Many consider E-cigarettes (ECs) to be a gateway to combustible cigarette use. Accordingly using PATH data, several studies have attempted to demonstrate that adolescents who use e-cigarettes at one point in time are more likely to use combustible cigarettes later. In 2017, for example, Bold et al. evaluated the trajectory of e-cigarette use among teens and found that those who used them were seven times more likely to go on to smoke combustible cigarettes than teens who did not. The authors therefore concluded that e-cigarette use predicted future cigarette use, while cigarette use did not predict future e-cigarette use. However, such a conclusion failed to take into account the fact that the strength of the association decreased over time and that the study did not control for other factors that might have more directly led to later cigarette use.

For these reasons, a recently published study examines the patterns that investigators use to assess the trajectory of EC and cigarette use over time. And, in an effort to account for other factors that may contribute to greater instances of later cigarette usage, the authors also evaluated the effects of mental health and alcohol and marijuana use. The study’s most significant findings are as follows:

First, the data shows that there are significant reciprocal associations between EC and cigarette use, which is to say that use of one does appear to be related to use of the other. In fact, the authors found that cigarette use and EC use had a bi-directional association over time, which means that as use of one product increased, use of the other increased as well. However, contrary to the findings in Bold et al., the trajectory of the newer data actually indicates a stronger association from cigarette to e-cigarette use. This could be for a variety of reasons, including that adolescents who already smoke are more comfortable with nicotine use, already have established sources to obtain tobacco or use e-cigarettes as a tool to quit or reduce the use of combustible cigarettes.

Second, the study’s inclusion of other possible factors that might lead to increased tobacco use found significant associations between both e-cigarette and combustible cigarette use and each of the third variables (alcohol, marijuana and mental health). For example, use of alcohol is associated with use of EC or combustible cigarettes. This indicates a complex interplay between the exposures, pointing to an association that cannot be attributable to just one factor. This means that any of these external factors are likely to interact, contributing to future substance use behaviors– a consideration not taken into account in the previous study.

However, on the individual level, the study shows no effect from these third variables on the trajectory of EC to cigarette use or cigarette to EC use. This means that a person’s increased alcohol use from one year to the next did not increase the risk of establishing new EC or cigarette use in subsequent years.  This study also found that third variables did not affect the amount of EC or cigarette use by an individual over time – meaning that a person’s increased alcohol use from one year to the next did not increase the use of EC or cigarette use in subsequent years.

Although Dunbar et al. offers an important refutation to the findings of previous studies that suggest that e-cigarettes are a gateway to future combustible cigarette use, the study has a few limitations that should be noted for the purpose of correcting them in future research.

First, the study was conducted in Los Angeles, California, which has an ethnic and racial demography that is not representative of the overall U.S. population. For example, a combined 66 percent of the study’s participants were either Hispanic or Asian—both of which are populations with lower than average smoking rates.  

The study also used mother’s educational attainment as a proxy for socioeconomic status, which may not adequately represent family socioeconomic status. Furthermore, 55 percent of respondents indicated their mothers had a college degree or higher, which is significantly more than the national average of 32.7 percent of women 25 or older. Finally, 84 percent of participants were attending college or trade school at the time of the final follow-up (average age was 19.33), which is also higher than the national average of 69.7 percent of high school graduates who enroll in college shortly after receiving their diploma. And finally, while the current U.S. smoking rate is 14 percent, it is important to note that smoking rates are between two and three times higher for people without post-secondary education, those who are uninsured or on medicaid, those below the poverty level and those who live in rural areas. In view of this, the study’s sample characteristics indicate that its findings are not generalizable to other poorer, more rural areas of the country, in particular.

Second, although the models used explore the impact of alcohol use, marijuana use and mental illness on subsequent use of cigarettes and e-cigarettes, the study does not directly assess the onset of use. This means that no clear temporal relationship can be established. So, although some assumptions can be made from the data, additional research would be necessary to establish direct patterns of transition between e-cigarettes to cigarettes, and vice versa. In its current form, however, the model is useful to describe potential mediation of the association of cigarettes and ECs by alcohol and marijuana use, or perhaps an additional common risk factor for using all three substances.

Despite these caveats, this study is significant in that it calls into question the claim that e-cigarettes act as a gateway to later combustible cigarette use in young people. Its highlighting of avenues for future research with respect to more direct relationships is also notable and helpful to the advancement of the field.