Ascorbic Acid: the benefits of vitamin C for the skin

Ascorbic acid, better known as vitamin C, is one of the most studied active ingredients in dermocosmetics. It is a powerful antioxidant, a cofactor required for collagen synthesis, and one of the most effective ingredients for evening out skin tone.

It is also one of the most fragile active ingredients: it oxidizes easily, requires a precise pH to work, and not all forms available on the market behave in the same way. Understanding these differences is what distinguishes a serum that works from one that turns yellow in the drawer.

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What is ascorbic acid?

Ascorbic acid is a water-soluble vitamin that the human body cannot synthesize: it must be obtained through the diet. Even when consumed regularly, however, the amount that reaches the skin through the bloodstream is limited—which is why topical application makes sense.

In the skin, it plays three distinct roles:

• antioxidant: it neutralizes free radicals generated by UV rays and pollution;
• enzymatic cofactor: it is essential for the enzymes that stabilize collagen fibers. Without vitamin C, collagen does not form properly;
• melanin modulator: it interferes with the activity of tyrosinase, the key enzyme involved in pigmentation.

There’s a little-known but decisive detail: vitamin C is also the “regenerator” of vitamin E. When vitamin E neutralizes a free radical, it is oxidized in turn and loses its effectiveness; vitamin C restores it to its active form. That’s why the two are almost always found together in a formula: on their own, they last less.

Concentration and pH: the two numbers that matter

These are the two parameters that determine whether a vitamin C serum really works, and they are almost never stated on the label.

The concentration

For pure ascorbic acid, the literature indicates a useful range between 8% and 20%. Below 8%, the effect is marginal; above 20%, no proportional benefit is observed, while the risk of irritation increases. The most common range in well-formulated serums is around 10–15%.

The pH

This is the most important technical point. Pure ascorbic acid penetrates the skin only at a pH below 3.5, because below that threshold it loses its electrical charge and can cross the stratum corneum.

The problem is that such a low pH is poorly tolerated by reactive skin, and on sensitive skin it can cause stinging and redness. It is precisely to overcome this trade-off that stabilized derivatives were developed.

Forms of vitamin C: they are not equivalent

When you read “vitamin C” on packaging, it may correspond to different molecules in the INCI, with different behaviors.

• Ascorbic Acid — the purest and most studied form. Maximum documented effectiveness, but it requires a low pH and is the most unstable.
• Sodium Ascorbyl Phosphate (SAP) — a stable derivative, active at a pH closer to that of the skin and well tolerated. It is converted into ascorbic acid in the skin, so its effects are more gradual. It is also appreciated for blemish-prone skin.
• Magnesium Ascorbyl Phosphate (MAP) — similar to the previous one, very gentle, with good data on evening out skin tone.
• Ascorbyl Glucoside — stable and well tolerated, with slow release.
• 3-O-Ethyl Ascorbic Acid — a newer, stable derivative with good penetration.
• Tetrahexyldecyl Ascorbate (THD) — a fat-soluble, highly stable derivative that penetrates the lipid component well but is expensive.

The practical rule: pure ascorbic acid is the most potent choice if your skin tolerates it and the formula is well designed. Stabilized derivatives are the most sensible choice for reactive skin, for beginners, or for products designed to last a long time after opening.

The oxidation problem

Ascorbic acid degrades when exposed to air, light, and heat. The sign is visible: the serum changes from transparent or pale straw yellow to deep yellow, then amber or orange. At that point, the active ingredient has transformed into dehydroascorbic acid and has lost much of its effectiveness.

Here are some practical consequences:

1. Choose opaque and airless packaging, or dark glass with a dropper;
2. Store the product away from light, not on the bathroom windowsill;
3. Don't buy oversized formats: vitamin C should be used up, not stored;
4. An orange-colored serum should be replaced; it is no longer active.

What to pair it with

Vitamin C, vitamin E, and ferulic acid

This is the most extensively documented combination in antioxidant skincare. Ferulic acid stabilizes vitamin C by slowing its degradation, and together the three molecules cover both the skin’s aqueous and lipid phases.

Vitamin C and niacinamide

The idea that they cannot be used together is a widespread myth, originating from experiments conducted at high temperatures and with pure ascorbic acid. We discuss it in detail in the article on using vitamin C and niacinamide together.

Use with greater care

Using it in the same application as retinoids or exfoliating acids may cause irritation, especially at first: it is better to alternate them, using vitamin C in the morning and a retinoid or acid in the evening.

Documented effects on the skin

Evens out skin tone and reduces dark spots

By partially inhibiting tyrosinase, vitamin C helps reduce discoloration and prevent it from forming. It is an active ingredient that works better for prevention than correction: established dark spots require more time, and other active ingredients should be combined with it.

Supports collagen and elasticity

As a cofactor for the enzymes that stabilize collagen, vitamin C directly contributes to maintaining the structure of the dermis, with effects on firmness and elasticity.

Protects against photodamage

Vitamin C is not a sunscreen and does not replace SPF. It works downstream: it neutralizes free radicals that filters cannot block. That is why its natural place is in your morning routine, under sunscreen.

How to incorporate it into your routine

Vitamin C should be applied in the morning to clean, dry skin, before moisturizer and sunscreen. A few drops of serum are enough.

Those with reactive skin can start by using it every other day or choose a stabilized derivative instead of pure ascorbic acid. Results in terms of radiance appear within a few weeks; improvements in dark spots and firmness require months of consistent use.

Vitamin C in LeLang products

The Ferulic + HA antioxidant serum combines vitamin C with ferulic acid, which slows its degradation, and low- and medium-molecular-weight hyaluronic acid, which softens the acidic component and keeps the skin comfortable.

The Perlage cream contains vitamin C together with microspheres of vitamin E released gradually, harnessing the regenerative synergy between the two.

For targeted action on dark spots, Spot Defence combines antioxidant ingredients with actives specifically targeting pigmentation.

You can also find LeLang products at partner pharmacies throughout Italy.

Frequently asked questions

My vitamin C serum has turned yellow: can I still use it?

A light straw-yellow color can be normal. If the color has turned orange or amber, the active ingredient has oxidized and lost its effectiveness: it is time to replace it.

Is it better to use it in the morning or evening?

In the morning, because its antioxidant action complements that of sunscreen during hours of exposure.

Does vitamin C cause photosensitivity?

No, this is a common misconception. It does not increase sun sensitivity; it is the molecule itself that degrades in light, not the skin becoming more vulnerable.

Can I use it if I have sensitive skin?

Yes, by choosing a stabilized derivative such as sodium ascorbyl phosphate, which works at a pH closer to that of the skin and is much better tolerated than pure ascorbic acid.