A lot of people assume a stinging serum is a working serum. It's an easy assumption to make, and it's mostly wrong.
What's actually happening
Ascorbic acid is, as the name suggests, an acid. Most serums sit at a low pH. That's deliberate, because the acid form is the one that penetrates. Low pH on skin can produce a brief tingle, particularly if your barrier is already a bit compromised.
A few seconds of mild tingling that fades: normal enough. Burning that persists for minutes, or redness still there twenty minutes later: not normal, and not a sign of efficacy.
The three usual causes
1. Your barrier is already struggling. This is the most common one and the least discussed. If you've been over-cleansing, using a foaming wash twice a day, exfoliating regularly, or you've had a run of hot showers, your skin's barrier is thinner than it should be. Everything stings on a compromised barrier. The serum is the messenger, not the problem.
2. The concentration is higher than you need. Twenty per cent is not twice as good as ten. Above a certain point you get diminishing returns and a much higher chance of irritation, especially if you're new to it. Starting lower and staying consistent beats starting high and abandoning it.
3. You're stacking acids. Vitamin C alongside a glycolic toner, an exfoliating pad and a retinol is a lot for one face. Each one on its own might be fine.
What to do
Take a week off actives entirely. Cleanser, moisturiser, sunscreen. Nothing else. If the stinging was a barrier problem, you'll know within about five days, because everything else will feel calmer too.
Then reintroduce the serum every third day rather than daily, on dry skin rather than damp. Damp skin absorbs more, which sounds good and isn't when you're trying to go gently. Build up from there.
If it still burns after that, it's probably not the product for you, and there's no prize for persisting.
Damp skin, and why it matters
Half the internet says apply serum to damp skin for better absorption. That's true, and it's also why the same serum stings more after a shower than it does at your desk. If you're sensitive, dry skin is the gentler option and the difference in outcome is small.
The bit worth saying plainly
If a product hurts, you will stop using it. Consistency does more for how skin looks than intensity ever will, so a gentler serum you actually apply four times a week beats a strong one that's been in the drawer since March.
General information, not personal advice. Persistent irritation, burning or a rash is worth a conversation with your GP or a dermatologist rather than a skincare brand.
