Useful info:

Contents of liquid solutions vary, but their common aspects include water and flavorings in a propylene glycol or glycerin base. Nicotine is also included in solutions intended to fulfill a nicotine replacement role.

Flavors

Hundreds of different flavor varieties are available. Some attempt to resemble traditional cigarette types, such as regular tobacco and menthol, and some even claim to mimic specific cigarette brands, such as Marlboro or Camel. A wide variety of food flavors are also sold, from the traditional (vanilla, coffee, cola) to the more exotic (strawberry daiquiri, Boston cream pie).

Nicotine

Liquid solutions containing nicotine are available in differing nicotine concentrations to suit user preference. Dosing nomenclatures are not standardized and vary by manufacturer, but tend towards the following rough figures:

• Liquids said to contain "low" doses of nicotine tend to correspond to a nicotine concentration of 6–8 mg/ml (milligrams of nicotine per milliliter of liquid).
• "Midrange" or "medium" doses tend to correspond to a nicotine concentration of 10–14 mg/ml.
• "High" doses tend to correspond to a nicotine concentration of 16–18 mg/ml.
• "Extra-high" doses tend to correspond to a nicotine concentration of 24–36 mg/ml.

Actual numerical nicotine concentration ratings are usually printed on liquid containers or cartridge packaging. Often, the standard notation "mg/ml" is shortened to a simple "mg".

Bases

Flavors and nicotine are dissolved in hygroscopic components, which turn the water in the solution into a smoke-like vapor when heated. Commonly used hygroscopic components include propylene glycol, vegetable glycerin, and polyethylene glycol 400 (often sold under the abbreviations PG, VG, and PEG 400, respectively).

Safety of liquid bases

All three liquid bases are common food additives used in a variety of pharmaceutical formulations. Propylene glycol, the current dominating liquid base, has been utilized in asthma inhalers and nebulizers since the 1950s, and because of its water-retaining properties, is the compound of choice for delivering atomized medication. The U.S. Food and Drug Administration (FDA) includes propylene glycol on its list of substances Generally Recognized as Safe (GRAS), and it meets the requirements of acceptable compounds within Title 21 of the Code of Federal Regulations.