Is 20% Vitamin C Too Strong? Who It Suits and How to Start

Bysachè Journal · 8 min read · 1 October 2026

Twenty per cent sounds bold. It is the number on the bottle that makes people pause: is this a glow-in-a-dropper, or a recipe for stinging, red skin?

If you’re wondering whether 20% vitamin C is too strong, the short answer is: not necessarily, but it sits at the very top of the useful range. A 2017 review notes that vitamin C generally needs to be above 8% to make a biological difference, that going above 20% doesn’t increase its effect and may cause some irritation, and that reputable serums sit between 10 and 20% (Al-Niaimi and Chiang, 2017).

Some people get on well with 20% vitamin C once their skin adjusts. Some skin won’t. To find out which group you’re in, patch test first, then build up slowly over your first month.

What “20% vitamin C” actually means

The percentage tells you how much vitamin C is in the formula. It doesn’t tell you how much reaches your skin, how long the formula stays fresh, or how your skin will respond.

The familiar 10 to 20% range draws on absorption studies by Pinnell and colleagues, published in 2001, which reviews still cite (Al-Niaimi and Chiang; Ravetti et al., 2019). So 20% isn’t a stronger tier beyond the normal range. It is the upper edge of it.

Bysachè’s Vitamin C 20% Brightening Serum sits at that edge. Its vitamin C is ascorbic acid, the L-ascorbic acid form, listed second after water, alongside Kakadu plum and green tea extracts.

10% vs 20% vitamin C: why strength isn’t the whole story

Both 10% and 20% sit inside the range reviews describe for reputable serums (Al-Niaimi and Chiang). And two serums can both say 20% and feel quite different on your skin. Four things explain most of the gap.

pH

L-ascorbic acid needs an acidic formula to get into the skin at all (Pullar et al., 2017). A pH below 3.5 is described as an effective way to improve both its stability and its penetration (Al-Niaimi and Chiang).

Form

Percentages only compare like with like. Vitamin C derivatives behave differently from L-ascorbic acid: phosphate forms are more stable but poorly absorbed through the skin, and while ascorbyl glucoside is stable and can penetrate, how quickly skin converts it to vitamin C isn’t known (Pullar et al.). A 20% derivative and 20% L-ascorbic acid are not the same thing. For the wider picture, see our complete guide to vitamin C serum.

Stability

Ascorbic acid is a very unstable vitamin that oxidises easily in water-based formulas. Heat, a higher pH, dissolved oxygen and traces of metal all speed up its breakdown, and early vitamin C products often turned yellow as it oxidised on contact with air (Ravetti et al.). It is also very prone to oxidising in light (Boo, 2022).

Packaging and storage

Reviewers list oxygen-impermeable packaging among the ways to protect it (Ravetti et al.). At home, the same logic applies: close the bottle after each use, keep it somewhere cool and out of direct light, and notice if the colour deepens over time.

The same goes when you travel. A capped bottle tucked inside a bag fares better than one left on a sunny windowsill or car dashboard. Our cotton Tote it up is roomy enough for a laptop and your daily essentials.

Who 20% vitamin C suits, and who should go slower

It is likely to suit you if

• Your skin is generally comfortable with active ingredients, or you have used a lower-strength vitamin C without trouble.
• You want brighter-looking skin, a more even-looking tone and everyday antioxidant support. Vitamin C is the most abundant antioxidant in human skin (Al-Niaimi and Chiang).
• You wear sunscreen every day. The American Academy of Dermatology (AAD) suggests applying vitamin C in the morning, after washing your face and before sunscreen (AAD).

Go slower if

• Your skin is sensitive and tends to sting, redden, burn or itch with new products.
• Your skin is currently dry, irritated or over-exfoliated.
• You already use retinoids or exfoliating acids. The AAD advises starting with one product, since using several at once can irritate your skin (AAD).
• Fragrance or essential oils have bothered you before. Fragrance is one of five common classes of cosmetic allergens named by the US Food and Drug Administration, and a label can list it simply as fragrance or perfume without naming each component (FDA). Our serum contains fragrance (listed as parfum), orange peel oil and lemon myrtle leaf oil.

We can’t promise it will suit sensitive skin. A patch test is the honest way to find out.

Product spotlight · Evens Skin Tone & Brightens

Vitamin C 20% Brightening Serum

L-ascorbic acid at 20%, the top of the range reviews describe, with Kakadu plum and green tea extracts and vitamin E, in a pump bottle. Use 1–2 pumps on clean skin and follow with sunscreen in the day. Contains parfum, orange peel oil and lemon myrtle leaf oil.

Shop the Vitamin C 20% Serum

How to start vitamin C serum: a simple four-week plan

Begin with a patch test. The AAD suggests applying a new product to a small spot, such as the underside of your arm or the bend of your elbow, twice a day for seven to 10 days. If there’s no redness, itching or swelling, you can go ahead (AAD).

Then build up gradually. Our serum’s directions suggest 1–2 pumps on clean skin, morning or evening; at night, apply it at least 30 minutes before bed, and follow with sunscreen during the day (product page).

• Finish with sunscreen. The AAD recommends broad-spectrum SPF 30 or higher (AAD).
• Change one thing at a time. Hold off on other new actives for these four weeks.
• Give it time. Most products take at least six weeks to work, and sometimes up to three months (AAD).

Vitamin C serum irritation: signs it’s too much, and what to do

In a small three-month trial with 19 participants, a vitamin C serum was applied daily and compared with a plain base, and the researchers checked for stinging, redness, dryness and other reactions (Traikovich, 1999).

Follow the AAD’s guidance: stop using a product that stings, burns or tingles, because those sensations mean it is irritating your skin (AAD).

Watch for:

• Stinging, burning or tingling after you apply it
• Redness that lingers
• Dry, tight or flaky patches
• An itchy, red rash, which is how allergic reactions to cosmetics most often appear (FDA), or any swelling

If you notice any of these:

1. Stop using the serum, and keep your routine to a gentle cleanser, moisturiser and sunscreen until your skin feels calm.
2. If you have a rash or swelling, gently wash the product off, don’t use it again, and apply a cool compress or petroleum jelly if needed. If it doesn’t settle, see a dermatologist (AAD).
3. Remember that vitamin C isn’t all or nothing. Reputable serums range from 10 to 20% (Al-Niaimi and Chiang), so a lower strength is worth considering.