Initiation Sequence of E-Cigarette and Cigarette Smoking among US Adolescents: A National Study.

BACKGROUND AND OBJECTIVES: The primary objectives were to: (1) examine the initiation sequence of e-cigarette use and cigarette smoking; (2) assess other substance use as a function of the initiation sequence of e-cigarettes and cigarettes; and (3) investigate the role of early e-cigarette initiation among US secondary school students. METHODS: Data were collected via self-administered questionnaires from independent 2015 and 2016 nationally representative cross-sectional samples of 8th grade, 10th grade, and 12th grade students (N = 36 410). RESULTS: The lifetime initiation sequence included: (1) e-cigarette use before cigarette smoking (1.7%); (2) e-cigarette use and cigarette smoking in same grade (4.2%); (3) cigarette smoking before e-cigarette use (6.1%); (4) e-cigarette use only (12.6%); (5) cigarette smoking only (3.6%); and (6) no e-cigarette use or cigarette smoking (71.8%). The risk of substance use was highest among dual users (regardless of initiation sequence), followed by cigarette smokers, e-cigarette users, and no e-cigarette use or cigarette smoking. The most prevalent initiation sequence of e-cigarette use involved initiating other substances before e-cigarettes, especially among older adolescents. Early initiation of e-cigarette use was associated with increased odds of substance use behaviors for all three age groups. DISCUSSION AND CONCLUSIONS: Adolescents who report a history of both e-cigarette use and cigarette smoking should be considered at high risk for substance-related problems. Early initiation of e-cigarette use is a signal for other substance use. SCIENTIFIC SIGNIFICANCE: Youth substance use prevention programs and prospective studies must take into account polysubstance use when addressing the relationships between e-cigarette use and cigarette smoking. (Am J Addict 2019;28:285-294).