Ascorbic Acid: the benefits of vitamin C for the skin

L-ascorbic acid, better known as vitamin C, is one of the most studied active ingredients in dermocosmetics. It is a powerful antioxidant, a cofactor essential 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 the same way. Understanding these differences is what sets apart a serum that works from one that turns yellow in the drawer.

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What is ascorbic acid?

L-ascorbic acid is a water-soluble vitamin that the human body cannot synthesize: it must be obtained through diet. However, even with regular intake, the amount that reaches the skin through the bloodstream is limited — which is why topical application makes sense.

It plays three distinct roles in the skin:

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

There’s a little-known but crucial detail: vitamin C is also the “regenerator” of vitamin E. When vitamin E neutralizes a free radical, it oxidizes 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 formulations: on their own, they don’t last as long.

Concentration and pH: the two numbers that matter

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

Concentration

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

pH

This is the most important technical point. LPure ascorbic acid penetrates the skin only at a pH below 3.5, because below that threshold it loses its electrical charge and can pass through 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. Stabilized derivatives were developed precisely to overcome this trade-off.

Forms of vitamin C: they are not interchangeable

When you see “vitamin C” on a package, it may refer to different molecules in the INCI, with different properties.

• Ascorbic Acid — the purest and most studied form. It has the strongest documented efficacy, but requires a low pH and is the least stable.
• Sodium Ascorbyl Phosphate (SAP) — a stable derivative that works at a pH closer to that of the skin and is well tolerated. It is converted into ascorbic acid in the skin, so its effects are more gradual. It is also popular for blemish-prone skin.
• Magnesium Ascorbyl Phosphate (MAP) — similar to the previous one, very gentle, with good evidence for improving skin tone evenness.
• Ascorbyl Glucoside — stable and well tolerated, with slow release.
• 3-O-Ethyl Ascorbic Acid — a newer, stable derivative with good penetration.
• Tetrahexyldecyl Ascorbate (THD) — an oil-soluble, highly stable derivative that penetrates the lipid component well, but is expensive.

The rule of thumb: 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.

If you need to choose a product, the guide on how to choose a vitamin C serum lays out the criteria to check on the label and packaging.

The oxidation problem

L'Ascorbic acid degrades when exposed to air, light, and heat. The change is visible: the serum goes from clear or pale yellow to deep yellow, then amber or orange. At that point, the active ingredient has turned into dehydroascorbic acid and lost much of its effectiveness.

Here are some practical implications:

1. Choose opaque, 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 bottles: vitamin C should be used up, not stored;
4. A serum that has turned orange should be replaced; it is no longer active.

What to combine it with

Vitamin C, vitamin E, and ferulic acid

It's the best-documented combination in antioxidant skincare. L'ferulic acid stabilizes vitamin C by slowing its degradation, and together the three molecules cover both the water-based and lipid phases of the skin.

Vitamin C and niacinamide

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

Use with more caution

L's combination in the same application with retinoids or alpha hydroxy acids can be irritating, especially at first: it's better to alternate them, using vitamin C in the morning and a retinoid or acid in the evening.

The documented effects on the skin

Evens skin tone and diminishes dark spots

By partially inhibiting tyrosinase, vitamin C helps diminish discoloration and prevent it from forming. It works better for prevention than correction: established dark spots take longer to treat, and other active ingredients should be used alongside it. Understanding the different types of hyperpigmentation helps clarify which ones skincare can address and which it cannot.

Supports collagen and elasticity

As a cofactor for enzymes that stabilize collagen, vitamin C plays a direct role in maintaining the structure of the dermis, supporting firmness and elasticity.

Protects against sun damage

Vitamin C is not a sunscreen and does not replace SPF. It works downstream, neutralizing free radicals that sunscreens cannot block. That's why it naturally belongs in your morning routine, under sun protection.

How to add it to your routine

Vitamin C is 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 acid. Results in radiance are visible 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 and ferulic acid, which slows its degradation, with low- and medium-molecular-weight hyaluronic acid, which softens the acidic effect and keeps skin comfortable.

The Perlage cream contains vitamin C alongside the microspheres of vitamin E for gradual release, harnessing the regenerative synergy between the two.

For targeted action on dark spots, Spot Defence pairs antioxidant ingredients with actives that specifically target pigmentation.

In the vitamin C ingredient guide, you'll find the full list of formulas that contain it. You can also find LeLang products at partner pharmacies across 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 it has turned orange or amber, the active ingredient has oxidized and lost its efficacy: it's time to replace it.

Is it better to use in the morning or evening?

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

Does vitamin C cause photosensitivity?

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

Can I use it if I have sensitive skin?

Yes, choose 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.

Is it still necessary if I eat lots of citrus fruits?

Yes. Vitamin C from your diet is distributed throughout the body, and only a limited amount reaches the skin: the skin is one of the last organs to receive it. LTopical application reaches skin concentrations that diet alone cannot achieve.

At what age should I start?

There is no set age. As an antioxidant, it works preventively: it makes sense to start when you begin intentional sun exposure, well before the first signs appear.