Vitamin C (L-ascorbic acid and derivatives)

What it is

Vitamin C, or L-ascorbic acid, is a water-soluble antioxidant that skin cannot make. Applied topically, it neutralises the free radicals from UV light, helps the enzymes that build collagen, and reduces the enzyme reactions that produce melanin¹. Pure L-ascorbic acid is unstable and only enters skin at a pH below about 3.5². Derivatives such as ascorbyl glucoside, magnesium ascorbyl phosphate (MAP) and tetrahexyldecyl ascorbate are more stable and less acidic, but they must convert to ascorbic acid in the skin, and in the one absorption study of MAP and ascorbyl palmitate, neither raised skin vitamin C levels^{1, 2}.

What the evidence says

Evidence: moderate for photoprotection alongside sunscreen and for collagen; limited for dark spots, where it has rarely been tested on its own, trials used 5 to 10% and results rest on patients' own ratings. Laboratory and small human studies used 3 to 20% L-ascorbic acid; results are best alongside sunscreen and vitamin E.

Photoprotection: a 15% L-ascorbic acid plus 1% vitamin E solution applied for four days gave about fourfold protection against UV redness, sunburn cells and DNA damage in pig skin³. That is a helper effect, not an SPF. Ageing and collagen: a 12-week placebo-controlled study of 10% vitamin C improved photoageing scores, and a four-month study of 3% increased the density of collagen-rich papillae¹. Dark spots: in a 16-week split-face trial of 16 women with melasma, 5% ascorbic acid gave good results in 62% against 93% for 4% hydroquinone by the women's own ratings, with no difference on colour measurement and side effects in one woman instead of eleven⁴. Absorption stops rising above about 20%², and reviews put the useful range at 10 to 20%¹. See dark spots and hyperpigmentation and sun protection for every skin tone.

How people use it

• For dark marks, a 5 to 10% serum in the morning on clean skin, then moisturiser and sunscreen; 10 to 20% L-ascorbic acid, the range used in photoprotection studies, stings more, so build up only if your skin stays calm.
• Formulas that add vitamin E and ferulic acid are more stable and protect better than vitamin C alone^{1, 5}.
• Sensitive skin: start with a derivative such as ascorbyl glucoside or MAP, and build up.
• Store it capped, cool and dark.

Cautions

• Stinging, redness and dryness happen mostly with strong, acidic L-ascorbic acid; strengths above 20% add irritation without benefit^{1, 5}. Patch test first.
• A fresh serum is clear or pale straw-coloured; once it turns dark yellow or brown it has oxidised, no longer works properly and can stain skin and clothing, so replace it⁵.
• It supports sunscreen; it does not replace it³.
• Pigmentation that does not respond, or any changing mark, deserves assessment; see when to see a dermatologist.