What is a serum and do I need one?


I try to research my video topics through several different sources, one of them being PubMed. If you are not familiar with PubMed, it is a tool that allows you to search through published articles in scientific journals, and I used it heavily when writing my thesis.

It is however not always the best tool for cosmetic research targeting the most basic questions like: What is a serum?


What is a serum? Doctor explains
What is a serum?


Why? Because in medicine, serum is part of the blood rather than something you put on your face. And similar to how it is with Toner, Tonic and Essence (read more about them here) there  is not much to be found in regards to skin care.

But that doesn’t mean I am not able to answer your questions, so let’s get to it!


What is a serum?

My favorite definition:

A serum is a skincare product you can apply to your skin after cleansing but before moisturizing with the intent of delivering powerful ingredients directly into the skin.

Of course “powerful ingredients” is not only limited to actives, if you have ever applied a humectant rich product to dehydrated and dull skin, you will know that sometimes basics can be the most powerful for sure. It is also important to note that “directly into the skin” still means “the upper layers of the epidermis”. Many serums have penetration enhancers like propylene glycol, ethanol, oleic and linoleic acids to help deliver on their claim of “delivering powerful ingredients directly into the skin”, but even with penetration enhancers, skin care will not affect the deep layers of skin like the Stratum basale or even the Dermis, that is reserved to either injectables or to medication.

The traditional serum is water based, but gooier than Essences (I wrote about the difference between Toners, Tonics and Essences here, if you are interested). More and more brands create oil based serums though, that are supposed to go into the place where a face oil would go, which makes it again a little difficult to draw a clear line between the different parts of your routine. In terms of when you should use what, a good rule of thumb is use a watery serum in the mornings and an oil based one at night (depending on your skin type).


The definition also gives an answer to the question: Why is a serum so expensive?

In theory a serum is the step in your skin care routine that delivers the highest concentration of beneficial ingredients, which reflects in the price.


What does a serum actually do?

Our skin has pretty basic needs, like hydration and protection from UV damage that stay the same, and other, more specific needs, that will change depending on your hormonal status, the season and weather conditions and maybe even our lifestyle choices over the last few days.

And if your cleanser and moisturizer are here to fulfill the basic needs, while they are the foundation of your skincare routine, a serum is here to react to the changes, to act as a boosting shot with whatever our skin craves.

A serum is particularly suited to this task because it is traditionally made up of smaller molecules and formulated to penetrate the skin rather than stay on top of it.


While there is basically a serum for every concern, I think you can divide them into three main categories:


Dr Dennis Gross ferulic acid + retinol brightening solution
Dr Dennis Gross ferulic acid + retinol brightening solution


Antiaging Serums

Think peptides, retinoids, anything that claims to stimulate collagen production or even out the skin tone.

These are the ones aimed at more mature skin.

A few examples are


pixi skintreats Vitamin C Serum Review
pixi skintreats Vitamin C Serum


Antioxidant Serums

Vitamin C, Vitamin E, Ferulic Acid, Ubiquinon – you have heard of them already.

Antioxidants are able to prevent free radicals from damaging our skin by neutralizing them, and free radicals are everywhere. They appear due to UV exposure, stress, pollution, exercise, even through basic skin cell functions in our bodies.

Prevention is key here, which is why I recommend you introduce them into your routine early on.

A few examples are:


Instytutum Super Serum
Instytutum Super Serum – very expensive, but worth it?


Hydrating Serums

Glycerin, Hyaluronic Acid, basically all the humectants that have one thing in common: When applied to dehydrated skin, they immediately make it look better.

Hydration plumps fine lines, gives a grown up glow and helps maintain a healthy skin barrier (read more about the skins barrier function here). Again something everyone benefits from and something that can be introduced into your routine early on.

A few examples are:


Can a serum replace a moisturizer?

In short: No.

More elaborate: That depends.

Traditional serums contain humectants and active ingredients and lack emollients and occlusives. Which means that while they increase moisture retention in the skin, they will not reduce Transepidermal Waterloss (TEWL), the loss of moisture through the skin, which leads to all the nice hydration evaporating again.

So for best results, you will need a serum and then a moisturizer to seal it in. In case you want to read up on that, here are my posts on humectants and emollients/occlusives.

The only exception? Very oily skins might be able to rely on their sebum to seal in the moisturizer.

With the hybrid serums coming to the market more and more though, this might change. Look at the ingredient list of the product marketed as serum: Does it contain humectants and actives only? Or does it actually contain emollients or even occlusives? The Inkey List Q10 for example is called a serum, but with the emollients it contains it is enough for me to be used as last step in my routine, only followed up by my sunscreen.

So as always: Look at the ingredient list and listen to your skin.


At what age should I start using a serum?

Serums were traditionally marketed at mature skin, but with preventative skincare becoming more important, that has changed.

I still think though that until the age of maybe 25 you don’t really need to invest in this step. Depending on your skin type (determine which one you have here) and your lifestyle choices, your midtwenties are the time when a hydrating or an antioxidant serum should be introduced into your routine.

Afterwards, you can gradually introduce the others (read here for a quick guide on which ingredients you need depending on your age).

I hope that answers all your questions, if not, feel free to ask away in the comments.


What is a serum, what does it do, where does it go in your routine and at which age do you need one? Doctor Anne explains everything you need to know!
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