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Vitamin C

aka Vitamin C, L-Ascorbic Acid (LAA, INCI: Ascorbic Acid), Sodium Ascorbyl Phosphate (SAP), Magnesium Ascorbyl Phosphate (MAP), Ascorbyl Glucoside, 3-O-Ethyl Ascorbic Acid (Ethyl Ascorbic Acid), Tetrahexyldecyl Ascorbate (THD ascorbate), Ascorbyl Tetraisopalmitate, Ascorbyl Palmitate

The short answer

Vitamin C is an antioxidant that small clinical trials link to brighter, more even-looking skin and softer-looking sun-related fine lines, with L-ascorbic acid having the most evidence. It is unstable, so packaging, pH and how fresh the product is matter as much as the percentage on the label.

Known for
Antioxidant serum staple for brighter, more even-looking skin; picky about pH and air.
Helps with
Dullness and uneven toneSun-related fine lines and roughnessAntioxidant backup under sunscreen
Evidence
moderate

Updated 5 Oct 2026 · Sources

Check first

Caution: Can sting (low pH)Caution: Oxidises: check the colourCaution: May stain skin or fabric yellowCaution: Never replaces sunscreen
  • Your skin is broken, sunburnt or very reactive; start with a gentler derivative such as sodium ascorbyl phosphate or tetrahexyldecyl ascorbate, or a lower strength.
  • You get hives or redness that doesn't settle; stop using it. Hives are rare but have been reported.
  • Your serum has turned orange or brown or smells metallic; it has oxidised and is past its best.

What it goes with

Pairs well

  • Vitamin E and ferulic acidIn pig-skin lab tests, adding 1% vitamin E and 0.5% ferulic acid to 15% L-ascorbic acid roughly doubled its protection against UV redness. That research was linked to SkinCeuticals through a consultant and a patent holder.
  • SPF50+ sunscreenVitamin C doesn't absorb UV, so it adds antioxidant support under sunscreen but never replaces it.
  • NiacinamideFine to use together; the old 'don't mix' rule traces to a 1940s colour-change test, not lost activity.
  • Tranexamic acidJapanese brightening formulas commonly combine vitamin C derivatives with tranexamic acid.
  • Hyaluronic acidA hydrating layer after a low-pH serum can make it feel more comfortable; no clash is known.
  • RetinolVitamin C in the morning and retinol at night lets you use both without stacking irritation.

Keep apart

  • Avoid: NiacinamideCosmetic chemists call this a myth. The original test saw a colour change when raw powders were mixed, not a loss of activity.
  • Avoid: RetinolNot a chemical clash, but combining them is more likely to irritate than either alone. Vitamin C in the morning, retinol at night.
  • Avoid: AHA (alpha hydroxy acids)Low-pH vitamin C plus exfoliating acids stacks irritation. Use them at different times of day or on different days.
  • Avoid: PeptidesCopper can break down L-ascorbic acid in chemistry terms. No skin study has tested the combination, but using them at different times is an easy precaution.
  • Avoid: Benzoyl peroxideA cosmetic chemist notes it can inactivate L-ascorbic acid. Use them at different times of day.

How it works

Vitamin C is an antioxidant: it donates electrons to neutralise the free radicals that UV generates in skin [3]. Lab research also shows it acts as a helper for the enzymes skin uses to form stable collagen, and that it interferes with an enzyme involved in pigment production [3]. Those mechanisms come mostly from lab work, so the page describes what human studies measured rather than promising the same in your skin.

In people, a 2023 systematic review of seven small randomised trials (139 volunteers) found treated skin looked smoother and less wrinkled, and objective measurements showed significant lightening of pigmentation, while hydration improved equally on vitamin C and placebo [2]. The reviewers note that 'long-term use may be needed to achieve noticeable changes'. A 2026 review of 44 studies calls L-ascorbic acid the most clinically validated form, with low-pH serums of 10% to 15% stabilised with vitamin E and ferulic acid showing the best stability, but many of its studies were small, open-label or industry-sponsored, and most pigment results involved microneedling or lasers as well [1].

There is an honest counterpoint. DermNet warns that many retail products contain very low concentrations that are not well absorbed, and that many claims lack rigorous testing [4]. Both views can be true: the ingredient can work, but a lot depends on the formula.

That formula is fussy. L-ascorbic acid stays stable below about pH 3.5, and in pig-skin studies benefit rose with concentration only up to about 20%, with a skin reservoir that lasted days [3][6]. It breaks down with water, light, air and higher pH, turning yellow, then orange, then brown [7].

Derivatives trade some evidence for stability. Sodium and magnesium ascorbyl phosphate are converted to ascorbic acid by skin enzymes [6]; ethyl ascorbic acid is more heat-stable but has little testing on human skin [6][9]; tetrahexyldecyl ascorbate is oil-soluble and works near skin's own pH [10]. They tend to be gentler, with less human evidence than L-ascorbic acid [1].

How to use it

  • Apply after cleansing and before hydrating serums and moisturiser, usually in the morning, then SPF50+ [7][8].
  • L-ascorbic acid serums used in studies are 10% to 20% at a low pH; if you're new, start under 10% [1][7].
  • Morning or night both work, because skin holds a reservoir for more than a day in lab tests [7].
  • Give it at least two to three months; reviewers say long-term use may be needed [2].
  • Keep it capped, out of the light and somewhere cool; in summer, the fridge is a good spot [7].
  • Replace it if it turns orange or brown or smells metallic; many brands suggest using it within about six months of opening [7].

FAQ

Yes. The warning comes from a 1940s lab test that saw a colour change, not a loss of activity, and cosmetic chemists call it a myth [7][11]. If the combination stings, use them at different times of day.

Products with vitamin c

Each links to Australian stockists, with the price range across shops.

Sources

General information only, not medical advice.