Study Design: The investigators will recruit 120 individuals who are long-term daily and exclusive users of nicotine-containing e-cigarettes. Participants will be asked to use their own e-cigarette as usual during the first week of the study and to then stop their use of e-cigarettes for 6 days. The total study duration will be 14 days. Participants will be instructed to continue abstinence from other tobacco and nicotine products during the entire study. The investigators will use an escalating payment system with bonuses based on breath and urine samples to encourage compliance.