In 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.

Inclusion Criteria:

• Healthy on the basis of medical history and limited physical examination.
• Current regular user of E-Cigarettes (≥ 15 days in the past 30 days)

Exclusion Criteria:

• • Medications

  • Use of medications that are inducers of nicotine metabolizing enzyme CYP2A6 (Example: rifampicin, carbamazepine, phenobarbital, and other anticonvulsant drugs).

  • Use of sympatholytic medications for cardiovascular conditions including hypertension (Example: beta and alpha-blockers).

  • Concurrent use of nicotine-containing medications (Example: nicotine patch, lozenge, gum).

  • Any stimulant medications (example: Adderall) generally given for ADHD treatment.

    • Pregnancy

  • Pregnancy (self-reported and urine pregnancy test)

  • Breastfeeding (determined by self-report)

  • Women of childbearing potential must be using an acceptable method of contraception

    • Inability to read and write in English
    • A known propylene glycol/vegetable glycerin allergy
    • Uncomfortable with getting blood drawn