To examine the longitudinal impact of time-varying factors on US youth’s trajectories of initiation and use of e-cigarettes and cigarettes during the transition from adolescence to young adulthood.

Longitudinal.

Nationally representative US survey, the Population Assessment of Tobacco and Health (PATH) Study.

2682 US youth (aged 16–17) at wave (W)1 of the PATH Study across six waves (2013–2020) into young adulthood (aged 22–23).

Unweighted longitudinal latent class analyses identified trajectory classes of e-cigarette and cigarette use, separately. Nationally representative weighted multinomial logistic regression analyses examined time-varying harm perceptions, substance use problems and tobacco product first tried as predictors of these trajectory classes.

Five e-cigarette classes (2013–2020; 41.5% Persistent Never Use, 12.6% W5 Initiation, 19.9% W3 Initiation, 15.2% Prior Initiation, 10.8% High Frequency Past 30-Day (P30D) Use) and five cigarette classes (2013–2019; 58.6% Persistent Never Use, 11.5% W4 Initiation, 10.9% W2 Initiation, 9.6% Prior Initiation, 9.5% High Frequency P30D Use) were identified. Time-varying harm perceptions and substance use problems were associated with trajectories of initiation and use for both products. Cigarettes, cigarillos, other combustibles and any smokeless tobacco as first product tried were associated with e-cigarette initiation and/or progression to high frequency use. E-cigarettes and hookah as first product tried were associated with later cigarette initiation. High Frequency P30D Cigarette Use was less likely if the first product tried was e-cigarettes, cigarillos, hookah or any smokeless tobacco product.

Results reinforce the need for identification and intervention of early substance use among younger adolescents and targeted public health messaging to address changing harm perceptions and prevent initiation among older adolescents.