The study is designed as a within-subjects pharmacokinetic comparison. Fourteen (14) e-cigarette users who are also current marijuana/THC users will switch between two THC e-liquid conditions, namely, (a) e-liquid solution containing 40% THC and no nicotine, and (b) e-liquid solution containing 40% THC and 0.6% (6 mg/mL) nicotine, during 2 outpatient visits. The outpatient days will comprise of a fixed-puffing regimen. Study days will be separated by at least 48 hours. The order of treatment (THC only and THC with nicotine) will be counterbalanced between subjects. Subjects will be blinded to the content of the e-cigarette (i.e. THC only vs THC + nicotine).