AHA, BHA and PHA: How Strong, How Often, What Format

If you've read that AHAs handle texture and BHAs handle congestion, you already know the easy half of the decision. The harder half is the one that actually decides whether an acid works for you: how strong the formula is, how often you use it, and how long it stays on your skin. Most people who give up on chemical exfoliation didn't pick the wrong acid family. They picked a sensible acid and ran it too hard.

So this guide treats AHAs, BHAs and PHAs as rungs on a tolerance ladder rather than three boxes to sort yourself into. The family question, which acid suits which skin type, gets answered properly in our guide to AHAs and BHAs: what they do and how they differ, so there's no sense repeating it. Read that one to pick a family. Read this one to sort out the dose, the format and what to do when things go wrong. That last part is what decides whether you're still reaching for the bottle in three months.

PHAs: the rung the usual AHA-vs-BHA framing leaves out

Polyhydroxy acids, mainly gluconolactone and lactobionic acid, are bigger molecules than glycolic or lactic acid, so they work into the surface layers more slowly. Slow is the whole point. They're also humectants, which means they hold water at the surface rather than only loosening dead cells.

The evidence behind them is better than their low profile suggests. A 12-week comparative study published in Cutis in 2004 put a gluconolactone regimen head-to-head against a glycolic acid regimen and found broadly comparable improvements in the appearance of photoageing, with the PHA regimen causing far less stinging and burning at both six and twelve weeks (Edison et al., Cutis 2004). One caveat for balance: that work came from researchers at NeoStrata, the company built around the original hydroxy acid research, so treat it as supportive rather than independent.

Two other findings earn their keep in practice. In a split-face study, 10% and 30% gluconolactone produced no significant difference in measured skin hydration, so the higher concentration simply didn't buy more (Jarząbek-Perz et al., Skin Research and Technology 2021). And gluconolactone didn't significantly increase sunburn cell formation in human skin in the study that looked at it (Bernstein et al., Dermatologic Surgery 2004), which is a meaningful departure from the AHAs. These are small studies, not landmark trials, so hold the conclusions loosely. They do all point the same way, though.

The practical upshot: a PHA isn't a weaker consolation prize. It's the sensible first rung for anyone whose skin flushes easily, or who has been stung by an AHA before. The section further down sets out exactly when to open there. Skintique's PHA skincare range is where to look if that sounds like you.

Format changes the dose more than the percentage does

Two products can both say "10% glycolic" and behave nothing alike. What varies is contact time, pH, and whatever else is in the bottle.

• Cleansers sit on the skin for maybe thirty seconds, then rinse away. That's the mildest way to bring an acid in, and genuinely useful as a maintenance step. Most acid cleansers are glycolic or salicylic, and the salicylic ones cope with the short contact time better, because the acid is oil-soluble and disperses into sebum rather than sitting on top of it. Even so, don't expect a rinse-off product to do the heavy lifting.
• Toners and pre-soaked pads are left on, but usually at lower concentration and buffered. Easy to overdo, because the ritual is quick and the bottle is large.
• Leave-on serums and creams give you the highest real-world exposure of the at-home formats. This is where a step up in frequency is most likely to tip into irritation.
• At-home masks and peels are short contact, high concentration, used now and then. Different job entirely: an event, not a routine.

If you're already irritated, changing format is often smarter than changing acid. Dropping from a leave-on glycolic serum to a glycolic cleanser keeps the ingredient and strips out most of the exposure. Browse the full exfoliation range by format rather than by percentage and you'll make better calls.

Do AHAs, BHAs and PHAs make your skin more sun sensitive?

"AHAs make you sun-sensitive" gets repeated everywhere without a timescale, which makes it useless. There's a specific answer. In a randomised, double-blind study, 10% glycolic acid at pH 3.5 applied once daily, six days a week, for four weeks measurably raised UV sensitivity, and that sensitivity had cleared within a week of stopping (Kaidbey et al., Photodermatology, Photoimmunology & Photomedicine 2003).

That gives you something to act on. If you've a beach holiday or a week outdoors coming up, pausing your AHA seven days beforehand is a reasonable, evidence-anchored plan rather than a guess. It also tells you the sensitivity isn't cumulative or permanent; it tracks with current use. Worth knowing the limits of the study, mind: it ran on the back rather than the face, and on a single acid at a single strength, so treat the seven days as a sensible rule of thumb rather than a precisely calibrated number.

None of that removes the need for daily protection. A broad-spectrum facial SPF is the non-negotiable companion to any exfoliating routine, and our guide to the best facial sunscreens in the UK covers how to choose one you'll actually reapply. Skipping SPF while you exfoliate for uneven tone is self-defeating: you spend the evening working on tone and undo it by lunchtime.

Where should the acid go in your week?

Think in weekly exposure, not per-application strength. A reasonable structure for most people:

1. Weeks one to two: twice a week, evenings, on clean dry skin, followed by your usual moisturiser. Nothing else new in the same fortnight.
2. Weeks three to four: if there's been no lingering redness or tightness, move to three times a week. Note that this is more frequency, not more concentration.
3. Beyond that: three to four times a week is a comfortable plateau for most people on leave-on formulas. Daily use is realistic only for gentle rinse-off or PHA formats, and it isn't automatically better.

Alongside other actives, the simplest reliable rules are these. Keep acids and retinoids on alternate evenings rather than stacking them. Put vitamin C in the morning and the acid at night, which sidesteps the compatibility question altogether. And treat niacinamide as a non-issue, because the old warning about not combining it hasn't held up. Our ingredient layering guide works through the trickier combinations.

The step-down protocol, for when it has gone wrong

Over-exfoliation rarely announces itself as "too much acid". It shows up as skin that stings when you apply things that never used to sting, a tight or shiny-tight feeling after cleansing, patchy flaking that tempts you to exfoliate more, and redness that no longer settles overnight. If you're reading the flaking as "I need more exfoliation", that's the trap.

Work down the ladder in this order, one change at a time:

1. Cut frequency first. Halve it. Give it two weeks before you judge, because skin turns over and settles on its own timescale, not yours.
2. Then change format. Leave-on to rinse-off, keeping the same acid.
3. Then change family. Step from an AHA to a PHA rather than stopping altogether. You keep some renewal without the sting, and you dodge the stop-start cycle that causes the problem again in six weeks.
4. Only then, pause entirely. Simplify to a gentle cleanser, a moisturiser and SPF while things settle.

Our guide to the skin barrier and why it breaks down explains what's happening underneath and how long the settling period usually takes.

Which strength should you start on?

This is a different question from "which family suits my skin type". For that, head back to the AHA and BHA guide, which sorts oily and congested skin, dull and uneven texture and everything in between. Once you've a family in mind, the starting dose depends far less on your skin type than on your history with acids.

• You have never used one: whichever family you've landed on, start in a rinse-off format twice a week. You're measuring tolerance, not chasing a result, and a fortnight spent finding out costs you nothing.
• An acid has stung, flushed or peeled you before: that's information about dose, not a verdict on the ingredient. Start a rung lower, either a PHA or the same acid in a lower-exposure format. Ranges built for easily flushed and reactive skin tend to sit at the gentler end by design, which saves you guessing.
• You are already using a retinoid: your routine is doing renewal work already. Add the acid on alternate evenings only, and open at a PHA rather than a leave-on glycolic. The combined weekly exposure is what matters, not either product on its own.
• You are between or after in-clinic treatments: follow the clinic's post-procedure advice first, and treat a PHA as the re-entry rung once you're cleared to resume actives.
• You are working on marks left behind after spots: the gentler end matters more than the strongest option here, because the appearance of those marks changes over months rather than fortnights, and only if you keep going. Mandelic acid and lactic acid are the usual starting points, alongside strict daily SPF.

The honest summary

The acid families aren't in competition, and there's no hierarchy where AHAs are the serious option and PHAs are the beginner's version. Glycolic, salicylic and gluconolactone are different tools with different exposure profiles, and the one that suits you is the one you can use consistently for three months without a fortnight off to recover.

If you're not sure which rung you're on, the Skintique skincare quiz narrows it down in a couple of minutes, and a virtual consultation is the better route if you've reacted badly to acids before. Persistent redness, stinging or a rash that won't settle after a fortnight without actives is worth raising with a GP or dermatologist rather than solving with another product.