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Vitamin C (ascorbic acid)

Vitamin C (ascorbic acid) in skincare: Antioxidant, collagen cofactor, tyrosinase inhibitor. What it does on skin, how strong the evidence behind the claim actually is, the concentrations in normal use, who reacts to it, and where it stands in pregnancy and sunlight. On a label it appears as Ascorbic Acid.

Pigment and brighteningAntioxidant, collagen cofactor, tyrosinase inhibitorEvidence: Moderate

On the label: Ascorbic Acid. Also called: L-ascorbic acid; derivatives include sodium ascorbyl phosphate, magnesium ascorbyl phosphate, ascorbyl glucoside, tetrahexyldecyl ascorbate.

This is an ingredient reference, not a diagnosis and not a treatment plan. It describes what substances are and what the published evidence supports; it does not tell you what your skin has or what to put on it. A rash that spreads, hurts, weeps, does not settle, or comes with feeling unwell is a reason to see a doctor or pharmacist, not to change serum.

What it is

The same vitamin C you eat, in the unstable acid form. It is an essential cofactor for the enzymes that stabilise collagen — the reason scurvy shows up in skin and wound healing — and a potent antioxidant. Its weakness is chemical: dissolved in water at the low pH it needs to penetrate, it oxidises steadily on contact with air, light and metal ions.

What it does on skin

Three jobs at once. It quenches the free radicals ultraviolet light generates, so it is used under sunscreen as a second line of defence. It supports collagen synthesis. And it slows the enzyme that starts melanin production, which is the basis for its use on dull tone and brown marks. None of those is dramatic alone; the case for it rests on doing all three at a low cost in irritation.

What the evidence is worth

Evidence: Moderate

The biochemistry is not in question. The clinical trials are small, short and often industry-run, so the honest grade is moderate: worth using, not worth expecting transformation from. The much-repeated formulation rules — 10-20% at low pH, with vitamin E and ferulic acid — come from a modest body of penetration work, and the derivatives that avoid the low pH have less evidence behind them, not more.

How it is used

Morning, on clean skin, before sunscreen. Pure ascorbic acid needs an opaque, airless or dark glass package; a clear dropper bottle is a design decision that costs you product. Once it turns yellow and then brown it has oxidised, and a deeply browned serum should be replaced rather than used up.

Typical concentration in normal use: 10-20% for pure ascorbic acid. That is a description of the market, not a recommendation, and it is not a dose.

Who reacts to it

Stinging at low pH is common on sensitive or compromised skin, and a small number of people find it triggers breakouts in a rich base. Oxidised product can transiently stain skin and light fabric orange. If pure ascorbic acid is intolerable, the buffered derivatives are gentler at the cost of a less certain effect.

Pregnancy and breastfeeding

Generally considered fine

Topical vitamin C is not a pregnancy concern and it is one of the actives usually kept when retinoids are stopped. Skin can be more reactive in pregnancy, so a lower-pH formula that suited you before may sting now.

Sunlight

No change

It is not photosensitising and it is not a sunscreen. Used in the morning it adds antioxidant protection under a sunscreen; used instead of a sunscreen it does close to nothing against ultraviolet damage.

Rules and labelling

Not restricted as a cosmetic ingredient. The interesting label question is which form you are buying: "vitamin C" on the front can mean ascorbic acid or any of half a dozen derivatives with different stability, different effective concentrations and different evidence, and only the INCI list tells you which.

What it goes with, and what it does not

The classic pairing is with vitamin E and ferulic acid for stability. It is a reasonable partner for sunscreen and for niacinamide. Using it in the same minutes as an exfoliating acid mostly adds stinging.

One thing people get wrong

The myth is that eating more oranges does the same job. Dietary vitamin C is essential and deficiency shows in skin, but plasma levels saturate: past a certain intake the extra is excreted, and skin concentrations do not keep climbing. The second myth is that niacinamide cancels vitamin C, which comes from old chemistry on heated solutions and is not a practical problem in modern formulations.

The nutrition side

Skin is fed from the inside as well as treated from the outside, and the two are far less interchangeable than either industry suggests. The relevant page on this site: Foods highest in vitamin C.

Sources: EU Cosmetics Regulation (EC) No 1223/2009 and its annexes, as published in the European Commission's CosIng database, for every statement about what is permitted, restricted or banned · opinions of the Scientific Committee on Consumer Safety (SCCS) and safety assessments of the Cosmetic Ingredient Review (CIR) for the concentration positions · standard dermatology reference material for mechanism, tolerability and contact-allergy patterns · checked 2026-09-06. Deep links are deliberately omitted where a stable public URL could not be confirmed.

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