June 24, 2026 5 min read
How to introduce an acid without wrecking your skin
Which acid to start with, how often to use it, what never to combine it with, and the signs that you should stop.
By Tobias Renwick, Head of Formulation
Exfoliating acids are the products people most often overdo. The label says a few nights a week, the first use goes well, and within a month it has become every night with a second acid on weekends. Then the stinging starts. This is how to avoid that.
What an acid does
Alpha hydroxy acids, the AHAs, loosen the bonds that hold dead cells to the surface of the skin, so those cells shed more evenly. Done at a sensible pace, the surface feels smoother and looks more even. Done too often, you remove surface faster than skin can replace it, and you are back to repairing your barrier.
Three numbers that matter
- The acid. Molecule size decides how quickly it moves into skin. Glycolic is the smallest and fastest. Lactic is in the middle. Mandelic is the largest and slowest, which is why it stings least.
- The percentage. More is stronger, but only alongside the third number.
- The pH. An acid only exfoliates in its free form, and how much of it is free depends on the pH of the formula. A 10% acid at pH 3.6 and a 10% acid at pH 4.5 are very different products.
This is why our cartons print all three, and why the 10% Mandelic Resurface also states its free acid value, 7.4%. If a label gives you a percentage and nothing else, you do not have enough information to build a routine.
Start with the gentlest one
If you have never used an acid, start with mandelic. It works more slowly than glycolic and stings less at the same percentage. It also tends to be the more comfortable choice for deeper skin tones, where irritation is more likely to leave a mark behind.
The Overnight AHA Peel, with glycolic at 8% and lactic at 4%, is the strongest thing we sell. It is for skin that already tolerates acids well, and its instructions say once a week to start. We wrote them that way on purpose.
The first four weeks
Before anything
Patch test on the inner forearm and wait 24 hours. If it stings and stays red, it is not for you.
Weeks one and two
One or two evenings a week, on clean and completely dry skin. Follow with a plain moisturizer. Sunscreen the next morning.
Weeks three and four
If your skin is calm, move to two or three evenings a week. That is the ceiling for the mandelic, not a starting point.
After that
Stay there. Results from an acid come from steady use over two months, not from frequency.
What never to combine
- Two acids on the same night. The Mandelic Resurface and the AHA Peel are never used together, and most people should simply pick one.
- The AHA Peel with retinoids or vitamin C on the same night.
- An acid over a retinol. Use them on alternate nights. Layering them is the fastest way to undo the tolerance you have built.
- An acid and no sunscreen. Freshly exfoliated skin is more exposed to the sun.
If you use a prescription retinoid, how an acid fits alongside it depends on you and on what has been prescribed. That is a question for your dermatologist, not for a label or for us.
Tingling is not the goal
A brief tingle for a few seconds is common. It is not a sign the product is working harder, and its absence is not a sign of failure. Burning, redness that is still there the next morning, tightness that spreads to products you used to tolerate: those are signs to stop. Rinse, go back to a gentle cleanser and a barrier cream, and wait until your skin is comfortable before trying again at half the frequency.
If acids are simply not for you
Some skin does not get along with leave-on acids at any frequency. That is not a failing. A rinse-off enzyme polish works by a different mechanism, stays on for three minutes and leaves nothing behind. It is what we suggest for the weeks you take off acids, and for the people who take off every week.
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- Low stock
Overnight Peel
Overnight AHA Peel
Glycolic 8% and lactic 4% at pH 3.5. Once a week, and we mean it
$54
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