AHA or BHA: which exfoliating acid suits which skin

The short answer

If your skin feels oily by the middle of the day and looks congested around the nose and chin, a salicylic acid product is the more sensible pick. If your skin looks dull or feels rough, an alpha hydroxy acid suits you better. Of the two common AHAs, lactic is the gentler place to start and glycolic is the stronger one. Sunscreen every morning is part of using either.

The rest of this is where that advice comes from, and where the evidence runs out.

AHA and BHA are different molecules

Glycolic and lactic acid are alpha hydroxy acids, usually shortened to AHA. They dissolve in water, so they work across the surface of the skin. Salicylic acid is a beta hydroxy acid, or BHA. It dissolves in oil, which is why it tends to be the one chosen for skin that feels oily.

They behave differently on the skin surface too. A review of salicylic acid as a peeling agent describes it as desmolytic, meaning it disrupts the junctions between skin cells rather than breaking down the keratin filaments between them (Arif, 2015). AHAs are widely used cosmetic ingredients that the literature describes as reducing the look of ageing, though that is a description of how they are used rather than a settled finding (Kornhauser et al., 2009).

One thing to hold in mind for everything below. Almost all of the research on these acids was done with clinic peels at strengths no shop product comes near, on people with diagnosed skin conditions, under supervision. We sell cosmetics. They are not the same thing and we are not suggesting they do the same job.

Glycolic or lactic

Glycolic is the smaller molecule of the two and is generally used as the stronger one. The clearest head-to-head we could find was a hospital trial in melasma, a pigmentation condition. Forty people were randomised, thirty-four finished, and a 50% glycolic peel produced a significantly larger fall in the pigmentation score than an 80% lactic peel over ten weeks. No statistically significant difference in side effects was detected, though a trial this small could easily miss one (Kadu and Laul, 2025).

That result is narrower than it looks. Two different acids at two different concentrations, one outcome, one condition, ten weeks, one hospital. It does not tell you which acid is better in a bathroom cabinet, and it says nothing about texture, dryness or everyday use. We are including it because it is the best direct comparison available, not because it settles the question.

Lactic acid is the larger molecule and is normally sold at gentler strengths, so it is the more reasonable place to begin if your skin is dry, reactive or new to acids. We do not stock a standalone lactic acid product, so treat that as general information rather than a recommendation to buy something.

If glycolic already sits well on your skin, night is the tidier slot for it, because it keeps the acid away from daytime sun. Medik8 Sleep Glycolic is made for that. Medik8 directs two to three nights a week, evenings only, a patch test before you start, and sunscreen the following morning. Those instructions are worth following as written.

Salicylic acid

Salicylic acid is the usual choice for skin that feels oily and looks congested. Two formats are common, and they are not equivalent.

A cleanser like Pore Cleanse Gel Intense is rinsed off, so contact time is short. Medik8 suggests morning and evening, and up to once a day if your skin is dry or sensitive. A leave-on tonic like Press & Clear stays on the skin, so it is the easier one to overdo. Medik8 suggests new users start once a day and watch how the skin responds. If your skin is dry or sensitive, use one of the two rather than both in the same routine.

On deeper skin tones, the evidence is thin but not discouraging. In a split-face trial of thirty-six people, of whom 94.5% were Fitzpatrick type IV to V, a 30% salicylic peel performed no differently from Jessner's solution, and 85.3% of participants rated the salicylic side good or very good (How et al., 2020). Burning, stinging and flaking were commonly reported. Thirty-six people cannot show that two treatments are equivalent, and those were clinic peels rather than shop products, so read it as reassurance about salicylic acid in general and nothing more.

If you have ever reacted badly to aspirin or salicylates, ask a pharmacist before using a salicylic acid product.

How often

Clinic strength glycolic peels disturb the skin barrier straight away. In a study on the forearms of thirteen healthy women, peels at 30%, 50% and 70% caused measurable barrier damage and redness immediately afterwards. Barrier function had recovered within 24 hours, and the redness took up to four days to settle. The authors concluded that repeating a superficial peel at two-week intervals gives the skin time to recover (Song et al., 2004). That was forearm skin and peel strengths, not a serum on a face, so it is a reason to leave gaps rather than a schedule you can copy.

Daily use has been looked at once, in a very small study. Six volunteers applied 12% glycolic acid for an hour a day, five days a week, for two weeks. Water loss through the skin increased significantly, hydration in the surface layer dropped significantly from day 11 to day 18, and the skin reddened quickly (Effendy et al., 1995). Six people is not proof of anything. It is a signal, and it points the same way as everything else here.

The practical version. Follow the directions on the product you actually own, because they genuinely differ between a rinse-off cleanser, a leave-on tonic and an overnight peel. Use one exfoliating product rather than three. Nothing we found supports layering an AHA and a BHA on the same night, or stacking acids with scrubs.

If you also use a retinoid

The glycolic study above also tested all-trans retinoic acid on the same schedule. Both increased water loss through the skin. The difference was afterwards. Water loss stayed raised on the retinoic acid sites once treatment stopped, and dryness and scaling persisted to day 18 (Effendy et al., 1995). Prescription retinoic acid is not the same as a cosmetic retinol, but the practical point carries.

If a retinoid is already part of your evenings, put the acid on different nights rather than doubling up, and go slower than you would with either on its own. Sunscreen every morning applies whichever one you used the night before.

Sun protection, which matters more here

This is the one place the two acids clearly differ. Fourteen people had sites on the mid back treated on weekdays for three and a half weeks. The glycolic acid 10% sites showed increased redness, increased DNA damage and increased sunburn cell formation after exposure to simulated sun. The salicylic acid 2% sites showed no significant change in any of the three (Kornhauser et al., 2009). Those strengths are in the same range as many cosmetic products, although the study used plain formulations on back skin.

Do not read too much into the salicylic half. Fourteen people, one concentration, one study, and finding no significant change is not the same as showing there is no effect. It is not a reason to skip sunscreen.

New Zealand summer UV is high, so daily sunscreen is the sensible baseline with either acid, and with an AHA we would treat it as non-negotiable. How you apply it matters more than the number on the bottle. Most people use too little and never reapply. As an example of what a full application looks like, Medik8 directs five pumps of Advanced Day Ultimate Protect for the face and five more for the neck and decolletage, as the last step of your morning routine, then reapplication every two hours in direct sun. A sunscreen you will wear properly every day does more for you than a stronger one you use sparingly.

What these ingredients cannot do

• The evidence behind them is weak. A Cochrane review covering 49 trials of six topical ingredients found that for salicylic acid and fruit acids, every comparison came out low or very low quality, limited by risk of bias and imprecision (Liu et al., 2020). The American Academy of Dermatology guideline gives salicylic acid a conditional recommendation, not one of its strong ones (Reynolds et al., 2024). They are useful cosmetic ingredients. They are not miracle ingredients.
• They do not change pore size. Pores do not shrink. What can change is the look of congestion and the smoothness of the surface.
• Purging is not something we can support. None of the studies here tested the idea that an initial breakout is a necessary or positive stage. Irritation is just irritation.
• They do not treat skin conditions. These are cosmetic products. If something is persistent, painful, spreading or getting worse, see a GP or a dermatologist rather than a shelf.
• Pregnancy and breastfeeding are outside what we can advise on. None of the studies above looked at it, and we are not qualified to reassure you either way. Ask your midwife or GP.
• Clinic percentages do not transfer. The 30% to 80% peels in these trials were applied by trained staff on set schedules. A stronger product at home is not a shortcut to the same result, and no study tested people applying those strengths to themselves.

When to stop

Tightness. Stinging when you apply anything, even a plain moisturiser. Redness that does not settle. Flaking. Skin that suddenly reacts to products it used to tolerate. The barrier and hydration findings above are the likely mechanism behind most of it.

Stop the acid completely. Use a plain moisturiser and sunscreen for a week or two until things feel normal. Then reintroduce at half the frequency you were using. Avoid acids altogether on skin that is broken, sunburnt or already irritated, keep them away from the eye area, and if you have a facial or a clinic peel booked, ask your therapist how long to pause beforehand.

Where to start

Oily and congested, go salicylic. Dry, dull or rough, go lactic first and glycolic later if you want more. Follow the directions on the product rather than a rule you read somewhere. Sunscreen every morning, reapplied when you are out. Give it six weeks before you judge it.

If you would like a hand choosing, come and see us in Parnell.