Abstract The Food and Drug Administration (FDA) is working to finalize a ban on menthol cigarettes. Finalizing a ban, navigating litigation from the tobacco industry, and implementing the ban will be a lengthy process. Some organizations have advocated that non-tobacco flavorings, including menthol, should also be banned from other tobacco products like e-cigarettes. Despite the wealth of cross-sectional and uncontrolled studies of menthol, we have very little prospective, experimental data investigating the impact of a menthol ban on tobacco use for menthol smokers in the United States, and even less evidence on how a menthol ban for e-liquids might interact with a cigarette menthol ban. If menthol is banned from both cigarettes and e-cigarettes, menthol smokers who otherwise would have tried to switch to e-cigarettes may continue to smoke cigarettes or quit using tobacco altogether. Thus, the impact of a cigarette menthol ban on changes in tobacco use may depend on whether menthol is banned in e-cigarettes. The proposed trial is a 2x2 between-subjects randomized controlled trial investigating the impact of a menthol bans for cigarettes and e-cigarettes on tobacco use patterns including cigarette smoking, e-cigarette use, use of medicinal nicotine, and cessation-related behaviors. Current menthol smokers (N=240) will complete a 1-week baseline period before being assigned to either menthol or non-menthol cigarettes and either menthol or tobacco-flavored e-liquid for 7 weeks. To model a ban, smokers will be instructed to only use their assigned products, and adherence will be assessed using self-report and a urinary biomarker for menthol. Changes in tobacco use patterns (cigarettes smoked per day, e-cigarette use, ability to abstain from smoking) will be assessed through electronic daily diaries (Aim 1). Secondary outcomes include cigarette and e-cigarette subjective effects, and cigarette and e-cigarette dependence (Aim 2). To assess the ability to abstain from smoking, participants will complete a 1-week practice quit attempt in Week 7, and we will assess the time to first smoking lapse. Self-reported tobacco use is corroborated by biomarkers for smoke and nicotine exposure (expired carbon monoxide, urinary cotinine). Finally, it is critical to understand how menthol regulation will impact the entire population of US menthol smokers, as well as the effect on vulnerable groups where menthol smoking is concentrated. Thus, we will use advanced statistical techniques to assess whether any baseline demographic or smoking variables moderate the treatment effect. Then, we will calibrate the treatment effects to the US population of menthol smokers, and model the impact on smoking and vaping attributable deaths and life years lost (Aim 3). The strong investigative team, coupled with our success using the proposed methodology, enhances the probability of success in achieving our aims. This project will be the largest, most rigorous clinical trial to dir...