Background:
Studies indicate that use of electronic nicotine delivery systems (ENDS) predicts subsequent cigarette use, but these are limited by a focus primarily on adolescents or examination of cigarette initiation/onset. Little is known about the impact of ENDS use in current (past 30-day) cigarette use or in changes in current use across young adulthood. This study examined if 1) ENDS use predicted trajectories of young adult current cigarette use over a two-year period.

Methods:
Data are from a five-wave 24-college study in Texas, USA. Wave 1 data were collected in Nov 2014-Feb 2015 and every six months thereafter. To examine the role of ENDS in current cigarette use, only students who reported never using cigarettes at Wave 1 were included, N=2577. At Wave 1, students were 18-25 year olds (m=19.7; sd=1.60); 67.7% female; 31.9%; non-Hispanic white, 27.4% Hispanic, and 40.7% another race/ethnicity. Growth curve modeling was used to examine predictors of the current cigarette use trajectory across the five waves.

Results:
Wave 1 ever ENDS use predicted a faster increase in the current cigarette use trajectory (OR= 2.87 [1.32, 6.25]) in the unconditional model (model with no covariates). However, this effect became non-significant when Wave 1 time-invariant covariates, including socio-demographics, family tobacco use, friend cigarette use, cigarette use susceptibility, and use of other tobacco (i.e., hookah, cigars, smokeless) products were added. Examination of the full model indicated that peer use (OR=1.44 [1.01, 2.06]), susceptibility to cigarette use (OR=4.49 [2.84, 7.10]), and other tobacco use (OR=2.40 [1.62, 3.56]) predicted an increasing cigarette use trajectory.

Conclusions:
Findings are in contrast to research indicating the ENDS use predicts cigarette initiation, but also extend it by showing that young adults who are susceptible to cigarette use and who use other tobacco products (besides ENDS) are at elevated risk for current cigarette use.