Objectives: E-cigarette use is increasing among young adults in the U.S. However, longitudinal research studies examining associations between e-cigarette use and combustible cigarette use among young adults are limited. This study assessed the relationship of e-cigarette use to smoking reduction and cessation among young adults. Methods: This is a secondary analysis of a randomized controlled social media-based smoking cessation trial comprising adults ages 18 to 25 who smoked cigarettes and engaged in heavy episodic drinking (N?=?179). Over 12 months, participants reported past month e-cigarette use with nicotine or tetrahydrocannabinol, cigarette quantity in the past week, quit attempts, and cessation strategies including nicotine e-cigarettes. Longitudinal regression models estimated associations between e-cigarette use, smoking reduction, and 7-day abstinence. Results: Past-month nicotine e-cigarette use prevalence ranged from 53.1 at baseline to 50.3 at 12 months. Over 70 of participants who reported past month nicotine e-cigarette use also smoked cigarettes (ie, dual use). Neither past month nicotine nor tetrahydrocannabinol e-cigarette use was associated with smoking reduction or cessation. However, use of nicotine e-cigarettes as a cessation strategy among participants attempting to quit (N?=?137) was positively associated with abstinence (adjusted odds ratio?=?2.47, 95 CI?=?1.20–5.09) and ≥50 reduction in cigarettes per week from baseline (aOR?=?2.36,95?CI?=?1.08–5.18), relative to other strategies. Conclusions: Nicotine e-cigarettes were significantly associated with improved tobacco use outcomes when used as a cessation strategy, but not when used apart from trying to quit smoking. Dual use may not be an effective path to achieve smoking cessation.
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