ABSTRACT: INTRODUCTION:Evidence for the health harms of e-cigarettes is growing, yet little is known about which harms may be most impactful in health messaging. Our study sought to identify which harms tobacco product users were aware of and which most discouraged them from wanting to vape. METHODS:Participants were a convenience sample of 1,872 U.S. adult e-cigarette-only users, cigarette-only smokers, and dual users recruited in August 2018. In an online survey, participants evaluated 40 e-cigarette harms from seven categories: chemical exposures, device explosions, addiction, cardiovascular harm, respiratory harm, e-liquid toxicity, and other harms. Outcomes were awareness of the harms ("check all that apply") and the extent to which the harms discouraged vaping (5-point scale; (1) "not at all" to (5) "very much"). RESULTS:Awareness of most e-cigarette harms was modest, being highest for harms in the device explosions category of harms (44%) and lowest for the e-liquid toxicity category (16%). The harms with the highest mean discouragement from wanting to vape were the respiratory harm (M = 3.82) and exposure to chemicals (M = 3.68) categories. Harms in the addiction category were the least discouraging (M = 2.83) compared with other harms (all p < .001). Findings were similar for e-cigarette-only users, cigarette-only smokers, and dual users. CONCLUSIONS:Addiction was the least motivating e-cigarette harm, a notable finding given that the current FDA e-cigarette health warning communicates only about nicotine addiction. The next generation of e-cigarette health warnings and communication campaigns should highlight other harms, especially respiratory harms and the chemical exposures that may lead to health consequences. IMPLICATIONS:E-cigarette health harms related to respiratory effects, chemical exposures, and other health areas most discouraged vaping among tobacco users. In contrast, health harms about addiction least discouraged use. Several countries have begun implementing e-cigarette health warnings, including the United States, and many others are considering adopting similar policies. To increase impact, future warnings and other health communication efforts should communicate about health harms beyond addiction, such as the effects of e-cigarette use on respiratory health. Such efforts should communicate that e-cigarette use is risky and may pose less overall risk to human health than smoking, according to current evidence.