Vitamin C

Vitamin C in skincare almost always means L-Ascorbic Acid, the pure, biologically active form. It is the version with the longest research history behind it, and it is also the most awkward to formulate: it is water-soluble, oxidises readily on contact with light, air and warmth, and needs a low pH to be absorbed properly.

That instability is why the category is full of derivatives. Sodium Ascorbyl Phosphate, Magnesium Ascorbyl Phosphate, Ascorbyl Glucoside, 3-O-Ethyl Ascorbic Acid and Tetrahexyldecyl Ascorbate are all attempts to make vitamin C more stable and more comfortable on skin. They are gentler and keep better, but each has to be converted to ascorbic acid once absorbed, and they are generally considered less potent than a well-made L-ascorbic acid product.

Packaging tells you a great deal. Vitamin C belongs in opaque or dark glass, airless pumps or single-use ampoules, never a clear jar. A serum that has turned deep yellow, orange or brown has oxidised, and while it is not harmful, it is no longer doing much. Some products are supplied as a powder to be mixed fresh for exactly this reason.

Typical strengths run from around 5% to 20% for L-ascorbic acid. Products commonly pair it with vitamin E and ferulic acid, a combination used because those ingredients help stabilise it and complement its antioxidant behaviour.

Reading a vitamin C label takes a little practice. A percentage on the front only means something if you also know which form is being used, since 10% L-ascorbic acid and 10% of a derivative are not comparable. Check the form named in the ingredients list, look at whether the packaging protects it from light and air, and note whether the product is water-based, which suits ascorbic acid, or anhydrous, which is how the oil-soluble derivatives are usually kept stable. Those three details tell you more than the headline figure.

Vitamin C is an antioxidant, and that is its primary job. Daylight, pollution and ordinary metabolism all generate free radicals in skin, unstable molecules that damage cell structures and contribute over time to the look of ageing and uneven tone. Ascorbic acid donates electrons to neutralise them before that damage accumulates. This is why it is a morning ingredient: it works alongside sunscreen, mopping up what gets through rather than filtering light itself.

It has a second, quite separate role in collagen production. Ascorbic acid is a required cofactor for the enzymes that stabilise newly made collagen, meaning the skin cannot properly build and maintain that supporting structure without it. This is a genuine biochemical requirement rather than a marketing claim, and it underlies the association between vitamin C and firmer, smoother-looking skin over the long term.

Its effect on the look of pigmentation comes from a third route: it interferes with tyrosinase, the enzyme that governs melanin production. That slows the formation of new pigment, so existing marks look gradually less defined while fresh ones are less likely to form. All three effects depend on the vitamin C actually reaching the skin in an unoxidised state, which is why formulation, pH and packaging matter as much as the percentage on the front of the bottle.

Vitamin C is a morning ingredient, and it is more sensitive to how it is stored than almost anything else in a routine.

• When: in the morning, on clean, dry skin, before moisturiser and sunscreen. Using it at night is not harmful, but it wastes the antioxidant benefit.
• Order in the routine: cleanse, vitamin C serum, wait a minute, then moisturiser, then SPF.
• Strength: 10–15% suits most people. 20% is not reliably better and is more likely to sting. If you are sensitive, start at 5–10% or choose a derivative.
• Sunscreen is the other half. Vitamin C supports sun protection; it does not replace it, and the tone benefits disappear without it.
• Pairs well with: vitamin E and ferulic acid, which are commonly included for stability, and with niacinamide, hyaluronic acid and peptides.
• Handle with care alongside: exfoliating acids and benzoyl peroxide, which can make the combination uncomfortable and, in the case of benzoyl peroxide, degrade the vitamin C. Separate them by time of day.
• Retinoids: vitamin C in the morning, retinoid at night is the straightforward arrangement.
• Watch the colour. Pale straw is fine. Deep orange or brown means it has oxidised and should be replaced.
• Store it away from light and heat. A bathroom windowsill is the worst place for it; a cool, dark cupboard is better.

Expect brightness and general radiance within weeks, but give dark marks two to three months.