If there’s one active ingredient you should add to your skincare routine (in addition to sunscreen, of course!) vitamin c serum for skin. It really truly works. There’s something to be said about a skincare ingredient that does what it says it’ll do—brighten, tighten, hydrate and lift—without too many side-effects or much sensitivity. 

It may sound perfect (and when it comes to efficacy, it is) but you need the product in the right consistency, formula, packaging and pH for it to really work. Here’s everything you need to know about the skincare must-have.

It boosts collagen production

This is Vitamin C’s most popular function. When applied to the skin, it directly triggers DNA to regulate and maintain collagen production. Collagen—a protein—is the building block that holds up skin cells. So a lack of it shows up as fine lines, hollowness, droopiness and sagginess. The loss of collagen reduces the skin’s structural support (there is less to hold it up!). It also causes a loss of definition along the contours of the face.

It’s hydrating

Some Vitamin C derivatives have a hydrating effect on skin. Research shows that it is able to decrease transepidermal moisture loss, which allows your skin to retain and store moisture better. 

It fades pigmentation

Have a black or purple scar left over from acne, sun damage or an injury? Struggling with melasma or sun spots? Vitamin C blocks the action of an enzyme called tyrosinase, which is the building block of the pigment melanin. No tyrosinase = less darkening of the skin, especially over time.

It offsets damage from the sun, pollution and free radicals

Vitamin C is a powerful antioxidant, which protects healthy skin cells by giving away electrons to free radicals, rendering them harmless to the skin. Free radicals are unstable molecules that are looking for other atoms or molecules to be whole again. This causes them to create oxidative stress on the cells. Oxidative stress results in the breakdown of collagen, the formation of dark spots and the exacerbation of sun damage. 

It can help soothe acne

Since Vitamin C is an anti-inflammatory ingredient, it is able to reduce redness and swelling from a particularly red and painful zit, making it an important ingredient in your anti-acne repertoire too.  

How should you pick a Vitamin C formula?

First, pick the active ingredient that’ll really work for you. For oily or normal skin, L-ascorbic acid is the potent form, giving you the best results in the least amount of time. However, it can be a little drying for sensitive skin. For those skin types, magnesium ascorbyl phosphate is a safer bet. Then, pick the right concentration. While it exists as low as 2% and as high as 20%, it is best to start low and then go up to prevent any sensitivity. Research shows that at 20%, it hits a plateau. So even if you use 30% Vitamin C, it won’t work any better. 

The pH is important too. If you have normal skin, look for a lower pH (like 3.5) that really ups the absorption level. For those with sensitive or very dry skin, a pH of 5 or 6 works better as it mirrors the skin’s natural pH.

In spite of all its benefits, Vitamin C is notoriously unstable. It is important to pick a well-formulated product housed in an opaque, dark bottle or tube that is air-tight, so it doesn’t get exposed to air. When it is oxidized it will thicken up or darken, which is a sign you should get rid of it, ASAP. 

How should you add Vitamin C to your routine?

Serums are designed to deliver a high concentration of actives and have smaller molecules, so they penetrate skin better. It is best to wear your Vitamin C serum under SPF in the day. Think of it as a safety net that prevents the free radical damage from any UV light that does penetrate through. 

The ingredient works best with ferulic acid and Vitamin E, both of which can help stabilize and reduce its pH balance for easier absorption into the skin. Hyaluronic acid is a good pairing too—apply the Vitamin C first, and then layer with HA in order to  fortify the skin’s barrier and lock in the moisture. It is best to skip layering it with retinoids. Both ingredients can cause irritation and can alter each other’s pH.