E-juice is the liquid used in e-cigarettes and vapes that gets heated into an inhalable aerosol. It typically contains four ingredients: propylene glycol (PG), vegetable glycerin (VG), nicotine, and flavorings. You might also see it called e-liquid, vape juice, or vape liquid. Despite the simple ingredient list, the ratio and type of each component changes the experience significantly.
The Four Main Ingredients
Propylene glycol is a clear, colorless liquid that acts as the primary carrier for both flavorings and nicotine. It’s thin in consistency, which helps it wick easily into the heating element of a vape device. PG is what delivers most of the flavor intensity and contributes to the sensation you feel in your throat when inhaling.
Vegetable glycerin is also clear and colorless but much thicker. It has a faintly sweet taste on its own, though that’s barely noticeable once mixed with flavorings. VG is responsible for vapor density. The more VG in the mix, the bigger and denser the clouds of vapor. People who enjoy producing large, visible exhales typically choose e-juices with a higher VG ratio.
Nicotine is what makes vaping satisfying for people transitioning from cigarettes. It comes in two forms (more on that below) and in a range of concentrations, from 0 mg for nicotine-free options up to 50 mg for high-strength formulas. Not all e-juice contains nicotine, but most does.
Flavorings are what give e-juice its taste. They come in thousands of varieties, from fruit and dessert profiles to tobacco and menthol. These flavoring compounds are typically suspended in a PG base before being blended with the rest of the liquid.
How PG/VG Ratios Change the Experience
The ratio of PG to VG in an e-juice isn’t just a technical detail. It directly shapes how the vapor feels and looks. Research published in Drug and Alcohol Dependence confirmed what vapers have long reported: higher VG concentrations produce significantly larger, more visible clouds, while PG contributes more to flavor sharpness.
Interestingly, throat hit (that slight kick at the back of your throat when you inhale) peaks with a 50/50 PG/VG blend rather than with either ingredient alone. The same study found that tongue sensation and perceived puff strength were also rated highest with mixed formulas. So if you want a balanced experience with decent flavor, visible vapor, and a noticeable throat hit, a 50/50 ratio is a reliable starting point. Cloud chasers lean toward 70/30 or 80/20 VG-heavy mixes, while people who prioritize flavor often prefer more PG.
Freebase Nicotine vs. Nicotine Salts
E-juice uses one of two nicotine types, and the difference matters more than most beginners realize. Freebase nicotine is the traditional form. It has a higher pH, which means it produces a sharper, more noticeable throat hit as the concentration increases. Most freebase e-juices come in strengths between 3 mg and 12 mg per milliliter. This form absorbs into your system more slowly, making it well suited for longer vaping sessions.
Nicotine salts combine nicotine with an organic acid (usually benzoic acid), which lowers the pH. The result is a much smoother inhale even at high concentrations like 25 or 50 mg. Your body also absorbs nicotine salts faster, so fewer puffs are needed to feel satisfied. This makes nic salts popular for people who want a quick nicotine fix or are using small, low-power devices like pod systems. If you’re a heavy smoker looking to switch, nic salts at 20 to 50 mg tend to replicate the nicotine delivery of a cigarette more closely than freebase at 3 mg.
How Vaping Turns Liquid Into Vapor
Inside every vape device is a small heating coil wrapped around a wick. The wick absorbs e-juice from a reservoir, and when you activate the device, the coil heats up and vaporizes the liquid on contact. Under normal conditions with a properly saturated wick, coil temperatures range from about 110°C to 185°C, depending on the power setting.
When the wick starts to dry out, as it does between refills or during heavy use, temperatures can climb well above 300°C. This is a problem because overheating causes the PG and VG to break down into harmful byproducts like formaldehyde and acrolein. That harsh, burnt taste you get from a dry hit isn’t just unpleasant. It’s a sign that thermal decomposition is happening. Keeping your wick saturated and replacing coils regularly reduces this risk.
Flavoring Safety Concerns
The flavoring compounds in e-juice are generally recognized as safe for eating. They’re the same chemicals used throughout the food industry. But “safe to eat” does not mean “safe to inhale,” and the distinction is important. When heated and aerosolized, some flavoring chemicals interact with lung tissue in ways that food-grade safety testing never evaluated.
A systematic review in the Journal of Toxicology and Environmental Health found that cinnamon, menthol, and strawberry flavors consistently showed the greatest harmful effects on lung cells in laboratory studies. Cinnamon flavoring contains cinnamaldehyde, which activates pain and irritation receptors in the airways. Diacetyl, a buttery flavoring compound, has been linked to bronchiolitis obliterans, a serious and irreversible lung condition sometimes called “popcorn lung” because it was first identified in workers at microwave popcorn factories. Diacetyl has been banned from e-liquids in the UK since 2016, and the EU has established a priority list of 15 potentially harmful additives that shouldn’t be included in e-cigarettes.
The bottom line from current evidence is that while e-juice flavorings are unlikely to cause immediate harm, their long-term effects when inhaled regularly are still not well understood. Certain flavor categories carry more documented risk than others.
Nicotine Strengths and How to Choose
E-juice nicotine levels are measured in milligrams per milliliter (mg/mL), sometimes displayed as a percentage. The standard strengths available are:
- 0 mg: Nicotine-free, for people who enjoy vaping purely for flavor
- 3 to 6 mg: Low strength, common among light smokers or experienced vapers who’ve gradually reduced their intake
- 10 to 12 mg: Medium strength, typical for moderate smokers
- 18 to 20 mg: High strength, suited for heavy smokers or those using nic salts in small devices
- 20 to 50 mg: Very high strength, almost exclusively nicotine salts in pod systems
Choosing the right strength depends on how much you smoked before, what device you’re using, and how often you vape. Starting too high leads to dizziness and nausea. Starting too low means constant cravings and chain vaping to compensate.
Storage and Shelf Life
E-juice has a shelf life of one to two years from the manufacturing date. The PG, VG, and nicotine components each remain stable for about two years when stored properly, though some flavorings can last up to five years. Heat, sunlight, oxygen, and temperature swings all accelerate degradation. Every time you open a bottle, oxygen enters and begins oxidizing the ingredients, which gradually breaks down nicotine and shifts the flavor profile.
You’ll notice expired or degrading e-juice by its darker color, thinner consistency, and muted or off-putting taste. Store your bottles upright in a cool, dark place, and keep them sealed when not in use.
What Steeping Does to E-Juice
Steeping is the practice of letting freshly mixed e-juice sit for days or weeks before using it. During this time, the ingredients bind together more fully, and volatile molecules (responsible for a harsh or overly sharp taste in fresh batches) evaporate out of the liquid. Oxidation also plays a role, triggering chemical changes similar to the Maillard reaction that browns a seared steak or ages a wine. The result is a smoother, more complex flavor and a noticeably darker color, especially in nicotine-heavy liquids. Not every e-juice benefits from steeping, but rich, dessert-style flavors almost always improve with a week or two of rest.
How E-Juice Is Regulated
In the United States, the FDA regulates the manufacture, import, packaging, labeling, advertising, and sale of e-cigarettes and their components, including e-juice. Anyone who mixes, manufactures, or modifies e-liquid, even a vape shop blending custom flavors behind the counter, is legally considered a manufacturer and must comply with federal requirements. To date, the FDA has authorized only 39 tobacco- and menthol-flavored e-cigarette products after rigorous scientific review. The vast majority of flavored products on the market either have pending applications or are sold without authorization, which has prompted enforcement actions and flavor bans in several countries including the US, Finland, Ukraine, and Hungary.