RecruitingPhase 1Drug trialComparative Pharmacokinetics and Pharmacodynamics of Synthetic NicotineIn a crossover study, experienced electronic cigarette users will vape 3 different forms of nicotine: natural (derived from tobacco), synthetic, or a 50:50 mixture of both natural and synthetic. The investigators will compare nicotine metabolism, cardiovascular effects, patterns of self-administration, and participants' feelings of craving/withdrawal and enjoyment.