C10 or C20: Which Vitamin C Strength Do You Actually Need?

The instinct is simple: if 10% vitamin C is good, 20% must be twice as good. With vitamin C, that's not how it works — and reaching for the higher number when your skin doesn't need it can leave you worse off, not better.

Both strengths do the same job. The difference is who they're for. Here's how to choose the one that's actually right for your skin.

First, what vitamin C is doing for your skin

Vitamin C is one of the most useful steps you can add to a routine. It's a potent antioxidant — it helps neutralise the free radicals generated by UV light and pollution, the everyday damage that even good sunscreen doesn't fully block. That makes it the natural daytime partner to your SPF: protection on top, antioxidant support underneath.

It also supports collagen, helps brighten a dull complexion, and improves the look of uneven tone and the dark spots the sun tends to leave behind. In short, it's a "treat" step — the work you do on top of daily protection.

(Worth saying plainly: vitamin C complements sunscreen, it doesn't replace it. Protect first, then treat.)

Why a higher percentage isn't automatically better

This is the part most people get wrong.

The active form of vitamin C, L-ascorbic acid, only absorbs into skin up to a point — research puts that ceiling at around 20%. Past it, you don't get more penetration or more benefit; you just get more of the downsides. Its effectiveness plateaus in roughly the 15–20% range, which means 20% isn't "double" 10% — the curve flattens out as you climb.

What does change as you go higher is the risk. Stronger formulas are more likely to cause redness or stinging on reactive skin, they tend to feel heavier, and they oxidise faster once opened.

So the honest rule is this: the best strength isn't the biggest number — it's the highest one your skin tolerates and that you'll actually use every day. A 10% you reach for every morning will do far more for your skin than a 20% that stings, pills under your moisturiser, and ends up at the back of the shelf.

C10 — who it's for

Our 10% is the smart starting point, and for many people it's the long-term option too. It's the right choice if you:

— are new to vitamin C, or building up tolerance

— have sensitive, reactive or dry skin

— have a deeper skin tone or are prone to pigmentation — gentler is genuinely better here, as pushing the strength too high can aggravate the very marks you're trying to fade

— want a lightweight serum that sits comfortably under everything else, every day

Don't read "10%" as "starter version." At this strength you still get real antioxidant protection and visible brightening over time — just with far less chance of irritation.

C20 — who it's for

Our 20% is the step-up: the most potent option within the range that actually works. Reach for it if you:

— already use vitamin C comfortably and your skin handles 10% with no fuss

— have resilient, non-sensitive skin

— are targeting more stubborn uneven tone, dark spots, or the early look of fine lines, and want the strongest effective dose

If you're moving up to 20%, introduce it gradually — patch test first, start every other day, and build up as your skin settles.

Still not sure? Here's the short version

Start with C10 if you're new to vitamin C, your skin runs sensitive, dry or reactive, you have a deeper or pigmentation-prone complexion, or you just want a fuss-free daily serum.

Choose C20 if you already use vitamin C happily, your skin is resilient, and you want maximum potency for stubborn tone or early fine lines.

Genuinely torn? Start at C10. You can always step up later — but you can't un-irritate skin you've pushed too hard, too fast.

How to use it (whichever you choose)

Morning, on clean skin, before your moisturiser and SPF.

Introduce slowly — every other day to begin with, especially at 20% — then daily once your skin is happy.

Keep it away from light and air. If your serum turns deep yellow or brown, it has oxidised and lost its potency.

Give it time. Vitamin C works gradually; the visible payoff in brightness and tone builds over weeks and months, not days.

Find your strength.