Ascorbic Acid

What it is

Ascorbic acid is the biologically active form of vitamin C, a water-soluble vitamin humans cannot make themselves. Skin needs it as an essential cofactor for the enzymes that cross-link collagen, and it also mops up free radicals generated by UV light and pollution. It is the most researched form of vitamin C in skincare, but also the least stable: it oxidises on contact with air, light and water, which is why formulations are acidic, airtight and often tinted.

What it does well

• Reduces the appearance of pigmentation and dullness

  A systematic review of trials in melasma and photoaging found topical vitamin C produced meaningful improvements in pigmentation scores, though usually less than hydroquinone and often as part of a combination.

  Moderate evidence

• Adds antioxidant protection on top of sunscreen

  Ascorbic acid neutralises UV-generated free radicals that sunscreen filters do not stop. It is a complement to sun protection, not a substitute for it.

  Moderate evidence

• Supports collagen synthesis in photoaged skin

  A double-blind placebo-controlled trial on photoaged skin found clinical and ultrastructural improvement, including changes consistent with new elastic tissue formation, after several months of use.

  Moderate evidence

Worth knowing

• Can sting or irritate at the low pH it needs to work

  Ascorbic acid must sit below roughly pH 3.5 to penetrate skin effectively, and that acidity causes stinging, redness or flaking in some people, especially at 15-20%.

  Matters most toPeople with sensitive skin, rosacea, or a compromised barrier, and anyone also using retinoids or exfoliating acids.

• Oxidises quickly once opened

  A product turning yellow, orange or brown has partly oxidised. Oxidised vitamin C is not harmful but is less effective, so airless or single-dose packaging matters more here than for most ingredients.

  Matters most toAnyone buying vitamin C in clear jars or droppers, or keeping a product open for many months.

Where the EU stands

Sources (3)

Every link goes to the original: a journal, a PubMed record or an EU document.

1. [1]
   Topical ascorbic acid on photoaged skin. Clinical, topographical and ultrastructural evaluation: double-blind study vs. placebo

   Experimental Dermatology · 2003 · doi:10.1034/j.1600-0625.2003.00008.x

   Six months of topical ascorbic acid produced statistically significant clinical improvement in photoaged skin versus placebo, with ultrastructural changes consistent with elastic tissue repair.

2. [2]
   Efficacy of topical vitamin C in melasma and photoaging: A systematic review

   Journal of Cosmetic Dermatology · 2023 · doi:10.1111/jocd.15748

   Topical vitamin C improved melasma and photoaging outcomes across the trials reviewed, with a favourable side-effect profile, but the authors noted small sample sizes and heterogeneous formulations.

3. [3]
   Stability, transdermal penetration, and cutaneous effects of ascorbic acid and its derivatives

   Journal of Cosmetic Dermatology · 2012 · doi:10.1111/jocd.12006

   Reviews the evidence that ascorbic acid needs a low pH and protection from air and light to penetrate and remain active, which is the core formulation challenge for this ingredient.