Two Chemical Forms
Nicotine in e-liquid comes in two forms: freebase and salt. Freebase nicotine is the unprotonated, alkaline form (pH 8-10) found in traditional cigarettes and lower-concentration e-liquids (typically 3-6 mg/mL for sub-ohm devices). It is more lipophilic, crossing the blood-brain barrier readily, but its harshness limits how much can be comfortably inhaled.
Nicotine salts are formed by combining freebase nicotine with an organic acid (typically benzoic acid), which protonates the nicotine molecule and lowers the pH to approximately 5.5-6.0. This makes the vapor smoother at high concentrations, enabling 20-50 mg/mL formulations common in pod systems and disposables.
Absorption: What the Christen Study Found
A landmark 2025 randomized crossover study by Christen et al. in Nicotine & Tobacco Research compared the pharmacokinetics of inhaled nicotine salt versus freebase using an e-cigarette. Key findings:
- Freebase 20 mg/mL formulations achieved lower blood nicotine concentrations than nicotine salt at the same concentration.
- 40 mg/mL nicotine salt yielded blood nicotine concentrations similar to cigarette smoking.
- The study concluded: "Nicotine salt formulations inhaled by an e-cigarette led to higher nicotine delivery compared to nicotine-free-base formulations with the same nicotine concentration."
This has a dual implication. On one hand, salt nicotine's efficiency means smokers can more easily transition to vaping with satisfying nicotine delivery. On the other, the smoothness eliminates the harshness cue that naturally limits consumption, potentially allowing users to exceed their intended nicotine dose without realizing it.
How the Christen Study Worked
The study used a randomized crossover design — each participant used both salt and freebase formulations on different days, with washout periods between sessions. Blood nicotine concentrations were measured at multiple time points after controlled puffing sessions. This crossover design controls for individual metabolic differences, making the comparison between salt and freebase more reliable than parallel-group designs.
Nicotine Concentration Regulations
| Region | Nicotine Cap | Regulatory Framework |
|---|---|---|
| European Union / UK | 20 mg/mL (2%) | Tobacco Products Directive (TPD) |
| Canada | 20 mg/mL (2%) | Federal + provincial flavor restrictions |
| United States (federal) | No federal cap | FDA PMTA process; state-level caps vary |
| US market (authorized products) | Up to 50 mg/mL (5%) | FDA PMTA authorization required per product |
