The Worry about Gateway Effects
Podlyfe is a company that’s dedicated to harm reduction. As we’ve discussed before, nicotine vapes are an increasingly important tool for “harm reduction”—directly reducing the health impact of smoking combustible tobacco. While vaping isn’t completely harmless, it’s significantly less harmful than smoking cigarettes, and the vapes we stock are designed specifically to be both appealing and satisfying to smokers. In other words, we want to help people quit.
We’re encouraged to see that it looks like it’s working. An increasingly common message from our customers is that our products are helping them stay tobacco-free, whether that’s because they found a favorite flavor, a vape that gives just the right “throat-hit,” or an e-liquid that delivers a familiar nicotine boost. And the scientific literature backs our users up, with recent studies showing that many habitual, lifelong smokers find e-cigarettes useful as an aid to quitting.
But there’s a common concern here: that even as it’s helping established smokers quit, vaping might also be acting as a “gateway” to smoking for new users. Some top researchers think that it is, others are equally sure that it isn’t. We’re going to give you a quick explainer on the issue: what it is, where it comes from, and what’s most likely going on.
The Gateway Theory
Back in the 1970s, some media figures started picking up on a scientific idea: that using a relatively safe substance, such as marijuana, might increase people’s risk of going on to use much more dangerous ones, like heroin. The concept of the “gateway drug” grew in popularity until, by the late 1990s, it was absolutely everywhere and had been sensationalized to the point of absurdity. Video games were described as “gateway drugs” to actual violence, and pornography as a “gateway” to promiscuity.
The gateway theory is serious, however, and in some specific cases we now know that the effect is real. Consistent use of some substances can alter how our brains and bodies respond to other drugs, potentially making them more addictive at lower doses.
When it comes to e-cigarettes, though, the idea is a little different. The concern is not that they will lead to the use of other drugs, but to the use of other forms of nicotine—especially smoking. The focus is almost always on younger users, so the gateway theory that we’re interested in is this: that there are some adolescents and teenagers who have no interest in smoking until after they start using e-cigarettes (which are, after all, cheaper, less harmful, and available in a greater range of flavours and designs). The worry, in other words, is that there are young people who start vaping and then switch to smoking, who wouldn’t normally smoke.
Obviously, this is a possibility we find very concerning. Our products are intended to help people stop smoking, not start. We keep track of the research, and are always looking for ways we can help reduce the risk of any potential gateway effects. (We’ll talk more about this at the end of the article!)
Worries about e-cigarettes as a gateway to smoking comes mostly from one big finding: that young people who do not currently smoke are more likely to start smoking if the currently use e-cigarettes. This pattern has been replicated in many studies, and applies to young people between the ages of 12 and 25. It’s been found in Europe, North America, and Asia, in large, stable studies. E-cigarette use is definitely correlated with smoking initiation.
And that means one of two things: either the gateway theory is right, and using e-cigarettes causes young people to start smoking, or both smoking and e-cigarette use are caused by common factors. That’s called the “common liabilities model.” Let’s dig into the differences.
Gateway Effect vs. Common Liabilities
We know that e-cigarette use is correlated with starting to smoke; the question is whether one causes the other. This is extremely important to get right, because we need to understand cause in order to protect people. For instance, bans on e-cigarettes are correlated with significant increases in cigarette smoking, as are public health warnings about the dangers of vaping. That’s because cigarettes and e-cigarettes are “economic substitutes”: if people can’t get one, they typically start buying the other. So the details matter, here.
And those details are fuzzy. For example, one study of high school students found that while e-cigarette use sometimes precedes cigarette use, and vice versa, what’s far more common is alcohol use preceding both. That would suggest either that alcohol is a “gateway drug,” or—more likely, according to the authors—that drug use in general is caused by a range of psychological, social, and genetic factors. Similarly, e-cigarette use is correlated with future habitual use of cigarettes, but also just with trying them once, so there isn’t a clear fit between intensity of vaping leading to intensity of smoking. And the relationship is definitely bi-directional, meaning that smoking predicts future vaping at least as much as vaping predicts future smoking.
So are e-cigarettes a gateway to cigarettes? The most thorough attempts to resolve that question are the big overview studies, or “meta-analyses.” There have been about half a dozen of them, attempts to collect all of the evidence on e-cigarettes as a risk factor for youth smoking, each using slightly different methods. They’re trying to pool all the data we have in order to get a definitive answer, and several of them—at least three—have concluded that e-cigarettes probably do have a gateway-type effect, at least for some young people, some of the time, under some conditions.
But the closer you look, the less clear the “gateway” relationship becomes. The problem is that establishing a causal relationship—like “e-cigarette use causes young people to start smoking cigarettes”—is incredibly difficult. Researchers have to account for hundreds of “confounds,” factors that affect both the use of e-cigarettes and the initiation of smoking. Those factors include age, gender, alcohol and substance use, parental smoking, e-cigarette and cigarette use among peer groups, genetic traits, personality traits, geographic location, exposure to relevant advertising, available spending money, accessibility of different products, school performance, anxiety and other conditions, and many, many others. Very few studies control for even a majority of these confounds. One meta-analysis estimated how strong even just one confound would have to be to explain the correlation between e-cigarettes and smoking initiation, describing the resulting score as “not high.” (Read about their estimation system here.) In other words, a single moderately strong common liability could explain the correlation between e-cigarette use and smoking, and there are dozens of these confounds that are not adequately accounted for in the data we have so far.
Other studies have reached the same conclusion in other ways. One re-analyzed a widely used dataset from the American National Institutes of Health, building a “smoking propensity score” for each participant based on the kinds of confounds we just mentioned. When they accounted for that score—meaning, they removed the effects of those variables from the model—it eliminated 73% of the link between e-cigarette use and smoking. That doesn’t mean there’s no link, it just means that the correlations prompting people to worry are probably caused by many, many interlinked factors.
The most comprehensive of these large studies similarly cautions strongly against a causal interpretation of the correlation data, noting that the more common risk factors you rule out, the smaller the correlation becomes. For instance, if you account for behavioural risk factors like alcohol and marijuana use, e-cigarettes become a much less powerful predictor of smoking initiation. The authors recommend using “negative controls” to help resolve the issue:
“For example, using an e-cigarette is unlikely to cause other risky behaviours such as the number of sexual partners a person has; if similar associations are seen between e-cigarette use and both smoking and number of sexual partners, it would indicate that the link may be caused by common underlying factors.”
The same researchers then carried out another large study doing exactly that. They used previous studies to estimate individuals’ genetic risk of smoking, then checked whether that genetic risk also predicted use of e-cigarettes, sexual promiscuity, and gambling. They found that the risk profiles were extremely similar. Another paper gives us a helpful summary:
“Khouja and colleagues argue that their results indicate that there may be a shared genetic aetiology for cigarette smoking and e-cigarette use and for socioeconomic position, externalising disorders in childhood, and risky behaviour. […] These “findings are consistent with other epidemiological evidence. This includes findings that the adolescents who are most likely to experiment with e-cigarettes are those who are at higher risk of smoking cigarettes (and using other drugs) because of traits such as sensation seeking, risk-taking, and oppositional behaviour.”
And that, once again, is the “common liabilities” argument. Crucially, almost no one—not even the most fervent advocates of the gateway theory—argue against the idea that youth smoking and youth e-cigarette use have common underlying causes and risk factors. The common liabilities theory is almost universally accepted.
It also provides a much more general, complete picture of youth nicotine use than the gateway theory does. For instance, most young smokers (about 85%) have never vaped, and there actually aren’t very many adolescents and teenagers who use e-cigarettes often but never smoke. It’s much more common for young people to switch or to use both.
If most of the pattern we’re seeing is caused by common liabilities, that also explains other evidence that seems to contradict the gateway theory. The rise in youth vaping, for instance, comes alongside a continued (and likely accelerating) decline in youth smoking rates. Similarly, the introduction of e-cigarettes to new parts of the United States was linked to drops in ever-smoking rates.
And perhaps most interestingly and closer to home, in 2020 Auckland University researches noted that although measures of e-cigarette use amongst 14-15 year olds has increased in recent years, all measures of cigarette use has decreased or remained static over time.
Although the proportion of students who had ever tried e-cigarettes in 2019 (37·3%), exceeded the proportion who had ever smoked (19·6%), daily use of nicotine products was low; Vaping (3·1%), cigarettes (2·1%), both vaping and smoking (0·6%). In 2019, daily use of e-cigarettes was extremely low in never-smokers (0·8%). 
There are a few take-home lessons from the evidence we have right now. First, the common liabilities theory is almost definitely true. There are clearly many psychological, social, and genetic factors that increase the likelihood of a young person starting to use cigarettes or e-cigarettes, and the observed correlation between e-cigarette use and smoking might be caused by those factors alone. (Especially bearing in mind the greater availability, lower cost, vastly greater variety, and far lower health risks that e-cigarettes offer.)
Another take-home point is that while evidence is still coming in on the gateway theory, it is, at most, true for some people, some of the time, under some conditions. Nicotine addiction is a real and serious concern, and if vapes are driving a significant increase in nicotine dependence, we should consider further legislative changes by the Ministry of Health to help protect children, adolescents, and teenagers.
At the same time, we are well aware of the complexity such legislation involves. The problem here is not limited to the potential for further restrictions on e-cigarettes to backfire by increasing smoking rates, as they have demonstrably done in several cases. There is also the need to keep e-cigarettes available as a harm reduction tool for established, habitual smokers, many of whom have come to rely on vaping as a means of staying tobacco-free; vaping is critical to achieving the New Zealand Governments aspiration of Smokefree 2025. This is especially important if, as some authors suggest, any potential gateway effect is having very little effect on youth smoking rates: if vaping is increasing long term nicotine use only very slightly, while simultaneously helping a large number of current smokers quit, then it remains a crucial tool for progressive public health.
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 Li et al. (2021), O’Leary, Polosa, and Volti (2021).
 Bell and Keene (2014).
 Kandel and Kandel (2014).
 See e.g., Berry et al (2019), Chien et al. (2019), Hair et al. (2021), Keller-Hamilton et al. (2021).
 Etter (2018), Stanbrook (2016).
 Friedman (2015), Katchmar, Gunawan, and Siegel (2021).
 Pesko et al. (2020).
 Ortega et al. (2021).
 Barrington-Trimis et al. (2018).
 Aleyan et al. (2019), Niaura et al. (2020).
 Ferkol et al. (2018), O’Brien et al. (2021), Soneji et al. (2017).
 O’Brien et al. (2021).
 Chan et al. (2021).
 Lee and Fry (2019).
 Khouja, Suddell et al. (2021), p. 13.
 Khouja, Wootton et al. (2021).
 Hall and Chan (2021, pp. 1-2).
 See e.g., Chapman et al. (2019).
 Mendelsohn and Hall (2020).
 Lee, Coombs, and Afolalu (2018).
 Creamer et al. (2021).
 Walker et al. (2020)
 Friedman (2015), Katchmar, Gunawan, and Siegel (2021), Pesko et al. (2020).
 Liu et al. (2020), Xu and Friedman (2020).
 Lee, Coombs, and Aflalu (2018).