Unit 3 · BHAs & PHAs
Why lipid solubility, not strength, decides between a BHA and a PHA
BHAs and PHAs sit at opposite ends of the exfoliation spectrum from AHAs — one engineered to reach where AHAs cannot, the other engineered to barely penetrate at all. By the end you should be able to select between salicylic acid, its gentler derivative, and the two major PHAs based on what the skin actually needs.
Learn · The BHA family
What defines a beta-hydroxy acid
BHAs have a hydroxyl group at the beta carbon position. The key difference from AHAs is lipophilicity: BHAs are oil-soluble, which allows them to penetrate the lipid-rich environment of the sebaceous follicle — where AHAs cannot reach effectively.
This makes BHAs the most targeted chemical exfoliant for acne, comedones and oily skin.
Salicylic acid
- Source
- Willow bark (Salix alba) — now produced synthetically
- Molecular weight
- 138 Da
- Typical OTC range
- 0.5–2% (TGA limit for OTC)
- Professional peel range
- 10–30%
- Key property
- Lipophilic — penetrates the sebaceous follicle
Salicylic acid is the primary and most clinically validated BHA. Its lipophilic nature allows it to dissolve within and penetrate sebaceous follicles — dislodging sebum plugs and desquamating follicular cells that form comedones. It is keratolytic, anti-inflammatory (inhibiting arachidonic acid and prostaglandin synthesis) and has mild antibacterial activity against C. acnes.
Salicylic acid is structurally related to aspirin — its anti-inflammatory mechanism shares the prostaglandin inhibition pathway.
Salicylic acid shares a structural relationship with aspirin and the same anti-inflammatory pathway. What does that mean for a patient who reports aspirin hypersensitivity?
Hold your answer before you open this. The value is in having committed to a position on cross-reactivity first.
A patient with aspirin hypersensitivity may also react to topical salicylate-based products. The structural relationship is clinically significant, not just a chemistry footnote.
This is a screening question worth asking before recommending a salicylic acid product or professional peel, not an assumption to make only after a reaction occurs.
Acne, inflammatory and comedonal: first-line BHA for all acne subtypes. Addresses comedone formation, active inflammation and bacterial load together — the most effective OTC option for comedone clearance.
Acne in darker skin types: carries a lower PIH risk than glycolic acid in Fitzpatrick IV–VI. Its anti-inflammatory property specifically reduces the post-acne inflammatory stimulus that drives PIH — a well-supported option for acne management in skin of colour.
Dry, sensitive skin: caution at higher concentrations — can be drying. Introduce gradually, with concurrent barrier support.
High-dose systemic salicylate is contraindicated in pregnancy. OTC topical concentrations (0.5–2%) are generally considered low risk, but guidance varies — advise patients to consult their treating physician. Professional-strength salicylic peels are typically avoided in pregnancy.
The key property that makes salicylic acid uniquely effective for comedonal and acne-prone skin is:
Select an option to commit. The reasoning appears afterwards.
Salicylic acid's lipophilic nature lets it dissolve within and penetrate the sebaceous follicle — the lipid-rich environment AHAs, being water-soluble, cannot reach effectively. This is what dislodges sebum plugs and desquamates follicular cells directly.
This single property explains why salicylic acid is the first-line BHA for every acne subtype: it addresses comedone formation, its anti-inflammatory action reduces active inflammation, and its mild antibacterial activity lowers bacterial load — all from the same follicular access point.
Learn · Betaine salicylate
A gentler BHA for patients who cannot tolerate salicylic acid
- Structure
- Salicylic acid bound to betaine, an amino acid derivative
- Typical OTC range
- 0.5–4%
Betaine salicylate is a gentler derivative in which salicylic acid is bound to betaine, imparting humectant and anti-inflammatory properties alongside the salicylate portion's BHA exfoliation. The result is a gentler exfoliating BHA with less irritation potential than free salicylic acid.
Betaine is a naturally occurring amino acid derivative found in sugar beets. Binding salicylic acid to it increases the overall molecular weight, which slows penetration relative to free salicylic acid — the same size-versus-speed relationship seen across the AHA family in Unit 2.
A gentler BHA option for patients who need follicular exfoliation but experience dryness or irritation with standard salicylic acid products. Also useful in sensitive or reactive skin with concurrent oiliness or comedonal congestion — a common combination in rosacea-prone or combination skin.
Learn · The PHA family
What defines a polyhydroxy acid
PHAs are a third generation of hydroxy acids. They share a similar mechanism to AHAs but have multiple hydroxyl groups, increasing their molecular size and reducing their penetration speed dramatically.
This slower penetration makes PHAs the most tolerable exfoliating acids — suitable for skin types that do not tolerate AHAs or BHAs. PHAs also function as humectants, since the multiple hydroxyl groups attract water, and as antioxidants.
PHAs are recommended for sensitive and post-procedure skin primarily because:
Select an option to commit. The reasoning appears afterwards.
PHAs share a mechanism with AHAs but carry multiple hydroxyl groups, which increases their molecular size and dramatically slows penetration. That slow penetration is what makes them the most tolerable exfoliating acid class.
This is the same size-versus-speed relationship you saw across the AHA family in Unit 2, taken further — PHAs are large enough that they sit primarily at the surface rather than penetrating deeply, which is exactly what sensitive or barrier-compromised skin needs.
Learn · Gluconolactone
Gluconolactone — the most widely used PHA
- Source
- Glucose oxidation product
- Key properties
- Exfoliant, humectant, antioxidant, metal chelator
- Typical OTC range
- 3–14%
Gluconolactone's large molecular size means it sits primarily at the stratum corneum surface — providing gentle exfoliation without penetrating deep enough to cause the erythema, stinging or barrier disruption seen with AHAs in sensitive skin. It also chelates metal ions that catalyse free radical production, providing a mild antioxidant benefit.
Clinical studies have shown gluconolactone to be effective as an exfoliant in rosacea patients — a skin condition where AHAs may be too stimulating. It has also been shown to be appropriate for use post-laser resurfacing and post-chemical peel as a maintenance exfoliant during recovery.
Rosacea, chronically sensitive skin: one of the few exfoliants appropriate for active or tendency-to-rosacea skin. Non-irritating, anti-inflammatory and gentle enough for regular use.
Post-procedure maintenance: a suitable introduction of exfoliation from week 3–4 post-ablative resurfacing or professional peel. Does not disrupt a recovering barrier the way AHAs can.
A patient with chronically sensitive, rosacea-prone skin wants to begin a gentle exfoliation routine to improve texture. The most appropriate acid family to recommend first is:
Select an option to commit. The reasoning appears afterwards.
Gluconolactone's large molecular size means it sits primarily at the surface rather than penetrating deeply — gentle enough to avoid the erythema and barrier disruption that AHAs can cause in rosacea-prone skin, while still delivering meaningful exfoliation.
It is one of the few exfoliants specifically studied and recommended for active or tendency-to-rosacea skin. Recommending an AHA here would ignore the tolerability trade-off that this unit has been building toward; recommending no exfoliation at all overlooks a genuinely appropriate option.
Predict · Post-procedure timing
Not every gentle acid is gentle enough for the same day
Gluconolactone can be introduced from week 3–4 post-ablative resurfacing. Lactobionic acid — the PHA covered next — is described in this module as gentler still. Before reading its full profile, commit to a position on timing.
Of gluconolactone and lactobionic acid, which can be introduced earlier in a post-ablative recovery timeline?
Hold your answer before you open this. The value is in having committed to a position on relative gentleness first.
Lactobionic acid. It can be introduced earlier in the post-procedure timeline than any other exfoliant in this module, and is often included in post-treatment skin recovery kits for that reason.
This follows directly from molecular size: lactobionic acid is a disaccharide — the largest commonly used hydroxy acid — which gives it the gentlest penetration profile of any acid covered so far.
Learn · Lactobionic acid
Lactobionic acid — the gentlest exfoliant in this module
- Source
- Oxidation of lactose (milk sugar)
- Key properties
- Exfoliant, powerful humectant, antioxidant, collagen support
- Typical OTC range
- 2–8%
Lactobionic acid is a disaccharide — galactose plus gluconate — the largest commonly used hydroxy acid, which gives it the gentlest penetration profile of all. Its numerous hydroxyl groups make it an exceptionally effective humectant, attracting moisture even more powerfully than glycerin.
Lactobionic acid also inhibits matrix metalloproteinases — the enzymes that degrade collagen — a mechanism directly relevant to anti-ageing and distinct from any AHA or BHA covered so far.
Very sensitive, reactive skin: the gentlest exfoliating option available, appropriate where even gluconolactone is not tolerated.
Dry, dehydrated mature skin: the powerful humectant and MMP-inhibiting properties make it particularly relevant for dry, mature skin seeking mild resurfacing with concurrent moisturisation and structural support.
Learn · Choosing between AHA, BHA and PHA
Three questions that guide the decision
When advising a patient on which exfoliant acid family to use, three questions guide the selection.
- Is the primary concern oily or congested skin?
- Choose a BHA — salicylic acid — first. It is the only acid family that penetrates the follicle.
- Is the skin sensitive, compromised or in recovery?
- Choose a PHA first — gluconolactone or lactobionic acid — for gentle, barrier-compatible penetration.
- Is the skin normal to oily, seeking surface renewal, texture or pigmentation?
- Choose an AHA, selected by skin tone and tolerance — lactic for sensitive or darker phototypes, glycolic for tolerant, photoaged skin.
Unit 3 summary
Clinical takeaways
- Lipophilicity, not concentration, is what makes a BHA a BHA. Salicylic acid's oil solubility lets it reach the sebaceous follicle where water-soluble AHAs cannot — this is the single property behind its first-line status for acne and comedones.
- Salicylic acid carries a real cross-reactivity risk. Its structural relationship to aspirin means aspirin-hypersensitive patients may react to topical salicylate products — worth screening for before recommending it.
- PHAs trade penetration for tolerability, and lactobionic acid trades the most. As the largest commonly used hydroxy acid, it can be introduced earlier post-procedure than any other exfoliant in this module.
- Acid family selection reduces to three questions. Oily or congested → BHA. Sensitive, compromised or recovering → PHA. Normal to oily seeking renewal or pigmentation improvement → AHA, chosen by tone and tolerance.