Stand in front of a shelf of hair products and you are reading a vocabulary, not a taxonomy. Serum. Oil. Tonic. Essence. Elixir. Treatment. The words sound like categories with rules behind them, and shoppers reasonably assume that a serum is a defined kind of thing in the way that a shampoo is.
It mostly is not. Some of those words describe a real difference in how a product is built. Others are marketing vocabulary with no fixed meaning at all, and one of them is the only word in the list that the US regulator actually recognises, which is not the one most people would guess.
Here is what each one tends to mean, what none of them are required to mean, and how to work out which you are holding regardless of what the front says.
There is no rulebook for these words
US cosmetic labelling has a requirement called the statement of identity: the front of the package has to say what the product is. The rule is broad about how. The FDA's labelling guide says the identity may be the common or usual name of the cosmetic, a descriptive name, or, when the nature of the cosmetic is obvious, a fanciful name, and that it may even be expressed as an illustration.1
Read that again, because it is the whole answer. "When the nature of the cosmetic is obvious, a fanciful name." If a shopper can tell roughly what it is, the name can be invented. There is no definition of serum to meet and no test to pass.
The front of the bottle has to tell you what kind of thing it is. It does not have to use any particular word to do it.
The FDA's own product category list makes the same point from the other direction. Under hair preparations, the secondary categories are conditioners, sprays, straighteners, permanent waves, rinses, shampoos, wave sets, other hair preparations, and one called tonics, dressings, and other hair grooming aids.2
Serum is not on that list. Neither is oil, nor essence, nor elixir. A leave-on scalp serum, filed with the regulator, is a tonic or dressing. The word on the front is the commercial name; the word in the filing cabinet is the older one.
The distinction that is real
Underneath the vocabulary there is one genuine division, and it is not subtle once you know to look for it. Is the product mostly water, or mostly oil?
That single fact decides almost everything else: how it feels, what it can carry, where it sits, and whether it can be worn under other things without turning greasy.
You do not have to take a brand's word for which it is, because the ingredient list settles it. Cosmetic ingredients present above one per cent must be declared in descending order of predominance, so the first ingredient is the bulk of the product.3 Water first means a water-based product whatever the front calls it. An oil or an ester first means an oil-based one.
Serum
In practice, a serum is usually a water-based product with a light texture and a relatively high concentration of whatever the brand considers its active material. The word arrived in hair care from skincare, where it came to mean something thinner than a cream and more concentrated than a lotion.
None of that is enforceable. There are serums that are almost entirely silicone, and serums that are water and glycerin with very little else. The word signals an intention about texture and focus, not a composition.
What it usually indicates in practice: water or a water-glycerin base near the top of the list, a light feel, and a format designed to be left on rather than rinsed.
Oil
The most honest word of the four, because it is the hardest to use dishonestly. If a product is called an oil and the first ingredient is water, something has gone wrong with the name.
Oils sit on the hair fibre and are good at what oils do: smoothing the cuticle, adding shine, reducing the friction that causes breakage. They are heavier, they do not absorb into skin the way a water-based product does, and they are generally a lengths-and-ends product rather than a scalp one.
Worth knowing: "oil" on the front tells you the format, not the ingredient. A hair oil can be argan, coconut, mineral oil, dimethicone or any combination. The list tells you which.
Tonic
The old word, and the only one of the four the FDA recognises as a category.2 Historically a tonic was a thin, usually alcohol-containing liquid applied to the scalp, and the word carries a slightly old-fashioned, barbershop association, which is roughly why the industry replaced it with serum.
Modern products called tonics are usually water-based and applied to the scalp rather than the hair. Functionally, a scalp tonic and a scalp serum are frequently the same kind of product with a different word on the front.
Essence
The least defined word of the lot. It arrived from Korean skincare, where it described a watery, lightweight step applied before heavier products. In hair care it has no settled meaning at all and tends to signal "thin and watery" and very little else.
If you see it, the ingredient list is doing all the work. There is nothing else to go on.
What this changes about shopping
Three habits are worth more than any of the vocabulary.
- Read the first ingredient. It tells you whether the product is water-based or oil-based, which is the only structural fact in the whole discussion.
- Work out whether it is for skin or for fibre. Scalp products and lengths products are built differently and go in different places. The word on the front often will not tell you, but the directions will.
- Ignore the category name when comparing. Two products called serums can be less alike than a serum and a tonic. Compare lists, not nouns.
What we call ours, and why
Both Eloura products are called serums, and by the standard above that word is doing exactly what the section on serum describes: water-based, light, leave-on, applied to the scalp. On the FDA's list they are tonics and dressings, which is the less flattering and more accurate word.
Peptide serum
Max Growth
Water first, then glycerin. Five peptides named one by one, tenth to fourteenth on the list. Leave-on, no rinsing, applied to the scalp rather than the lengths. 1 fl oz.
See Max Growth
Botanical serum
Max Density
Water, glycerin, then rosemary leaf extract third, ahead of every other extract. Water-based and leave-on, left on the scalp for one to two hours before washing. 2 fl oz.
See Max DensityNeither is an oil, which is why neither is meant for your ends, and why neither will do the shine-and-slip job an oil does. That is a limit of the format, not a modesty.
The thing none of these words changes
One last point, because format words sometimes get used to imply more than a cosmetic can claim. Under US law a cosmetic is a product intended for cleansing, beautifying, promoting attractiveness or altering the appearance, and a product intended to affect the structure or any function of the body is a drug.4
Calling something a serum, a treatment or an elixir does not move it across that line. The category is decided by what the product is sold as doing, not by the shape of the bottle or the word on the front.
If you want the next layer down, what the claims on the front are allowed to mean is where most of these questions get settled, and where each format sits in a routine is the practical companion to this one.
Sources
- US Food and Drug Administration, Cosmetics Labeling Guide. The statement of identity, and the allowance for a descriptive or fanciful name.
- US Food and Drug Administration, Cosmetic Product Categories and Codes. The secondary categories under hair preparations, including "tonics, dressings, and other hair grooming aids".
- US Code of Federal Regulations, 21 CFR 701.3, Designation of ingredients. Descending order of predominance above one per cent.
- US Food and Drug Administration, Is It a Cosmetic, a Drug, or Both? (Or Is It Soap?). The definitions of cosmetic and drug under the FD&C Act.
Eloura products are cosmetics. They are not intended to diagnose, treat, cure or prevent any condition. This article is general information about product labelling and is not medical advice.
