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

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