18.7Effects of e-cigarette use on smoking

Last updated: June 2024
Suggested citation
: Greenhalgh, EM, & Scollo, MM. 18.7 Effects of e-cigarette use on smoking. In Greenhalgh, EM, Scollo, MM and Winstanley, MH [editors]. Tobacco in Australia: Facts and issues. Melbourne: Cancer Council Victoria; 2024.   https://www.tobaccoinaustralia.org.au/chapter-18-e-cigarettes/18-7-effects-of-e-cigarette-use-on-smoking  

 

At the heart of the debate over e-cigarettes is the extent to which the potential benefits of making available a product that may help people who smoke to quit might be outweighed by several potential risks. Risks identified to date include uptake by never-smokers, gateway effects, dangers associated with dual use, discouragement from smoking cessation, relapse to or resumption of smoking, renormalisation of tobacco use, and allowing the tobacco industry to influence decision-making in public health (see Section 18.8 for a discussion of potential population-level risks and benefits).1-3

People who use e-cigarettes (vapes) report mixed reasons for vaping. Dual users of tobacco and e-cigarettes commonly report vaping to try and help them cut down or quit smoking, because they perceive it as cheaper and/or healthier than smoking, or because they can vape in places that smoking is not allowed or acceptable4 (see Section 18.12). As indicated in Section 3.36, health risks associated with smoking are unlikely to be reduced by e-cigarettes if users continue to smoke. Crucial to assessing the potential public health benefits of increasing use of e-cigarettes, then, is whether such use will increase or decrease the likelihood of a person who smokes eventually quitting tobacco.5 That is, whether use of e-cigarettes increases or decreases

  • the likelihood of people who smoke making quit attempts
  • the likelihood of a given quit attempt being successful
  • the likelihood of reducing use of tobacco eventually leading to quitting
  • the likelihood that use of e-cigarettes among ex-smokers prevents relapse.

Smoking poses a very significant risk to health both in the immediate and the longer term, but the most serious diseases emerge only with long-term use. While two of every three long term people who smoke in Australia will die from smoking, quitting prior to age 45 reduces mortality risk close to that of people who have never smoked.6 Given that complete cessation of use of any nicotine product is the option associated with least harm, it follows that the optimal approach for younger people who smoke at least in the first instance would be to attempt to quit without continuing use of nicotine.

This section summarises research examining the relationship between e-cigarette use and smoking, including:

18.7.1 A ‘gateway effect’ to tobacco smoking among young people

Along with the health and addiction risks of vaping among young people (see Section 18.6.2), of great concern to public health experts is the robust body of research demonstrating a clear relationship between e-cigarette use and the uptake and escalation of smoking.7 Major reviews published by Public Health England,8 the US National Academies of Sciences, Engineering, and Medicine,9 the CSIRO10 and NHMRC11 in Australia, and the Scientific Committee on Health, Environmental and Emerging Risks (SCHEER) in Europe12 have all concluded that e-cigarette use increases the risk of smoking among young people. A major review and meta-analysis funded by the Australian Government Department of Health found that never smokers who used e-cigarettes had about three times the odds of smoking initiation compared with non-e-cigarette users—see Figure 18.7.1, below.13 Additional longitudinal studies,14-25 and several less extensive reviews26 and meta-analyses,27-34 have similarly concluded that e-cigarette use is associated with an increased risk of cigarette smoking. Longitudinal research in the UK found that among 10–25-year-olds, the likelihood of e-cigarette users transitioning to smoking rose over time from 14% in year one to 27% in year five. Those aged 14–17 were more likely to make this transition than older adolescents and young adults.35

Figure 18.7.1 Forest plot and random-effects meta-analysis for the adjusted odds of smoking initiation at follow-up among never smokers and current e-cigarette users at baseline compared with never e-cigarette users at baseline. aOR, adjusted OR; REML, Restricted Maximum Likelihood
Source: Baenziger et al.13

 

While the relationship between vaping and smoking uptake is clear, the question of causation continues to be debated. E-cigarette use could be a marker in young people who would have gone on to smoke regardless; it is possible that the relationship between vaping and smoking could be explained by other variables, such as social—environmental factors, or a common genetic vulnerability.28,29,36-41 Research in the US,42 China,43 and Australia44 does demonstrates that adolescents who are susceptible to smoking are significantly more likely to report e-cigarette use; however, several studies have also noted conventional cigarette uptake among e-cigarette users who would generally be considered least at risk of smoking.45-50 E-cigarette use is associated with an increase in smoking, even among adolescents not susceptible to smoking.47 Several studies have suggested that the relationship may be bi-directional, such that smoking increases the risk of e-cigarette use, and e-cigarette use increases the risk of smoking.35,51-56

Several factors may contribute to the relationship between smoking uptake among e-cigarette users. Compared with older teenagers, onset of e-cigarette use at a younger age appears to be associated with increased odds of smoking.57-59 The relationship between e-cigarette use and smoking uptake may also be stronger among boys than girls.60,61 As noted above, e-cigarette use is more common among people susceptible to cigarette smoking.42-44,62 Other studies have found that factors such as more frequent vaping,63 higher nicotine content,10 device type,64 mental health symptoms,65,66 White race (compared with Black),67,68 and middle socioeconomic status (compared to low),67 increase the likelihood of transitioning from e-cigarette use to cigarette/dual use. Use of e-cigarettes among youth and young adults may increase consumption of conventional cigarettes and duration of smoking.9,69,70 E-cigarette susceptibility and use may also predict uptake of other types of tobacco products,18,71,72 as well as marijuana, alcohol, and other drug use over time.73-82

Several possible mechanisms of how e-cigarette use may act as a ‘gateway’ to smoking have been proposed. E-cigarette use could normalise nicotine use and smoking behaviours more generally, leading to the renormalisation of smoking. It could also cause users to develop nicotine addiction,34,45 with research showing that the use of e-cigarettes with higher concentrations of nicotine has a stronger association to later cigarette use.10,83

In contrast to the gateway theory, some commentators have suggested that, rather than acting as a ‘gateway’, e-cigarettes may be acting as a ‘diversion’ among young people. That is, e-cigarette use partially or wholly replaces cigarette smoking, and may therefore accelerate the decline in youth cigarette smoking.84 They note that increases in use of non-cigarette tobacco/nicotine products has increased substantially in the US84 and UK,85 alongside a decline in cigarette smoking. Several studies from the US have concluded that a long-term decline in smoking prevalence among youth and young adults accelerated after e-cigarette use became more widespread, which the authors argue demonstrates that e-cigarette use has not increased youth smoking at the population level and may support the diversion theory.86-88 Another contended that data on adolescent tobacco/nicotine use from 2000 to 2019 in the US is more consistent with a diversion effect than a catalyst/gateway effect.89 Authors of a recent meta-analysis of longitudinal studies argued that including only the sub-cohort of never smokers means that only a very small proportion of smoking uptake is explained and the diversion theory is not adequately examined in these studies.90 Critics of diversion theory however argue that continued declines in smoking might alternatively be attributed to the continuing and long-term effects of current and historical comprehensive tobacco control programs.91,92 To put it another way, declines may have been accelerating due to the cumulative and social contagion effects of previous decades of policy, and may have been steeper still without e-cigarette use. Several US studies conclude that there was a slower decline in current cigarette smoking among young people following the introduction of e-cigarettes, suggesting that e-cigarettes are not acting as a substitute for cigarettes.93,94

Some researchers have argued that any gateway effect of vaping to smoking is likely to only have a small impact on youth smoking prevalence at the population level.95 A recent cohort study in the US examined the risk of smoking initiation after vaping and additionally, the risk of smoking becoming an established, regular habit. In line with the gateway theory, it found that ever e-cigarette users were about twice as likely, and current users about 4.5 times as likely, to report subsequent established smoking (defined as 100 cigarettes or more in the respondent’s lifetime and smoking currently). This represented non-significant ‘adjusted risk differences’ (calculated as risk with e-cigarette use minus risk without e-cigarette use, adjusted for covariates) of 0.60 percentage points for ever users and 1.79 percentage points for current users.96 Nonetheless, with very recent large increases in vaping among young people in many countries, even small increases in the absolute risk of smoking will translate to thousands of additional young people smoking as a result of vaping. Any changes in the prevalence of smoking among young people—and whether these changes could potentially renormalise tobacco use among adolescents—are likely to be intensely studied over the coming years.

18.7.2 Effects on attempts to quit smoking

Many people who smoke report using e-cigarettes as a cessation aid.97,98 In Australia in 2022–2023, about two in five people who smoke and had ever used e-cigarettes cited quitting or cutting down smoking as one of their reasons for vaping.4 A randomised controlled trial in Australia conducted between 2017 and 2019 reported that e-cigarettes were chosen more frequently than nicotine replacement therapy (NRT) by trial participants (71.1% vs. 29.0%),99 and recent research in New Zealand also found e-cigarettes to be the most popular cessation aid.100 Perceptions that e-cigarettes are less harmful than cigarettes predict their use as a cessation aid.101

A number of studies have found that people who smoke who use e-cigarettes are more likely to attempt to quit conventional cigarettes,102-112 though a recent meta-analysis found conflicting results depending on whether the studies were retrospective (which showed increased quit attempts) or prospective (which showed no differences). The authors call for more robust research that can better establish causality.113 Users of e-cigarettes may be more motivated to quit smoking  than those who do not use them,10 with research showing that dual users who were predominant vapers (compared with predominant smokers) were more likely to report using e-cigarettes as a cessation aid.114 Though, use of e-cigarettes among people who smoke unmotivated to quit may also prompt quitting thoughts and behaviours.115,116 Research in the US found no association between use of e-cigarettes and attempts to quit tobacco smoking among teenagers,117 while among adults, use of e-cigarettes to quit smoking was associated with more frequent quit attempts among continuing smokers (i.e., those that had tried but failed to quit).112

18.7.3 Effects on success in quitting smoking

Major reviews of the effectiveness of e-cigarettes as a cessation aid have been published since 2018 by Public Health England,8 the US National Academies of Sciences, Engineering, and Medicine,9 the NHMRC11 and CSIRO10 in Australia, the Scientific Committee on Health, Environmental and Emerging Risks (SCHEER),12 the US Preventive Services Task Force (USPSTF),118 and the US Surgeon General.119 Overall these major reviews have generally concluded that the use of e-cigarettes containing nicotine may be associated with increased smoking cessation. A systematic review and meta-analysis commissioned by the Australian government and published in 2022 that was considered by Australia’s National Health and Medical Research Council11 concluded that there is ‘limited evidence’ that, in the clinical context in combination with best-practice counselling and supportive care, freebase nicotine e-cigarettes may be more efficacious for long-term (i.e., >four months) smoking cessation than NRT, and than no intervention or usual care.120

Several meta-analyses of clinical trials (which provide the highest level of evidence) have examined the effectiveness of e-cigarettes for cessation,31,121-124 104,105 most of which conclude that users of e-cigarettes were more likely to successfully quit than users of NRT31,122,123,125,126 and than those in control conditions (e.g., usual care, counselling, or non-nicotine e-cigarettes).31,122,123 RCTs published since these reviews have concluded that nicotine e-cigarettes both in combination with counselling127 and ‘unguided’128 may promote smoking cessation. Though several studies have found that among people who smoke receiving behavioural cessation interventions, self-selected use of e-cigarettes  (i.e., e-cigarettes were not being studied but were used by some participants) could potentially undermine successful cessation.129,130 The most recent in a series of Cochrane reviews that examine e-cigarettes for smoking cessation was published in 2024, and concluded that there was high certainty evidence that users of nicotine e-cigarettes were more likely to successfully quit than those who used NRT,131 though critiques of the Cochrane review argue that the limited number and methodical issues of included studies do not warrant the strength of its conclusion (i.e., high certainty evidence).132,133 Another recent Cochrane reviewing the evidence on all of the major pharmacotherapies for smoking cessation found that the benefits of nicotine e-cigarettes were comparable to those of varenicline and cytisine (see Section 7.16) and likely similar to combination NRT.134

A number of studies have attempted to assess whether some patterns of use of e-cigarettes may be more helpful in assisting users achieve smoking cessation than others. Studies generally find that sustained and frequent e-cigarette users are more likely to reduce cigarette consumption135 or successfully quit105,119,136-154 than those who use the products short-term or intermittently. Infrequent e-cigarette users may be less likely to quit, with research showing an association between non-daily use and both lower motivation to quit136 and lower rates of successful cessation.139,144,145,151 This may be partly explained by research showing that dual users who vape more frequently are more likely than those who vape less frequently to cite quitting or reducing smoking as a reason for vaping.114,155

A recent systematic review of the role of flavours in quitting found low certainty evidence that there is no association between use of flavoured vapes and increased quit intentions or attempts compared to tobacco-flavoured vapes. It also found very low certainty evidence that using non-tobacco flavours did not increase quitting success compared to using tobacco/unflavoured vapes, nor did use of non-menthol and non-tobacco flavours compared to tobacco and menthol-flavoured vapes.156 A secondary analysis of the Cochrane review of e-cigarettes for smoking cessation similarly did not find any clear evidence of an association between flavours and cessation, though noted the paucity of data.157 Use of disposable e-cigarettes may be associated with a lower likelihood of quit attempts and successful cessation compared with other product types.146,147,158,159 Studies have also begun examining the effectiveness of combining e-cigarettes with cessation medications, with some small studies suggesting this approach shows promise.160-162

A number of population-level studies published in recent years have explored the association between e-cigarette use and quit rates in the ‘real world’ (i.e., outside of clinical trials).143,163-166 That is, whether recent declines in smoking prevalence in some countries can be attributed to the increasing use of e-cigarettes. Initial reviews of research up to 2019 did not find evidence that e-cigarettes increase quitting at the population level.144,167 Several more recent studies in England have suggested that e‐cigarette use is associated with increased rates of successful quit attempts and/or overall quit rates.8,165,168,169 Cross-sectional research in Australia,170,171 Germany,172 and in the EU173 has found an association between e-cigarette use and smoking cessation, though research in Italy has not.174 Initial evidence from the longitudinal PATH study in the US found that e-cigarette use was not associated with increased cessation among adults who smoke compared with non-e-cigarette users175-178 or compared with users of NRT or cessation medications.177,179 However the most recent of these analyses found that while rates of quitting were no higher among e-cigarette users in earlier survey years (2013/14–2015/16), between 2018/19 and 2021 rates of quitting were significantly higher among people who vaped. The authors speculate that increases in frequency of use and changes in product features may account for these differences over time.180 In contrast to these studies, longitudinal research in Aotearoa New Zealand found that those who vaped were just as likely to transition to smoking as those who smoked were to transition to vaping. The authors suggest that declining smoking prevalence is better explained by other factors such as public education campaigns and increased tobacco taxes.181 Research in Taiwan182 and the US183 also notes that e-cigarette use does not appear to be associated with smoking cessation among adolescents.

Some have argued that the increased risk of prolonged dual use among those assigned to e-cigarettes in clinical trials has not been adequately acknowledged or reported.132,184 A meta-analysis published in 2022 notes that the use of e-cigarettes for cessation is more likely to lead to extended dependence on nicotine and long-term e-cigarette use,185 the long-term health risks of which are largely unknown (see Section 18.6). Authors of one review suggest that that e-cigarettes could be considered on a prescription basis with clinical supervision, and after considering potential benefits as well as risks such as these.144

18.7.3.1 Effects on success in quitting smoking during pregnancy

A number of recent studies have examined the efficacy of e-cigarettes as a cessation aid during pregnancy (for a discussion of their safety, see Section 18.6.1). An RCT in the US suggests that the timing of e-cigarette uptake may affect smoking cessation. It found that existing e-cigarette users before pregnancy had a higher smoking abstinence rate in late pregnancy than NRT users, while new e-cigarette users had a similar smoking abstinence rate to NRT users.186 An RCT in England and Scotland found that e-cigarettes appeared to be more effective than NRT in helping pregnant people who smoke to quit; however treatment adherence and validated abstinence were low.187,188 Longitudinal research in England found that e-cigarette use during pregnancy may be associated with a lower likelihood of returning to smoking postpartum.189

Guidelines in some countries support the use of e-cigarettes as a cessation aid during pregnancy as a last resort for those unwilling or unable to use other medications.190 Due to insufficient information on their effects on foetal development and obstetric outcomes,  Australian guidelines do not recommended e-cigarettes as a smoking cessation aid for people who are pregnant or breastfeeding.191

18.7.4 Effects on relapse among ex-smokers

Given the higher likelihood of long-term e-cigarette use among those who use the products as a smoking cessation aid,192 it is important to examine whether such use prevents or facilitates relapse to smoking. Data from the ITC Four Country study showed no differences in relapse in 2018 between ex-smokers who were vaping in 2016 compared to those who were not vaping.193 However, longitudinal studies in France,164 the US,194-199 and the UK200 have found an increased risk of relapse among former smokers who use e-cigarettes. Some studies have noted differences in relapse risk depending on frequency of vaping and smoking history, with the risk appearing to be higher among long-term quitters than among those who have recently stopped smoking164 and among intermittent users compared with those who vape daily.201 200 A systematic review and meta-analysis published in 2021 found evidence that that ex-smokers who use e-cigarettes have more than twice the odds of relapse compared with ex-smokers who have not used e-cigarettes.13 Another similarly concluded that ex-smokers who used e-cigarettes—particularly long-term ex-smokers—had an increased risk of relapse to smoking compared with non-e-cigarette users.202 A further recent industry-funded review concluded that findings on the risk of smoking relapse among ex-smokers who use e-cigarettes are mixed and inconsistent.203

Studies examining the effects of quitting e-cigarettes on smoking have produced mixed findings. One study using PATH data in the US found a possible risk of relapse among ex-smokers in the short-term following cessation of e-cigarettes,204 suggesting that NRT may be beneficial to support former smokers who are stopping vaping. In contrast, a secondary analysis of data from an RCT examining vaping cessation found that quitting e-cigarettes was associated with smoking cessation, even though smoking was not a focus of the trial.205

18.7.5 Cutting down as a step towards quitting

Reduced daily consumption of conventional cigarettes in users of e-cigarettes have been noted in many studies,110,135,193,206-211 and reviews;125,212,213 with some authors suggesting that vaping may play an important role in reducing harm among people who smoke who belong to population groups disproportionately affected by smoking (e.g., low socioeconomic status groups).214,215 However, it will be important to continue examining whether reduced cigarette consumption among those who both vape and smoke increases their chances of smoking cessation.  

Even following substantial reductions in cigarette consumption, evidence suggests that the health benefits of reduced consumption are limited. Although people who smoke who substantially reduce the number of cigarettes smoked per day reduce their exposure to some harmful and potentially harmful constituents, these decreases are limited and much smaller in magnitude than reductions following complete cessation.216 Several large cohort studies have found that people who smoke who reduce their consumption do not significantly reduce their risk of premature death.217,218 Smoking just 1–4 cigarettes per day significantly increases a person’s risk of dying from smoking-related disease.219 The modesty of the health benefit from smoking reduction is often attributed to compensatory smoking: those who cut down tend to inhale each cigarette more deeply, and smoke more of it.

A number of studies have shown that dual use of NRT and cigarettes can help alleviate the issue of compensation,220-222 leading some to advocate long-term use of NRT in combination with cutting down as a harm reduction strategy.223,224 Researchers have suggested that it may be possible that the use of e-cigarettes while smoking could similarly reduce intake of nicotine and toxins from each cigarette. However, data on the long-term safety of e-cigarettes is lacking, and they may be less safe than existing, licenced NRT products.103  216,225  And physiological factors beyond compensation are also relevant to the extent to which risk reduces with reduced consumption. Even low levels of exposure to tobacco smoke cause profound changes in cardiovascular function (See Section 3.36 for further detail).

The main benefit of cutting down seems to be its role as a step toward quitting.217,226 A 2015 review exploring the usefulness of cutting down determined that people who smoke who reduce the number of daily cigarettes smoked are more likely than those who do not cut down to attempt to quit and actually achieve smoking cessation. This is particularly the case when the quit attempt is combined with NRT.227 A 2019 Cochrane review concluded that cutting down may result in similar quit rates to going cold turkey, and that reduction‐to‐quit interventions may be more effective when pharmacotherapy is used, particularly fast‐acting NRT or varenicline.228 NRT-assisted reduction appears to be an effective intervention for achieving sustained smoking abstinence, particularly for people who smoke not currently intending to quit (or quit abruptly),229,230 and is effective and cost effective compared to no quit attempt.226 E-cigarettes may similarly prove to be a useful form of NRT in combination with smoking reduction as part of ‘cutting down to quit’, but further research is needed on the efficacy of the products in this context.227 A pilot RCT in Australia that compared abrupt and gradual smoking cessation alongside the use of e-cigarettes among people receiving alcohol and other drug treatment found that while both groups found the intervention feasible and acceptable, adherence was lower in the gradual condition.231

18.7.6 Prolonged dual use of cigarettes and e-cigarettes

Dual use refers to the concurrent use of conventional tobacco products and e-cigarettes.  Dual use is actively promoted by some e-cigarette manufacturers, especially those that also manufacture tobacco products, as a way for people who smoke to by-pass smokefree regulations,232 and this is a commonly cited reason for vaping..233,234 Other people who smoke use both products as a means of cutting down the number of cigarettes they smoke each day, and/or as an intended pathway to complete smoking cessation.4,5,235 However, many people who smoke are unsuccessful in transitioning fully from smoking to vaping, despite initial intentions to quit smoking.233  

Research in the US has found that continued dual use is a relatively common pattern among adults,236-238 and that among dual users, most predominantly smoke (rather than heavy or light use of both, or predominant vaping).114 Completely switching from dual use to exclusive e-cigarette use appears to be relatively uncommon,239 with several longitudinal studies finding that the majority of dual users transitioned over time to either continued dual use or exclusive smoking.198,237,240-242 The increased risk of dual use has been flagged as a concern in clinical trials among people who smoke who are allocated to e-cigarettes.243

While a reduction in health risks seems likely for people who smoke who completely substitute tobacco cigarettes for e-cigarettes,244,245 the benefits for those who continue to use some tobacco cigarettes are much less certain; as noted above, even ‘light’ cigarette smoking carries substantial health risks—see Section 3.36. Among those who exclusively smoke tobacco cigarettes, cutting down the number of cigarettes consumed may not reduce toxic exposure if people who smoke compensate by drawing more deeply on the cigarettes they do smoke. There is some evidence that supplementing reduced cigarette consumption with an alternative source of nicotine might reduce toxicant exposure and hence mortality among people who smoke who continue to smoke at a reduced rate.217,218,246 However, there is no available evidence whether or not long-term e-cigarette use among people who smoke changes morbidity or mortality compared with those who only smoke tobacco cigarettes.9 Exposure to various toxicants may also vary with the patterns of use; i.e., among dual users who predominantly smoke compared with those who predominantly vape.247 Use of e-cigarettes has the potential to introduce independent or additive health risks.10,248 Targeted smoking cessation interventions for dual users have been developed with the understanding that most will continue to smoke249 —see Section 18.11.

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