AHAs, BHAs & PHAs

What it is

What it does

• Dissolves the "glue" between dead corneocytes, accelerating surface turnover.
• Improves skin texture, tone, and product penetration.
• Clears congested follicles (BHA in particular).
• Fades superficial pigmentation.
• Hydrates indirectly — lactic and PHAs are also humectants.

Evidence strength

Strong. AHAs and BHAs have decades of RCTs on photoaging, acne, and pigmentation. PHAs have growing data, mostly for sensitive populations.

Best for

• Glycolic 5%–10% — texture, dullness, fine lines.
• Lactic 5%–10% — dry, dehydrated skin that needs gentle exfoliation.
• Mandelic 5%–10% — sensitive or deeper skin tones; lower irritation, anti-pigment.
• Salicylic 0.5%–2% — oily, congested, blackhead-prone skin.
• PHAs 4%–10% — rosacea, eczema-prone, post-procedure.

Who should avoid it

• Active barrier impairment.
• People already on a retinoid + ceramide-rich routine who don't need more.
• Salicylic acid in very high topical doses during pregnancy — see below.

Ideal concentration range

The "right" percentage depends on the acid and the vehicle. Two 10% glycolic toners can behave very differently because of pH and buffering. Free-acid value matters more than the label percentage. Practical rule: start at the lower end, twice a week, with sunscreen.

Best ingredient pairings

• Hyaluronic acid + ceramides — buffer water loss after exfoliation.
• Niacinamide — soothes the post-acid sting.
• Sunscreen daily — AHA users are 2× as photosensitive for up to a week.

Ingredients to avoid combining

• Retinoids in the same slot as a strong acid — alternate nights.
• Two acids in the same step (e.g., glycolic toner + lactic serum) unless you know what you're doing.
• Physical scrubs on acid days.

Pregnancy considerations

AHAs are generally considered acceptable at OTC concentrations. Salicylic acid is acceptable at the typical 0.5%–2% leave-on or wash-off levels — avoid high-strength peels and prescription-grade products. Confirm with your obstetrician.

Irritation potential

Moderate. Glycolic and high-strength lactic sting most. PHAs sting least. Tingling for 30–60 seconds is normal; redness or burning beyond that means too strong, too often, or both.

Beginner guidance

1. Cleanser → lactic 5% or salicylic 2% twice a week → moisturizer → SPF in the morning.
2. Increase to 3–4 nights only after 2 tolerable weeks.
3. Stop if peeling, redness, or sting persists.

Advanced guidance

• Layer mandelic in the morning + low-dose retinoid at night for tone work.
• Use salicylic + niacinamide for oily, congested skin maintenance.
• Reserve glycolic peels (10%–30%) for occasional use under good barrier conditions.

Best products containing it

Research notes

• Ditre et al., 1996 — glycolic acid 25% improves photoaging.
• Multiple RCTs on salicylic 2% vs benzoyl peroxide in mild acne.
• PHA studies in sensitive skin and post-procedure populations.

Related

• Acne concern guide
• Aging concern guide
• Acne-prone routine