BACKGROUND AND AIMS: It is unknown whether young adults who vape nicotine and have poor mental health have greater risk of smoking initiation than expected based on individual risks of vaping and mental health alone. This study aimed to estimate the joint association of vaping and mental health symptoms with smoking initiation among young adults, and test for additive interaction between vaping and mental health in smoking initiation risk.

DESIGN: Using five waves of the Population Assessment of Tobacco and Health (wave 1, 2013-2014; wave 2, 2014-2015; wave 3, 2015-2016; wave 4, 2016-2018; wave 5, 2018-2019), we estimated risk differences (RD) for the association of time-varying and time-lagged vaping and internalizing (e.g., anxiety, depressive) and externalizing (e.g., inattention/hyperactivity) mental health symptoms with cigarette smoking initiation at follow-up, over four 1-year intervals. We calculated interaction contrasts (IC) to estimate the excess risk of smoking initiation attributable to the interaction of vaping and mental health symptoms.

PARTICIPANTS: A total of 6908 cigarette-naïve individuals aged 18-24 years in the United States.

MEASUREMENTS: Exposures included current (past-30 day) vaping and internalizing and externalizing mental health symptoms (high vs moderate/low symptoms). The outcome was smoking initiation (ever cigarette use) after 1 year.

FINDINGS: The per-interval risk of smoking initiation was 7.6% (1039 cases/13 712 person-intervals). Compared with noncurrent vaping and moderate/low mental health symptoms, adjusted RDs for current vaping and high mental health symptoms were 17.2% for internalizing and 18.7% for externalizing symptoms. The excess risk attributed to interaction of current vaping and high externalizing symptoms was IC = 11.3%, with inconclusive findings for internalizing symptoms.

CONCLUSIONS: There is possible, but inconclusive, superadditivity between vaping and mental health in risk of smoking initiation among young adults in the United States.