E-juice (also called e-liquid or vape juice) is the fluid used in electronic cigarettes and vaporizers. It typically contains a blend of vegetable glycerin, propylene glycol, nicotine, and flavorings. When heated by a small coil inside the device, the liquid turns into an inhalable aerosol. That’s the basic idea, but the details of what’s actually in the bottle, how different formulations affect the experience, and what to look for on a label are worth understanding.
What’s Inside the Bottle
The two main ingredients in virtually all e-juice are vegetable glycerin (VG) and propylene glycol (PG). A large analysis of e-liquid compositions found glycerin in 94% of products and propylene glycol in 86%. Both are thick, slightly sweet liquids used widely in food, cosmetics, and pharmaceuticals. Vegetable glycerin is derived from plant oils and gives e-juice its viscosity. Propylene glycol is thinner and carries flavor more effectively.
Beyond those two bases, nicotine appears in about 88% of e-liquids. Water shows up in roughly 45%, ethanol (alcohol) in 23%, and a compound called triacetin, used as a solvent and mild sweetener, in about 15%. The remaining ingredients are flavorings, which can number in the dozens depending on the recipe.
How PG/VG Ratios Change the Experience
E-juice labels often list a PG/VG ratio like 70/30 or 50/50. This ratio matters most for throat hit, the slight catch at the back of your throat that mimics the sensation of smoking. In controlled testing, a 70/30 PG/VG blend produced a noticeably stronger throat hit than both a 50/50 blend and pure VG. Higher PG means a sharper, more cigarette-like sensation.
VG-heavy blends are often marketed for bigger vapor clouds, but this perception doesn’t always hold up. In the same study, participants didn’t rate cloud production differently across PG/VG ratios. The device itself, its power output, and coil design likely matter more for vapor volume than the liquid ratio alone. What VG does reliably contribute is a smoother, denser mouthfeel. If you find high-PG liquids too harsh, shifting toward a higher VG blend softens the experience.
Freebase Nicotine vs. Nicotine Salts
E-juice comes in two nicotine formats, and they behave quite differently. Freebase nicotine is the traditional form: it’s chemically unbound, which means it crosses the membranes of your lungs quickly for rapid absorption. That speed comes with a trade-off. Freebase nicotine feels harsher on the throat, especially at higher concentrations, which is why freebase liquids typically top out around 3 to 12 mg/mL (0.3 to 1.2%).
Nicotine salts pair nicotine with an organic acid (benzoate, lactate, or similar) to create a protonated form that’s gentler on the throat. This allows much higher concentrations, commonly 20 to 50 mg/mL (2 to 5%), without the burning sensation. The trade-off is that salt nicotine absorbs more slowly through lung tissue than freebase. In practice, though, nicotine salt devices still deliver nicotine efficiently, which is why they became popular with people transitioning from cigarettes who wanted a stronger hit per puff without the harshness.
How E-Juice Becomes Vapor
Inside every vape device is a heating coil wrapped around or pressed against a wick, usually made of cotton or silica. The wick draws e-juice from the reservoir to the coil through capillary action. When you activate the device, the coil heats up and turns the liquid into a fine aerosol.
Under normal conditions with a well-saturated wick, coil temperatures range from about 110°C to 185°C. That’s actually below the boiling point of propylene glycol (188°C), meaning the liquid aerosolizes rather than truly boiling. When the wick isn’t fully saturated, as happens with a partially filled tank, temperatures climb to 145 to 334°C. A completely dry coil can spike to over 1,000°C, which is why “dry hits” taste burnt and are worth avoiding. The wick’s ability to keep the coil wet turns out to be the single most important factor in keeping temperatures in a safe operating range.
Flavoring Ingredients and Concerns
Flavorings make up a small percentage of e-juice by volume but account for most of the chemical complexity. A single flavor profile might combine a dozen or more individual compounds. Most are food-grade additives considered safe to eat, but inhalation safety is a separate question since the lungs process chemicals very differently than the digestive system.
Diacetyl, a buttery flavoring compound, is the most studied concern. It was detected in 39 of 51 flavored e-liquids tested in one analysis. Diacetyl exposure by inhalation has been linked to damage in the cells lining the airways, specifically reducing the number of ciliated cells that sweep debris out of the lungs. The compound has been banned from e-liquids in the UK since 2016. The EU maintains a priority list of 15 potentially harmful additives that includes diacetyl, along with compounds like guaiacol, geraniol, menthol, and ethyl maltol. Cinnamaldehyde, the compound that gives cinnamon its flavor, also has sufficient evidence of harm when inhaled. In the U.S., there is no specific ban on these flavoring compounds in e-liquids, though the FDA regulates e-cigarette manufacturing, labeling, and marketing overall.
Nicotine Strength: Choosing a Level
Nicotine concentrations are listed in milligrams per milliliter (mg/mL) or as a percentage. The two scales convert directly: 10 mg/mL equals 1%. What strength works for you depends mostly on how much you currently smoke or vape and what type of device you’re using.
Freebase e-liquids in the 3 to 6 mg/mL range suit people using higher-powered devices that produce more vapor per puff, since each puff delivers a larger total dose. The 9 to 12 mg/mL range works for moderate smokers using smaller devices. Nicotine salt liquids at 20 to 50 mg/mL are designed for low-power, pod-style devices that produce less vapor. Using a high-concentration salt liquid in a high-powered device would deliver an uncomfortably large dose of nicotine, so matching strength to device type matters.
Storage and Shelf Life
E-juice has a shelf life of roughly one year from production, and most bottles carry a printed expiration date. Over time, the nicotine oxidizes and loses potency, while volatile flavor compounds either break down or evaporate. You might notice a color shift toward amber or brown as this happens, along with a muted or “off” taste.
Heat, light, and air exposure accelerate all of these changes. Storing bottles in a cool, dark place with the cap tightly sealed gives you the longest usable life. Nicotine-free e-juice tends to last somewhat longer than nicotine-containing liquid, since nicotine oxidation is one of the primary drivers of degradation. If a bottle has been sitting open or in a warm car for weeks, it’s likely past its best quality even if it hasn’t hit the printed date.
Regulatory Status in the U.S.
The FDA classifies e-juice as a tobacco product and regulates its manufacture, import, packaging, labeling, advertising, promotion, sale, and distribution. Anyone who makes, mixes, modifies, or imports e-liquid must comply with federal manufacturer requirements. As of now, the FDA has authorized 39 tobacco- and menthol-flavored e-cigarette products after rigorous scientific review, including toxicological assessments. The vast majority of flavored e-liquids on the market have not gone through this authorization process, which remains a point of ongoing enforcement action.