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. 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. 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. 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.