Ingredient · Antioxidant
Vitamin C Serum: What It Does and When to Use It
Also known as: L-ascorbic acid, Ascorbic acid, THD ascorbate
Vitamin C is the one active that earns its place specifically in the morning. It is an antioxidant, so it works alongside sunscreen rather than instead of it — sunscreen blocks UV, vitamin C mops up some of the damage from what gets through.
What it does
- Neutralises free radicals generated by UV exposure and pollution.
- Brightens overall tone and helps fade pigmentation over time.
- Supports collagen production, which is part of its anti-ageing reputation.
- Measurably improves the protection you get from sunscreen when layered underneath it.
Which form to choose
L-ascorbic acid is the pure, best-studied form. It is also the least stable — it oxidises on contact with light and air, losing potency as it goes. Effective concentrations run from 10% to 20%; above that you gain little and irritate more.
Derivatives such as sodium ascorbyl phosphate, magnesium ascorbyl phosphate, and tetrahexyldecyl ascorbate are more stable and gentler, but generally less potent. For sensitive skin that finds L-ascorbic acid stinging, they are a sensible trade.
How to tell it has gone off
A fresh L-ascorbic acid serum is clear or very pale straw. As it oxidises it turns yellow, then orange, then brown.
Deep yellow means it is past its best. Orange or brown means you are applying something that no longer does the job, and oxidised vitamin C can itself be mildly irritating.
Making it last
- Buy opaque or dark glass packaging, ideally with a pump rather than a dropper.
- Store it away from the bathroom window and out of direct light.
- Buy smaller bottles more often rather than a large one you use for a year.
How to use it
- Morning, on clean dry skin, before moisturiser and sunscreen.
- Start every other day if you have sensitive skin — the low pH of L-ascorbic acid can sting at first.
- Give it eight to twelve weeks before judging its effect on tone.
Pairs well with
- Sunscreen — the pairing that makes vitamin C worth using
- Niacinamide — the old conflict advice does not hold for modern formulas
- Hyaluronic acid — helps offset the dryness some formulas cause
Use with care alongside
- Retinol — many people are fine, but split morning and night if you react
- Strong exfoliating acids in the same step
Common questions
Morning or night?
Morning. Its main job is antioxidant defence against UV and pollution, which only matters during the day, and it improves the protection your sunscreen gives.
Can I use vitamin C with niacinamide?
Yes. The claim that they cancel out comes from old research on raw ingredients at high temperatures, not finished products. If the combination stings, use one in the morning and the other at night.
My serum has turned orange — is it still usable?
It has oxidised and lost much of its potency. It is not dangerous, but you are no longer getting the benefit, and oxidised vitamin C can be mildly irritating. Replace it.
Not sure if vitamin c is right for your skin?
A scan grades what your skin actually needs and builds the routine around it — including which actives to use together and which to keep apart.