDS = 2.58), participants switching from freebase to salt (10.1%, EDS = 2.52), participants using freebase in three waves (34.9%, EDS = 2.18), and participants changing back and forth (2.4%, EDS = 2.11). After controlling for smoking status, participants stably using nicotine salt and participants switching from freebase to salt reported significantly higher EDS than those stably using freebase (p < 0.01).
Conclusions
Over an 18-month period, people consistently using nicotine salt liquids and participants switching from freebase to nicotine salt were more likely to have a higher ENDS dependence than those consistently using freebase liquids. Understanding how switching between nicotine formulations relates to ENDS dependence can inform nicotine formulation and concentration regulations that may impact addiction.
Introduction
Approximately 4.5% of U.S. adults reported electronic nicotine delivery systems (ENDS) use every day or some days in 2021 (U.S. CDC, 2023). ENDS heat and aerosolize a nicotine-containing liquid to produce an aerosol that can deliver nicotine to the bloodstream and the brain (Baldassarri et al., Citation2018). Camara-Medeiros et al. (Citation2021) found more than half of young adults (aged 16–25) who use ENDS regularly (at least weekly) in Canada reported that they were very or somewhat addicted to e-cigarettes. In this study, respondents who used ENDS with higher nicotine concentration, used ENDS daily, had used ENDS for more than 1 year, and formerly smoked cigarettes had a greater likelihood to report they felt that they were addicted compared to their respective counterparts.
ENDS dependence risk may be associated with nicotine formulation and strength. The liquids used in ENDS include nicotine in one of two formulations, either freebase or nicotine salt. In the marketplace, most freebase liquids contain nicotine concentrations below 20 mg/mL (Goniewicz et al., Citation2019; Cohen et al., Citation2022). Liquids with nicotine concentrations higher than this are more likely to contain nicotine salt (Loukas et al., Citation2022) since freebase nicotine liquids with high nicotine concentration are perceived by users as harsh, bitter, and less appealing (DeVito & Krishnan-Sarin, Citation2018). By adding organic acids to liquids, nicotine formulation is changed from a freebase to a protonated salt which is chemically more neutral and reduces unpleasant nicotine taste and irritation in the upper airways (Hajek et al., Citation2020). Therefore, nicotine salt liquids may facilitate greater nicotine delivery by improving the sensory experience of vaping and making it easier to inhale high concentrations (Hajek et al., Citation2020). This is particularly relevant to people who are new to ENDS (Hammond et al., Citation2023).
Greater nicotine strength, meaning the liquid contains a higher concentration of nicotine, may facilitate greater nicotine absorption and be associated with higher ENDS dependence (Foulds et al., Citation2015). Earlier research did not present a simple answer as to whether higher concentrations of nicotine is associated with greater ENDS dependence, as users can alter nicotine delivery by their volume and frequency of puffing (Browne & Todd, Citation2018; Piper et al., Citation2020). It is also important to note that some of this research was conducted prior to the emergence of ENDS containing nicotine salts. Several studies suggest that young people using high-nicotine salt-based ENDS were more likely to report perceived addiction to vaping (Goniewicz et al., Citation2019; Boykan et al., Citation2019; Messina Case et al., Citation2020), with one study finding no association (McKelvey et al., Citation2018). Loukas et al. (Citation2022) examined ENDS dependence trajectory among young adults in Texas from 2014 to 2019 (Loukas et al., Citation2022). They observed a flat trajectory from 2014 to 2017; however, it was significantly positive after 2017 indicating an increase in ENDS dependence which may result from the popularity of ENDS containing nicotine salts.
There is limited knowledge of ENDS dependence using validated scales and specifically considering the association with nicotine formulation longitudinally. Furthermore, little is known about the longitudinal use patterns of type of nicotine formulation among adults and how switching between freebase and nicotine salt liquid relates to ENDS dependence. In addition, many adults who use ENDS have a history of cigarette smoking (Crespi et al., Citation2023). This may complicate efforts to explore the correlation between ENDS dependence and nicotine formulation of liquids. This study fills the research gap by examining longitudinal changes between nicotine formulation; and exploring whether maintenance or change in nicotine formulation over time is associated with ENDS dependence controlling for cigarette smoking status.
Methods
Study sample and protocols
This study used data from waves 3 (September–November 2021), 4 (July–September 2022) and 5 (February–April 2023) of the Vaping and Patterns of E-cigarette Use Research (VAPER) study, a longitudinal cohort study among adults (age 21+) living in the U.S. who used ENDS at least 5 days/week. Participants were recruited using Craigslist ads posted on the gigs and jobs boards. Participants who provided valid responses and indicated interest in doing the follow-up survey in the previous wave(s) were invited via text and email to take the follow-up surveys. Participants reported their ENDS use behaviors and submitted photos of their most commonly used ENDS liquid via online surveys hosted by REDCap. Liquid characteristics were coded using photos and online searches of manufacturer, academic, retail, and review sites. When liquid characteristics were not available from photos or online searches, survey responses were used (Crespi et al., Citation2022). Rigorous data review and cleaning procedures were applied upon completion of coding to ensure data quality (Hardesty et al., Citation2023). Among 1219 participants who responded validly in wave 3, 731 reported still using ENDS at least 5 days per week and provided valid responses in wave 4; among these, 543 reported still using ENDS at least 5 days per week and provided valid responses in wave 5. The sample of this study is the group who provided valid responses in all three successive waves (n = 543) to explore the longitudinal change in use of nicotine formulation and ENDS dependence. There were no significant differences between the study sample and other participants in wave 3 in terms of sociodemographic characteristics (including region, age, income, race, and Hispanic origin) (p > 0.05). The Virginia Commonwealth University Institutional Review Board (IRB) approved the study (HM20015004), with the Johns Hopkins Bloomberg School of Public Health IRB relying on the VCU IRB as the IRB of record (IRB9277).
Measurements
ENDS dependence
ENDS dependence was assessed by a 4-item E-cigarette Dependence Scale (EDS) score (Morean et al., Citation2018), which has been validated previously using a sample of adults who either use e-cigarettes exclusively or who use both e-cigarettes and cigarettes (Morean et al., Citation2019). The score was derived by calculating the mean of the following four items: (1) I find myself reaching for my e-cigarette without thinking about it, (2) I drop everything to go out and buy e-cigarettes or e-juice, (3) I vape more before going into a situation where vaping is not allowed, and (4) when I haven’t been able to vape for a few hours, the craving gets intolerable. Potential responses included: 0-never, 1-rarely, 2-sometimes, 3-often, and 4-almost always. A greater EDS score indicates a greater level of ENDS dependence. Participants reported their ENDS dependence in each wave. The change in ENDS dependence was assessed by the difference of EDS scores between waves.
Nicotine formulation
Nicotine formulation (1-freebase; 2-nicotine salt) was determined by coding participants’ submitted photos if available (W3: 83.2%; W4: 81.8%; W5: 82.7%). Otherwise, self-report data were used (W3: 16.8%; W4: 18.2%; W5: 17.3%). The switching status of nicotine formulation was classified as: (1) using nicotine salt liquids in three waves, (2) using freebase liquids in three waves, (3) changing nicotine formulation from freebase to salt (freebase-freebase-salt; freebase-salt-salt), (4) changing nicotine formulation from salt to freebase (salt-salt-freebase; salt-freebase-freebase), and (5) changing nicotine formulation back and forth between waves (salt-freebase-salt; freebase-salt-freebase).
Other variables
Sociodemographic variables based on self-report data include geographic location (Northeast, Midwest, South, West), gender (man, woman, non-man/woman, prefer not to answer), age (21–29, 30–44, 45+), age of first use of ENDS (<21, 21+), race (White, other single race, multi-racial, prefer not to answer), ethnicity (Hispanic/Latino, non-Hispanic/Latino, prefer not to answer), income ($0–$39,999, $40,000–$59,999, $60,000–$99,999, $100,000+, prefer not to answer), and sexual identity (heterosexual or straight, non-heterosexual or non-straight, prefer not to answer). Smoking status was assessed by three items: (1) how many cigarettes have you smoked in your entire life?; (2) have you smoked a cigarette in the past 30 days?; and (3) How many days in a typical week do you smoke cigarettes? Participants who smoked less than 100 cigarettes in their lifetime were classified as people who have never smoked; those who smoked 100+ cigarettes in their lifetime and did not smoke in the past 30 days and smoked 0–3 days or 4–7 days/week were classified as people who formerly smoked; and those who smoked 100+ cigarettes in their lifetime and smoked in the past 30 days were classified as people who currently smoke.
Statistical analysis
All analyses were conducted using SAS (Version 9.4, SAS Institute, Cary, NC). Descriptive statistics across sociodemographic variables, smoking status, and ENDS dependence were provided for the overall sample and by nicotine formulation switching status. The Rao-Scott Chi-square tests were used to assess differences in categorical variables. A series of analyses of covariance (ANCOVAs) were conducted, with the five-category nicotine formulation switching status variable entered as a fixed factor, smoking status as a covariate, and ENDS dependence in each wave and changes in ENDS dependence between waves entered as the respective dependent variables. Nicotine concentration is highly correlated with nicotine formulation (Cohen et al., Citation2022). The mean of nicotine concentration in liquids with nicotine salt was 44.5 mg/mL which was significantly greater compared to freebase liquid (6.6 mg/mL, p < 0.0001) in wave 5. Thus, nicotine concentration was not used in ANCOVAs when exploring the association between longitudinal nicotine formulation use and ENDS dependence. Sociodemographic characteristics and smoking status that were significantly correlated with dependent variables (age, age of first use of ENDS, smoking status) were eligible to be included as covariates in the ANCOVAs. Age and age of first use of ENDS were not included due to a significant correlation with the independent variable. Post-stratification survey weighting for gender/age/race ranging from 0.34 to 1.90 was applied based on the data from the 2019 Tobacco Use Supplement to the Current Population Survey (TUS-CPS) to ensure representativeness to U.S. adults frequently using ENDS although sociodemographic characteristics of our sample was generally similar to those who use ENDS daily in the 2019 TUS-CPS. The weighted n’s scaled to our sample size were reported for all analyses, and pvalue were adjusted to account for the large sample size resulting from the implementation of the survey weights. Approximately 2.4% of nicotine formulation data was missing in one or two waves. The pairwise deletion method was used to deal with the missing data. For each analysis, a 2-sided p < 0.05 was used to determine statistical significance.
Results
Most of the participants (80.8%) were less than 45 years old, White (90.1%) and non-Hispanic (93.5%); and 37.6% had an annual income less than $40,000 (). About 70% of participants reported that they formerly smoked (70.4%) and 17.5% reported that they currently smoke.
Table 1. Weighted sample characteristics by nicotine formulation switching status from wave 3 (September-November 2021) to wave 5 (February-April 2023) of the VAPER study (N = 543).
Over 80% of participants reported using the same nicotine formulation for their most commonly used liquids in three waves with 49.3% (n = 264) stably using nicotine salt liquids and 34.9% (n = 187) stably using freebase liquids. Among participants who changed the nicotine formulation for their most commonly used liquids between waves (n = 85, 15.8%), 64.3% (n = 54) changed the nicotine formulation from freebase to salt (freebase-salt-salt: n = 31; freebase-freebase-salt: n = 23); 20.2% (n = 17) changed from salt to freebase (salt-salt-freebase: n = 11; salt-freebase-freebase: n = 6); and 15.5% (n = 13) changed the nicotine formulation back and forth between waves (freebase-salt-freebase: n = 8; salt-freebase-salt: n = 5).
Age and age of first use of ENDS varied significantly by nicotine formulation switching status (p < 0.01) (). Compared to those who used freebase liquids in all three waves (September–November 2021 to February–April 2023), significantly greater percentage of participants who used nicotine salt liquids in all three waves were less than 30 years old (34.3% vs 13.8%) and first use ENDS before 21 years old (25.2% vs 11.3%) (p < 0.001).
ANCOVA models regarding ENDS dependence in specific waves showed that associations with ENDS dependence differed across nicotine formulation switching status groups. As shown in , after controlling for smoking status, compared to those who used freebase liquids in all three waves (September–November 2021 to February–April 2023), participants who used nicotine salt liquids in all three waves reported significantly higher ENDS dependence in wave 3 (September–November 2021) (2.45 vs 2.06, p < 0.001), wave 4 (July–September 2022) (2.46 vs 2.14, p = 0.001) and wave 5 (February–April 2023) (2.59 vs 2.18, p < 0.001); and compared to those who used freebase liquids in all three waves (September–November 2021 to February–April 2023), participants who changed nicotine formulation from freebase to salt reported significantly higher ENDS dependence in wave 4 (July–September 2022) (2.47 vs 2.14, p = 0.040) and wave 5 (February–April 2023) (2.52 vs 2.18, p = 0.018). ANCOVA models regarding changes in ENDS dependence between waves showed that there was no significant difference across nicotine formulation switching status groups after controlling for smoking status ().
Table 2. ANCOVA models comparing ENDS dependence as a function of nicotine formulation switching status (N = 535).
Discussion
This study examined how switching between freebase and nicotine salt liquid relates to ENDS dependence using a longitudinal survey design. From September 2021 to April 2023, most adults (82.0%) using ENDS frequently did not change the nicotine formulation of their most used liquid. People who stably used nicotine salt liquids reported significantly higher ENDS dependence compared to those who stably used freebase liquids.
Relative to freebase, nicotine salt formulation can reduce the bitter and harsh sensation in the mouth and throat and deliver nicotine concentration at a higher level, especially to people who have never smoked or experimented with ENDS (Leventhal et al., Citation2021). Nicotine salt liquids typically have concentrations about twice that of freebase liquids (Harvanko et al., Citation2020; Talih et al., Citation2020). Cohen et al. (Citation2022) observed a larger difference between nicotine salt and freebase liquids in nicotine concentration, with most nicotine salt liquids having nicotine concentrations around 50 mg/mL and most freebase liquids having nicotine concentrations in the 3–10 mg/mL range. Nicotine formulation is also an important factor driving consumer preference (Ma et al., Citation2023). The rapid growth of JUUL in the late 2010s is in part attributable to the use of nicotine salts in its products (Ma et al., Citation2023). Currently, the use of nicotine salt is very prevalent, and many ENDS manufacturers offer products containing nicotine salt (e.g., RJ Reynolds’ Vuse Alto, EVO and PVG2’s Puff Bar, NJOY’s Ace) (Harvanko et al., Citation2020). Pennings et al. (Citation2023) examined liquids with nicotine salts in the Netherlands and found that nicotine salt liquids may be more attractive and addictive than freebase liquids because of higher nicotine concentrations and reduced throat irritation.
Our findings suggest that consistently using nicotine salt liquids over a 18-month period is associated with higher ENDS dependence compared to those consistently using freebase liquids. In addition, among people who changed nicotine formulation of their most used liquids, 64.3% switched from freebase to nicotine salt. Therefore, regulating nicotine formulation of liquid may have an impact on individual behaviors and public health. Currently, regulations to limit nicotine delivery are exclusively addressing nicotine concentration (Snell et al., Citation2021; Vardavas, Citation2022; Campaign for Tobacco-Free Kids, Citation2022). For example, the nicotine concentration of ENDS sold in EU cannot exceed 20 mg/mL (European Union, Citation2014). Given the high sensory appeal and heightened addictiveness of nicotine salts compared to freebase, more research is needed to further explore the impact of nicotine salt on ENDS dependence which can inform specific regulations related to the use of nicotine salts, acid ingredients, or the extent of nicotine protonation. In addition, these regulations need to balance both reducing the abuse liability among people who have never smoked and helping people transition completely from cigarettes to ENDS and then stopping to use ENDS.
Using ENDS at an earlier age may have a greater likelihood of ENDS dependence (Vogel et al., Citation2018). This study also finds that people who were younger and first used ENDS before 21 years old are more likely to consistently use nicotine salt liquids over an 18-month period and report a higher ENDS dependence. Approximately 2.1 million U.S. middle and high school students currently used ENDS in 2023 and 61% used disposable ENDS that typically contain nicotine salt (Birdsey et al., Citation2023).
It is important to consider that nicotine concentration or nicotine formulation is not consistently communicated to users on packaging, devices or in advertisements. A recent study found that most English-language advertisements for ENDS in the U.S. did not include nicotine-related content such as indicating nicotine strength; and when it was provided, the presentation of nicotine concentration varied by using mg, mg/mL, or % nicotine (Kennedy et al., Citation2023).
This study presented longitudinal data from a recent sample across the U.S. that reflect ENDS liquids people use and ENDS dependence under the rapidly evolving ENDS market and recent regulations. By using longitudinal data, it is more likely to explore the effect of maintenance or switching between nicotine formulations on ENDS dependence over an 18-month period than a cross-sectional study. In addition, post-stratification weights were applied to improve representativeness. We also employed a range of data integrity procedures and used both photo and self-report data of liquids to optimize data quality (Hardesty et al., Citation2023). One limitation is that this study collected and reported detailed liquid data about participants’ most commonly used liquid, not other liquids that participants may use; we took this approach to keep the survey to a manageable length for participants. Another limitation is the sample size of the group switching nicotine formulation between waves back and forth is small. In addition, we were unable to control all potential confounders such as puff duration and volume that influence nicotine delivery (Blank et al., Citation2020), length of time used ENDS, etc. because we did not collect puff topography data in this study. Because of the high correlation between nicotine concentration and nicotine formulation, this study is not able to discern whether the change in ENDS dependence is induced by nicotine concentration, nicotine formulation or the interaction of both. In addition, the span between wave 4 and 5 is shorter compared to the span between wave 3 and 4 (5 vs. 9 months) which may have an impact on the transitions observed. Another limitation is that this study did not collect data on those who discontinued vaping until wave 5.
Conclusion
This study used longitudinal data at three time points from September 2021 to April 2023 to provide evidence of patterns of longitudinal nicotine formulation use and how maintenance or switching between freebase and nicotine salt liquid relates to ENDS dependence within a changing marketplace. Over an 18-month period, 49% of participants stably used nicotine salt liquids and were more likely to have a higher ENDS dependence than those stably using freebase liquids; about 16% switched the nicotine formulation of their most used liquids. In response to patterns of nicotine formulation use and its association with ENDS dependence, future research can explore possible changes in toxicity and addiction among people who transition from cigarettes to ENDS and people who have never smoked, and predict the potential impact that different regulations on nicotine formulation may have on these outcomes.
Disclosure statement
The authors declare that they have no competing interests, financial or otherwise, related to the current work. JEC is a paid consultant in litigation against a tobacco company.
Additional information
Funding
References
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