What Is A Serum? Here's Your Guide To The Skin Care Powerhouse

Contributing writer By Andrea Jordan
Contributing writer
Andrea Jordan is a beauty and lifestyle freelance writer covering topics from hair and skincare to family and home. She received her bachelor's in Magazine Journalism from Temple University and you can find her work at top publications like InStyle, PopSugar, StyleCaster, Business Insider, PureWow and OprahMag.
woman applying a serum
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Welcome to modern skin care, where a basic routine consists of dozens of products, all in the hopes of achieving that youthful glow. Yes, gone are the days when a basic cleanser and moisturizer were enough to have gorgeous skin. These days there are pre-cleansers, oils, scrubs, retinols, acids, and the increasingly popular serums that make up a well-rounded skin care regimen. And the truth is, if you're not using a serum (or two), you are missing the mark because these potent potions are the beloved secret to tackling major complexion woes like dark spots, acne, redness, and even fine lines and wrinkles.

But if you've ever wondered what a serum actually is, don't fret you're not alone. That's where we are here to help. We tapped two board-certified dermatologists to get the scoop on serums, what they are, why we need them, and how to use them.

Keep reading for an explainer on serums 101. 

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What is a serum? 

According to board-certified dermatologist Dendy Engelman, M.D., "Serums are light, thin skin care products filled with active ingredients." She continues to say the main purpose of incorporating a serum into your routine is to ensure that these skin actives penetrate the skin barrier for the ingredients to be as effective as possible.

How, you might ask? "Serums tend to have smaller molecules, which allow the ingredients to do deeper into the skin," she says.

So even though you may have the same active in your cleanser and moisturizer, the serum is your best bet for getting the best results from your favorite ingredients. 

Serum vs. moisturizer 

Even though serums and moisturizers may have similar effects on the skin, depending on your formula, the two are not synonymous. And if we're being honest, you shouldn't choose one over the other. Instead, these two products should be used together.

"Serums are the workhorse products that target specific skin concerns," Engelman says, "whereas moisturizers act as a barrier for the skin, locking in moisture." And even though both can fix skin issues, the purpose of the serum is to address a specific issue you want to target and the moisturizer is to, well, keep the skin moisturized and healthy. 

Serum vs. oil 

Now, you may be thinking, serums and oils have practically the same consistency. But that doesn't mean the two are interchangeable. Engelman says that oils are used to soften the skin and help trap moisture into the skin so it doesn't seep out. And even though they are conditioning, most oils aren't formulated with skin care actives that target skin concerns. In fact, in most cases, you'll want to use an oil that is 100% pure and steer clear of peculiar blends. 

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Why are they so expensive? 

If you've ever perused a beauty aisle, you may have noticed that serums typically come with a hefty price tag, although there are a slew of wallet-friendly options becoming available these days. Marmur says serums are usually more expensive because of the powerful active ingredients that you won't find in a standard moisturizer or other skin care products. These actives are the secret to addressing specific concerns, so the quality of ingredient can drive up the price tag. 

But take note, Marmur says, expensive doesn't always mean better. "Compare the ingredients of products at different price points to see the difference," she suggests. "Less expensive products can sometimes contain less effective ingredients or a lower percentage of those ingredients," which is something to be mindful of when choosing the best formula for you.  

So, when do I apply a serum? 

The struggle is real, and we get it. When you have multiple products that make up your skin care routine, it can be confusing to know which order is most beneficial. Serums, since they contain potent actives, should be applied to clean, freshly washed skin—that way, they have more opportunity to penetrate the skin and do their work.

According to board-certified dermatologist and founder of MMSkincare Ellen Marmur, M.D., skin care products should be applied from thinnest to thickest. But take note, that doesn't include your cleanser. Your cleanser should always go first, no matter how thick or thin it is. "If you choose to use a serum, it's usually applied after cleansing the skin and before the moisturizer," Marmur says. 

Engelman agrees that thinnest to thickest is the go-to rule of thumb because it allows each layer to be fully absorbed by the skin. "If you put a serum on top of a moisturizer, the serum won't be fully absorbed since the smaller molecules cannot penetrate the larger ones." So, when in doubt, stick to thick creams as your last step and lightweight serums in the middle. 

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How often should I use a serum? 

There's no general rule for how often you should or shouldn't use a serum, but it's always best to use as directed. "You can use serums day and night as long as they don't have harsh active ingredients like retinol," Marmur suggests. "If you're using a serum with a harsh ingredient or anything that can make you extra sensitive to the sun, you should use it at night and never forget sunscreen." And as always, it's always better to be safe than sorry, so be sure to read the directions on your preferred serum before applying. 

The takeaway.

We get a lot of questions around serums. And so even though it's become a pretty standard skin care item, it's not always well understood. But once you take the time to figure it out, you'll understand how important the product is.

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