The so-called “gateway effect” is the subject of much debate between supporters of tobacco harm reduction and tobacco control advocates. In the context of e-cigarettes, the gateway effect describes the potential for individuals to transition from using e-cigarettes to smoking combustible cigarettes—particularly among people who never used combustible cigarettes before using an e-cigarette.

Although e-cigarettes may act as a gateway to smoking for some, this fact will not significantly affect the health of the population if the number of combustible cigarette users who quit smoking with the aid of e-cigarettes is greater than the number of non-smokers who begin smoking after using e-cigarettes. That is to say, determining the magnitude of the gateway effect influences the calculation of e-cigarettes’ net benefit to public health.

Most evaluations of gateway effects focus on youth and young adult populations. McMillen et al. contribute an analysis focused on adults. They also include “distant” former smokers—people who have abstained from cigarettes for at least five years—in the study to assess the effect of e-cigarette use on the odds of smoking relapse.

In the realm of gateway effect studies, the authors’ study design is better than most. The authors’ thoughtful methodological choices minimize confounding, which gives the results more merit than many similar studies. The results also demonstrate that research and prevention efforts must take into account significant differences between subpopulations. This study is perhaps most valuable, however, for establishing e-cigarettes’ net impact on the population’s health.

Although the study has some limitations, it presents useful information about the likelihood of adult e-cigarette users and experimenters transitioning to combustible cigarettes. Based on previous research, it is not surprising that the overwhelming majority of never smokers and distant former smokers studied—98.5 and 95.7 percent, respectively—had never tried an e-cigarette at baseline. And although the fact that some never smokers and distant former smokers are experimenting with or consistently using e-cigarettes is not ideal, this study shows that the majority of these users do not go on to establish combustible cigarette use.

Among both never smokers and distant former smokers, 7 and 9 percent (respectively) reported smoking a combustible cigarette during the past 30 days. At face value, this may seem to indicate that a significant number of subjects transitioned from e-cigarette to combustible cigarette use. Yet since the subpopulation evaluated accounts for no more than 4 percent of study participants—in this case, between 67 and 207 individuals—then in the context of this study, fewer than 10 individuals surveyed in each of these two groups reported past-30-day smoking at follow-up.

The study did find that distant former smokers and never smokers who had experimented or consistently used e-cigarettes had higher odds of trying or establishing use of combustible cigarettes than their counterparts who abstained from e-cigarette use. However, there are some important considerations to take into account when interpreting the magnitude of this association. For one, it is impossible to control fully for common liability in gateway effect studies. “Common liability” refers to the idea that many of the risk factors for trying and establishing e-cigarette use are the same as those for establishing combustible cigarette use. This may be especially relevant for the group of never smokers who reported smoking at least one puff of a cigarette at follow-up, since the willingness to try e-cigarettes suggests that individuals within this group would also be willing to try other substances, even if they do not establish consistent use.

Moreover, although this study is longitudinal, adjusts for several strong confounders, and uses an adult population less prone to establishing smoking behavior (about 90 percent of smokers begin smoking by age 18, and 98 percent start by age 26), common risk factors could be confounding the relationship between e-cigarette use and future smoking. This is especially true for younger adults, whom the study finds have significantly higher odds of transitioning to combustible cigarettes than their older counterparts. Unfortunately, the sample studied was too small to analyze the association further than dividing it into participants ages 18-34 and participants ages 35 or older. Since young adults (ages 18-25) have higher odds of establishing tobacco use than older adults, being able to separate this subpopulation from the 18-34 group may have proven useful.

Despite the study’s limitations, its findings are particularly useful to public health efforts, since we cannot accurately assess e-cigarettes’ public health value without determining their net effect on the number of people who smoke combustible cigarettes. That is, if the number of current smokers who quit by using e-cigarettes exceeds the number of never and former smokers who establish combustible cigarette use during the same period, the availability of e-cigarettes has a net positive impact on the health of the population—even if it may not have a positive health impact on every individual.

While the best-case scenario would be for no distant former smokers or never smokers to begin using e-cigarettes, the focus should be on making sure that these individuals do not begin smoking combustible cigarettes. Members of the health care community can contribute to this effort by tailoring their messaging to the proper populations and refraining from using a standard one-size-fits-all approach to tobacco control that fails to dissociate nicotine dependence from the act of smoking combustible cigarettes.

Even with these changes, some people outside of the target market will establish e-cigarette use, as would occur with any product. But from a public health perspective, the overall impact on the population should be our primary measure of e-cigarettes’ value. McMillen et al. provide new information that helps reveal that impact.