PROJECT SUMMARY/ABSTRACT This proposal is in response to PAR-21-138: Method to Extend Research in Time (MERIT) Award Extension Request, which includes a 2-year MERIT extension request to support new data collection focused on cannabis-tobacco co-use and the implications of cannabis use on tobacco cessation outcomes. This MERIT proposal represents a timely and logical extension of the NCI-funded parent MERIT award (R37 CA237245, PI McClure) entitled, “Determining the impact of cannabis use and severity on tobacco cessation outcomes: A prospective tobacco treatment trial.” The overall goal of the funded parent R37 MERIT award was to conduct a prospective tobacco treatment study evaluating the impact of cannabis co-use on tobacco cessation outcomes, focused specifically on those smoking cigarettes. In the parent study, participants who co-use cigarettes and cannabis are being oversampled 2:1 to cigarette-only participants. The parent study is designed to evaluate how cannabis use may serve as a barrier to successful tobacco cessation and how cannabis use changes during a tobacco cessation attempt. Several important observations have emerged during the course of the parent award as well as in the tobacco-cannabis co-use literature more broadly, prompting the proposed 2-year MERIT extension. Specifically, the use of e-cigarettes (vaped nicotine products; electronic nicotine delivery systems [ENDS]) has increased among youth and adults in the United States (US). These trends in non-combustible forms of tobacco use have been observed in the parent R37 award. Within the parent study, we are excluding a large number of interested callers due to their use of e-cigarettes primarily or in combination with cigarettes at a frequency that is exclusionary. While the parent study is focused on combustible tobacco use (cigarette smoking), which is the most harmful of tobacco products, the ubiquity of e-cigarettes cannot be ignored, particularly as it relates to cessation and the underlying relationship with cannabis co-use. Therefore, this 2-year MERIT extension proposes to enroll a third group of adults (ages 18-40; N=70) who are using e-cigarettes daily or near daily (co-use of cigarettes is allowable; <10 days out of the past 30) and are also using cannabis. We will leverage the methods, measures, and procedures being used as part of the parent R37 to complete this e- cigarette cessation trial with a focus on a co-use population to extend the generalizability of study findings. The sample of e-cigarette-cannabis co-use participants is the main difference in this MERIT extension. This 2-year MERIT extension will allow for the exploration of e-cigarette and cannabis co-use characteristics and the impact of cannabis co-use on e-cigarette cessation. New data collection proposed here falls within the scope of the parent grant and represents an important extension of the original aims. The inclusion of those using tobacco through both cigarettes and e-cigarettes wil...