Examining Perceptions of Uncertain Language in Potential E-Cigarette Warning Labels: Results from 16 Focus Groups with Adult Tobacco Users and Youth.

E-cigarette use among youth presents a public health risk. Yet, cigarette smokers who substantially reduce their smoking or switch completely from traditional combustible cigarettes could benefit. As science about e-cigarettes is continually emerging, any potential warnings are likely to contain uncertain language. Hedged verbiage may impact decision making. To assess reactions, we conducted 16 online focus groups; 8 with youth ( = 32, grouped by gender and by vaping experience) and 8 with adult tobacco users ( = 37, grouped by smokers, dual users of e-cigarettes and cigarettes, and former smokers who switched to e-cigarettes). Each focus group viewed and discussed 8 potential warnings messages. We conducted an inductive thematic analysis of the reactions to warning messages that contain uncertain language. Respondents' reactions were often negative, but varied based on specific usages of uncertainty, existing beliefs about uncertainty in law and science, and smoking/vaping use patterns that supported the use of uncertainty related to e-cigarettes. Many youth (and some adults) believed that uncertain language enabled audiences to minimize the likelihood of harm or interpreted it as meaning there are both healthy and unhealthy e-cigarettes. This qualitative study provides evidence that the use of types of uncertain language, the frequency of that use, and/or the selection of particular words in warnings, might not achieve the intended public health aims of increasing understanding of risk, deterring youth uptake, and/or facilitating a substantial switch from cigarettes. The use of certain types of uncertain language appears to have significant potential to bring unintended consequences. Suggestions for research and policy are included.