Vitamin C serums: strength, form, and which to choose

Updated August 2026

The mistake most people make

A patient brought me her vitamin C last week. It was the colour of weak tea. She had been using it for three months thinking it was working.

By the time most vitamin C reaches you, it is already oxidised. That brown tint is not a colour, it is rust. L-ascorbic acid is wildly unstable. Preserving it properly is expensive: the right pH, dark glass, vitamin E to protect it. That is why a real vitamin C is never cheap.

The question is not whether to use it. It is which one is right for your skin.

That comes down to two decisions almost nobody is helped to make. Which form, and at what strength. Get them wrong and you own an expensive bottle of rust, which is roughly where the market wants you.

The three forms that matter

L-ascorbic acid is pure vitamin C and the most studied form by a wide margin. Two drawbacks: it needs a low pH to be absorbed at all, which is why high-percentage serums sting reactive skin, and it is unstable. Pinnell and colleagues in Dermatologic Surgery (2001) found a pH below roughly 3.5 is needed for meaningful penetration, and that going beyond 20% does not increase absorption. That explains the sting, and tells you a 30% serum is a number, not a benefit.

Tetrahexyldecyl ascorbate (THD) is an oil-soluble derivative that solves both at a cost. It needs no low pH, so it does not sting, and it is far more stable. The cost is speed: it is converted in the skin, so it works gradually, and the evidence base is smaller. For reactive skin, and anyone whose last three serums went orange, that trade is worth making.

Ascorbyl glucoside and the other derivatives sit in the middle: more stable, gentler, milder. Fine for very sensitive skin, but also where cheap products hide, at a low percentage with vitamin C on the front.

Strength: why 20% is not four times better than 5%

The percentage on the bottle is the most misread number in skincare, and the industry knows it.

Absorption does not scale indefinitely. It rises, then plateaus, and past roughly 20% you gain nothing, while irritation keeps climbing because higher percentage means lower pH.

For most people 10 to 15% is the useful range, and that is where I put almost everyone. A reactive skin at 20% will stop using it, which delivers nothing.

pH, packaging and why your serum turned orange

Three things kill vitamin C: light, air and heat. Which makes the bottle part of the product. I look for opaque or amber glass, an airless pump or ampoules, and L-ascorbic acid paired with vitamin E and ferulic acid.

Use the colour as your expiry date. Clear or pale straw is fine. Amber or brown means it is finished, whatever the box says. It does your skin no harm. It also does nothing else.

What to pair it with, and what to keep apart

Retinol: fine, but separated. Vitamin C morning, retinoid night, because two irritating actives in one sitting is how barriers get damaged (retinol vs retinal). Niacinamide: the internet's favourite incompatibility, largely a myth from lab conditions at high heat (the longer version). SPF: the pairing that matters, because vitamin C for tone without daily sunscreen is work undone. The art of layering and you've layered everything and it still falls flat cover sequencing.

Choosing by skin type

Sensitive or reactive: THD or a mild derivative, low percentage, two or three mornings a week to begin. Not 20% L-ascorbic acid, whatever the reviews say (more here). Pigmentation: 15 to 20% if tolerated, daily, for a full season, with rigorous SPF (before you buy another pigment product, why brightening creams keep failing you). Loss of firmness: 15%, or THD alongside a retinoid. Oily: a lighter aqueous serum.

The four we stock, compared

Also: Sente Defense C at £130 for barrier-focused skin, and iS Clinical C Eye Serum Advance at £68 for the orbital area.

On cost: a bottle lasts two to three months used daily, so £64 is roughly £280 a year and £206 closer to £900. Both are defensible. Only one is if you will not use it every morning. Browse Serums, or by brand: Obagi, Revision, Dermaceutic, iS Clinical.

How to introduce it without irritation

What I tell patients: three mornings a week for a fortnight, on dry skin, before moisturiser. Daily if comfortable. Sunscreen every time.

If it stings beyond a minute, the form or strength is wrong. Not the ingredient. Drop the percentage or move to THD rather than abandoning vitamin C.

Give it eight to twelve weeks. Antioxidant defence is invisible by design, and tone changes are gradual enough that a photograph in consistent light shows what a mirror cannot.

A vitamin C serum is not a transformation. It is a small daily deposit against the thing that will otherwise age your skin faster than anything else. Buy the one you will finish, keep it out of the light, and put sunscreen over it.

The four are here.