Topical acids — complete patient guide:
Retinoids, retinol, azelaic acid, AHA, BHA — explained in one place

Why these 5 topical "acids" are the foundation of acne / aging skincare

The five topical active families — retinoids, retinol-class precursors, azelaic acid, alpha hydroxy acids (AHA), and beta hydroxy acids (BHA) — are the most evidence-based ingredients in dermatology for acne, photoaging, hyperpigmentation, and overall skin renewal. Each works on a different molecular pathway, so they're often combined or rotated based on indication and skin type.

1. Topical retinoids (Tretinoin / Adapalene / Tazarotene / Trifarotene)

Mechanism: Bind to retinoic acid receptors (RAR α/β/γ) → modulate gene transcription → normalize keratinocyte differentiation, reduce comedone formation, anti-inflammatory effects, increase dermal collagen.

Generations:

• Tretinoin (1st gen) — gold standard for acne + photoaging; available 0.025%, 0.05%, 0.1%
• Adapalene (3rd gen, RARβ/γ-selective) — gentler, photo-stable, OTC in US (0.1%); now first-line per AAD 2024
• Tazarotene (3rd gen) — strongest; for psoriasis + acne
• Trifarotene (4th gen, RARγ-selective) — most selective; for back/chest acne

How to start: Pea-sized amount, every 3rd night for 2 weeks, then every other night, then nightly. Apply on dry skin only; wait 20 min after washing. Always pair with daily SPF 30+.

Side effects: Retinoid dermatitis (redness, peeling, dryness) in first 4-6 weeks is normal. Pregnancy: contraindicated (Category C/D depending on agent).

2. Retinol-class precursors (Retinol / Retinaldehyde / Retinyl esters / Bakuchiol)

These are cosmetic-grade precursors that the skin must convert to retinoic acid in 1-2 enzymatic steps before they're active. Less potent than prescription retinoids but better tolerated.

Conversion order (and relative potency):

• Retinyl palmitate → retinol → retinaldehyde → retinoic acid (active)
• Retinaldehyde is ~ 1/10–1/20 the potency of tretinoin but 5-10× retinol
• Retinol is ~ 1/100 the potency of tretinoin
• Bakuchiol — plant-derived non-retinoid that activates similar genes; useful in pregnancy as a "retinoid alternative" though evidence is weaker

Use case: Beginners, sensitive skin, pregnancy (bakuchiol only), maintenance after stopping prescription retinoids.

3. Azelaic acid

A naturally-occurring dicarboxylic acid with three independent mechanisms:

• Anti-inflammatory (downregulates kallikrein 5)
• Anti-microbial (against C. acnes)
• Tyrosinase inhibitor (lightens pigmentation)

Strengths: 15% (Finacea, OTC) or 20% (Skinoren, prescription).

Best for: Acne + melasma + rosacea — uniquely positioned to handle multiple concerns. Pregnancy-safe (Category B), making it the #1 retinoid alternative.

4. Alpha Hydroxy Acids (AHA): Glycolic / Lactic / Mandelic

Mechanism: Water-soluble, work on the surface by loosening intercorneocyte bonds → exfoliation. Stimulates underlying collagen synthesis at higher concentrations.

Comparison:

• Glycolic acid (smallest molecule, 76 Da) — penetrates deepest, most irritating; 5-10% OTC, 30-70% in-clinic peels
• Lactic acid (90 Da) — humectant + gentler; for dry/sensitive skin
• Mandelic acid (152 Da) — slowest penetration, best for sensitive / darker skin (less PIH risk)

pH and free-acid percentage matter more than total concentration. AHA at pH 3.5-4 has substantial free-acid activity; pH > 6 means most is neutralized.

5. Beta Hydroxy Acid (BHA): Salicylic Acid

Lipid-soluble, so it penetrates into oily pores — uniquely effective for blackheads, whiteheads, and oily/combination skin. Anti-inflammatory at higher pH.

Strengths: 0.5-2% in OTC (Paula's Choice 2% BHA, CosRX), up to 30% in clinical peels.

Salicylates and aspirin allergy: rare but possible cross-reactivity; large surface application can theoretically cause systemic salicylism (avoid in children).

Combining acids — what works, what doesn't

Pregnancy & breastfeeding safety

How to start (beginner protocol)

1. Week 1-2: Pick ONE active. Apply every 3rd night, on dry skin, after a heavy moisturizer ("buffering" reduces irritation).
2. Week 3-4: If tolerated, increase to every other night.
3. Week 5+: Daily evening use if no irritation.
4. Always: Daily SPF 30+ broad-spectrum in the morning. Stop and consult a dermatologist if persistent erythema, burning, or worsening acne occurs after week 6.

Common questions

Q: How long until I see results? Initial benefit at 4-8 weeks; significant improvement at 12 weeks; continued improvement up to 6-12 months.

Q: Should I expect a "purge" period? Some patients experience accelerated comedone surfacing in weeks 2-4 with retinoids — this is comedone evacuation, not new acne. Persists 2-6 weeks then improves.

Q: Can I use multiple acids together? Yes, but rotate days (Mon/Wed/Fri retinoid; Tue/Thu AHA) rather than stacking nightly. Daily simultaneous use significantly increases barrier disruption risk.

Q: When should I see a dermatologist? If acne persists >3 months on OTC, if scarring develops, or if you have moderate-severe baseline disease. Prescription tretinoin/adapalene + oral medication may be needed.

Note: All claims based on AAD 2024 acne guideline (Reynolds et al., JAAD 2024), Plewig & Kligman Acne and Rosacea 4th ed (2024), and Mukherjee et al. retinoid review (Clin Interv Aging 2006). See Chinese version for full reference list.

Common questions (frequently asked on online forums)

2026 supplement: new drugs and guidelines

Two notable additions to topical acne therapy in recent years, both formally included in AAD 2024:

• Trifarotene 0.005% cream (Aklief): 4th-generation topical retinoid that selectively targets the RAR-γ receptor (the most abundant subtype in skin),approved for both facial and truncal (back, chest) acne. Traditional retinoids were studied mainly on facial acne; consider Aklief when the back/chest is involved.
• Clascoterone 1% cream (Winlevi):the first topical androgen-receptor inhibitor, FDA-approved in 2020. Locally suppresses androgen signaling at the pilosebaceous unit — works in both sexes, applied twice daily to the full face. Important for male patients who previously had no topical anti-androgen option. Not yet available in Taiwan; may require self-pay or overseas purchase. AAD 2024 classifies as a conditional recommendation (high-quality evidence, but cost is a limitation).

References

1. Reynolds RV, Yeung H, Cheng CE, et al. Guidelines of care for the management of acne vulgaris (2024 AAD).J Am Acad Dermatol. 2024;90(5):1006.e1-e30. [Source]
2. National Institute for Health and Care Excellence. Acne vulgaris: management (NG198). London: NICE; 2021 (last updated April 2026). NICE first-line therapy uses fixed-combination "retinoid + BPO" or "retinoid + topical antibiotic", consistent with AAD 2024.[Source]
3. Mukherjee S, et al. Retinoids in the treatment of skin aging. Clin Interv Aging. 2006;1(4):327-348. [Source]
4. Schulte BC, et al. Azelaic acid: evidence-based update. J Drugs Dermatol. 2015;14(9):964-968. [Source]
5. Tang SC, Yang JH. Dual effects of alpha-hydroxy acids on the skin. Molecules. 2018;23(4):863. [Source]
6. Tan J, et al. Trifarotene 50 µg/g cream for moderate facial and truncal acne — phase 3.J Am Acad Dermatol. 2019;80(6):1691-1699. [Source]
7. Hebert A, et al. Topical clascoterone cream 1% for acne — phase 3 trials.JAMA Dermatol. 2020;156(6):621-630. [Source]
8. UpToDate: Topical retinoids. Accessed 2026. [Source]