AHAs and BHAs — which acid, and how often

The difference

Water-soluble, or oil-soluble.

An exfoliating acid does less than the word suggests. It scrubs nothing off. It loosens the bonds holding dead cells together, so skin sheds what it was going to shed anyway — sooner and more evenly.

AHAs are alpha hydroxy acids, and they are water-soluble. Glycolic, lactic and mandelic are the three you will meet. They work across the top layers. A pore is lined with oil, and oil and water do not mix, so an AHA largely stays out of one.

BHA means salicylic acid, and it is oil-soluble. It mixes with sebum rather than being repelled by it, so it reaches the pore lining where congestion sits.

That is the decision rule, and it is more useful than the marketing on either bottle. Texture, roughness and dullness are AHA territory. Blackheads, bumps you feel more than see, and an oily central face are a BHA question.

The limit on both: an acid works on the top few layers and nothing deeper. No strength changes that.

The four

Glycolic, lactic, mandelic, salicylic.

Three AHAs and one BHA cover almost everything on a shelf. What separates the AHAs is mostly molecule size — smaller travels faster, which cuts both ways.

Glycolic acid: The smallest molecule of the three, from sugar cane, with the longest research history. It penetrates fastest — the most effective on surface texture, and the most likely to sting, redden and be started too strong. Most people meet acids through glycolic, and over-exfoliation the same way.
Lactic acid: A larger molecule, so it works more slowly and shallowly. It is also a humectant — it holds water — so it tends to be less drying than glycolic at comparable strength. Often the more sensible starting point, and often skipped for sounding weaker.
Mandelic acid: The largest of the three, from bitter almonds, and the slowest to penetrate — which is why it suits skin that reacts to the other two. It also complicates the split above: unusually for an AHA it has some oil solubility, so it sits partway between the camps.
Salicylic acid: The BHA in practice. Oil-soluble, so it reaches where the AHAs do not, and related to aspirin, which is part of why it tends to settle inflammation as well as clear a pore lining. The more relevant acid for congestion and oiliness; the less relevant for dryness.
PHAs, and the blends: Polyhydroxy acids — gluconolactone, lactobionic — are bigger molecules again, gentler and slower. More useful: many toners, pads and serums blend three or four acids at undeclared proportions, so the label tells you what is in it, not how much.

Strength on the front of a bottle is half the story. The pH an acid is formulated at matters as much as the percentage, and is rarely printed.

The common problem

What we see more than anything else.

Of everything that arrives here self-inflicted, over-exfoliation is the one. Not a skin type and not a condition — a routine, built carefully by someone paying attention.

Acids give immediate feedback. Skin is smoother the next morning, so more looks like it would be better, and it escalates slowly enough that no single day goes obviously wrong. By the time it is uncomfortable, the acid is the last thing suspected.

Things that never stung now sting: The most reliable sign, and the one worth acting on. Moisturiser, sunscreen, sometimes plain water. A barrier doing its job does not report on ordinary products.
Shiny rather than glowing: A tight, slightly waxy sheen that catches light differently. It gets read as hydration, or as the look people chase, which is why it goes unquestioned.
Tight within a minute of washing: Not dryness later in the day — immediately, before anything is applied. Skin short of surface lipids loses water fast.
Redness that takes longer to settle: A flush that used to fade in ten minutes and now takes an hour. The settling time says more than the redness.
Flaking that comes with tightness: Fine, loose flaking on skin that also feels tight, rather than skin that feels dry and comfortable. The most misread sign here.
Breakouts that started after the routine got busier: Congestion appearing weeks into a more active routine, not before it. The timing is the useful part, and the detail people leave out.

What helps

Subtraction, and time.

Stop the actives rather than reducing them. Tapering keeps the input going while the barrier is trying to rebuild against it. What goes back on is a plain moisturiser, a cleanser that does not foam much, and sunscreen.

Expect weeks, not days — and the first week can look worse as accumulated flaking lifts. That is where most people give up and reach for something.

Do not buy a repair product to fix it. What resolves this is the absence of input, and every new bottle is one more variable in a routine that already has too many.

None of that is something we sell. The answer is frequently to buy nothing, book nothing and wait — which is also why it goes under-recognised. No part of the industry has a reason to say it.

Frequency

How often, and what not to run on the same night.

What follows is convention — the directions formulators print on their own products. It is not a plan, and the difference matters.

Most leave-on acids are built around two or three nights a week. Plenty of labels say nightly. Nightly sells more of the bottle, and for a lot of skin two nights does what seven would at less cost to the barrier.

Acids and retinoids are conventionally kept off the same night. Both act on the same top layers by broadly the same route, so running them together does not double the result. It doubles the irritation and brings forward the point where you have to stop. The usual handling is alternating nights. A few products are built to be layered; most are not, and their directions say so.

That applies to a prescription retinoid your doctor prescribes as well, and there the directions are theirs, not ours. Bring the details to a consultation so a plan is built around it rather than over the top of it.

Count the acid you have forgotten. A salicylic cleanser, an exfoliating toner, a scrub and a serum is four products, and people routinely call that using one acid. A cleanser is a shorter exposure than a leave-on, and it is not nothing.

Sunscreen is not optional alongside an AHA. AHAs increase sun sensitivity, which in Australia settles it.

What this page will not do is pick for you. Which acid, at what strength, on which nights, next to what else, depends on what your skin is doing and what is already in your routine — and a page cannot see either. That selection is what an assessment is for.

The loop

Flaking is not a sign to exfoliate more.

The instinct is reasonable. There is loose material on the surface, and exfoliants remove it.

But flaking has several causes and only one is a build-up of surface cells. An over-worked barrier flakes. So does skin short of water, and so do conditions that belong with a doctor.

Where the cause is over-exfoliation, exfoliating removes the layer that was rebuilding. It appears to work — smoother that evening — then returns worse. Every round hands you a day of evidence for the wrong conclusion, which is how it runs for months.

Some flaking is a different question entirely. Scaly rather than fine, edged, itchy, or on the scalp, hairline, eyebrows or sides of the nose — that pattern needs identifying, and that is a GP's job. We describe what we see and refer. We do not name it.

Acids narrow considerably in pregnancy, and which ones is a conversation with your nurse and your GP, not something a page should settle. See skin in pregnancy.

Questions

Asked about acids.

Answered the way they would be answered in the room.

What is the difference between AHA and BHA?

Solubility. AHAs — glycolic, lactic, mandelic — are water-soluble, so they spread across the surface and loosen the bonds between dead cells at the top. BHA means salicylic acid, which is oil-soluble, so it mixes with sebum and reaches inside a pore instead of being repelled by it. Texture and dullness point to an AHA; congestion and oiliness to a BHA.

Is glycolic or salicylic acid better for blackheads?

Salicylic, generally, and the reason is mechanical rather than a matter of strength. A blackhead sits inside a pore lined with oil. Glycolic is water-soluble and mostly stays on the surface; salicylic is oil-soluble and gets into that environment. Glycolic still helps the roughness around it, which is why the two often appear together. Neither clears congestion for good; both need to keep being used.

Can you use AHA and retinol together?

Conventionally they are kept off the same night. Both act on the same top layers by broadly the same route, so running them together does not double the benefit — it doubles the irritation and brings forward the point where you have to stop. Alternating nights is the usual handling. If a doctor prescribed your retinoid, their directions set what goes alongside it.

How often should you use a chemical exfoliant?

Most leave-on acids are built around two or three nights a week rather than nightly, whatever the label suggests. Count what you have forgotten, too: an acid cleanser, an exfoliating toner and a treatment pad are three exposures, not zero. The right frequency depends on the acid, its strength and what else you use.

What are the signs of over-exfoliated skin?

The most reliable one is ordinary products starting to sting — moisturiser, sunscreen, sometimes water. Alongside that: a tight, shiny look often mistaken for hydration, tightness within a minute of washing, redness that takes far longer to settle, fine flaking on skin that also feels tight, and congestion that appeared after the routine got busier. It develops slowly, so the acid is rarely suspected.

Does exfoliating help flaky skin?

Sometimes, and sometimes it drives the problem. Flaking has several causes and only one is a build-up of surface cells. If the cause is an over-worked barrier, exfoliating strips the layer that was rebuilding — it looks better that evening and returns worse. Flaking that is scaly, itchy, edged, or on the scalp or brows is a different question, and naming it belongs with a GP.

Which acid is best for sensitive skin?

Mandelic is usually the gentlest AHA, being the largest molecule and the slowest to penetrate; lactic is the next step up. But skin that reacts to everything often reacts because of what is already on it, so a gentler acid treats the symptom rather than the cause. Sometimes the honest answer is that no acid is the right next step.

Where this leads