Introduction:
In recent years, studies have highlighted the upward trend in e-cigarettes use among adolescents, as well as the ability to e-cigarette use to lead to subsequent conventional cigarette use.

Objectives:
The study aims was two-fold:

1. to examine the progression from e-cigarettes use to conventional cigarette use
2. to analyze the differences between dual users and only cigarette users, in a Spanish adolescent population.

Methods:
Data were obtained from the ESTUDES, a representative survey of addictive behaviors of Spanish adolescents aged 14-18, which comprise 38,010 adolescents (Mage=15.69; SD=1.188; 51.4% females).

Results:
Lifetime e-cigarette use increased the likelihood of subsequently conventional cigarette use in 1.581 times (95%CI 1.485 - 1.683), and the odds of conventional cigarette use in the last month 1.517 times (95%CI 1.403, 1.641). Further, dual users (i.e., e-cigarette and conventional cigarette use) compared to only cigarette smokers show greater severity in tobacco patterns, evidenced by a higher percentage of daily smokers (25.51% vs. 12.14%), an earlier age of smoking onset (13.75 vs. 14.67), a greater number of cigarettes per day (5.96 vs. 4.97), and a lower risk perception of e-cigarettes (12.18% vs. 18.66%) and cigarettes (91.30% vs. 94.02%) (p-values < .001).

Discussion:
These findings call for the need to conduct several changes in Spanish regulatory law, such as, the regulation of advertising, marketing, taxes, and consumption permits, alongside the prohibition of sale to adolescents and the presence of flavors.