Topical Acne Therapy: Hitting the Four Pathogenic Steps

The four things that go wrong in a pore

Acne is not one problem; it is four, stacked inside a single pilosebaceous unit. Every hair follicle on the face drains a sebaceous (oil) gland — together the pilosebaceous unit. Acne begins when four events collide there. First, follicular hyperkeratinisation: the cells lining the pore stop shedding cleanly and instead stick together into a plug, forming the microcomedone — the seed of every lesson. Second, excess sebum: androgens (which is why acne flares at puberty) drive the oil gland into overproduction, feeding the plug and the bacteria. Third, Cutibacterium acnes (the organism formerly called Propionibacterium acnes) — a normal skin commensal that overgrows in this oily, blocked, oxygen-poor pocket. Fourth, inflammation: C. acnes and the trapped debris trigger the immune system, turning a quiet blackhead into a red papule, a pustule, or a deep nodule. Read those four in order and every drug that follows has an obvious target.

Topical retinoids: the cornerstone

If you learn one acne drug class, learn this one — it is the backbone of almost every regimen. Topical retinoids are vitamin-A derivatives that bind nuclear retinoic-acid receptors and reprogram how keratinocytes mature and shed. That single action hits step one directly: they normalise follicular keratinisation, unstick the plug, and are comedolytic — they empty existing comedones and, crucially, prevent new microcomedones from forming. They are also anti-inflammatory, which reaches step four. The family runs tretinoin (the original), adapalene (better tolerated, more photostable, so it can be layered with benzoyl peroxide), tazarotene (the most potent and most irritating), and trifarotene (a newer, selective agent studied for the trunk as well as the face). Because they stop the seed lesion forming, retinoids are the drug you keep using for maintenance long after the visible spots clear — the foundation, not the finishing touch.

The price is tolerability. Retinoids reliably cause a retinoid reaction early on — dryness, redness, peeling and stinging — worst in the first few weeks before the skin adapts. The fix is technique, not abandonment: a pea-sized amount, at night, on dry skin, a bland moisturiser over it, and starting every other night if needed. They increase photosensitivity, so a daily sunscreen is part of the prescription, not an optional extra. And one point students must never blur: this is where topical acne care touches the Isotretinoin chapter and the wider retinoid story in Foundations. Oral isotretinoin is famously and powerfully teratogenic, demanding strict pregnancy prevention. The topical agents are not absorbed in remotely the same way, but the profession is deliberately cautious — topical retinoids are conventionally avoided in pregnancy, and "retinoid" should immediately make you ask about pregnancy in any woman of child-bearing potential.

Benzoyl peroxide: the resistance-proof antibacterial

Benzoyl peroxide (BPO) attacks steps three and four. It releases reactive oxygen radicals inside the follicle that are directly bactericidal to C. acnes, and it is mildly keratolytic, helping unblock the pore. Its single most important property is one word: no resistance. Because it kills bacteria by non-specific oxidation rather than a precise molecular target, C. acnes cannot evolve around it — unlike antibiotics. That is exactly why BPO is the essential partner drug: whenever a topical (or oral) antibiotic is used for acne, benzoyl peroxide is added alongside it to suppress the emergence of resistant organisms. The main nuisance is that BPO bleaches — it will lighten coloured towels, pillowcases and clothing on contact, a genuinely useful thing to warn patients about — plus dryness and irritation, and rarely a true allergic contact dermatitis.

Topical antibiotics: never fly solo

Clindamycin and erythromycin work — but the way we use them is dictated entirely by resistance. Topical antibiotics — chiefly clindamycin, and older erythromycin — target steps three and four: they suppress C. acnes and have a genuine anti-inflammatory effect independent of killing bacteria. The problem is the same one that haunts all of antimicrobial therapy: used alone and repeatedly, they breed resistant C. acnes, and resistance to topical acne antibiotics is now widespread. This is why the modern rule is that a topical antibiotic is never prescribed as monotherapy for acne. It is always combined — most often with benzoyl peroxide (which shields it from resistance) and frequently with a retinoid (which adds step-one comedolytic action). This is the same antimicrobial-stewardship logic taught in the Antimicrobials chapter, applied to a cream: use the narrowest effective approach, protect the drug from resistance, and never deploy an antibiotic where it can select for resistance unopposed.

Azelaic acid: the quiet all-rounder

Azelaic acid is the underrated multitasker of topical acne care, touching three of the four steps at once. It is antibacterial against C. acnes, comedolytic (helping normalise keratinisation), and anti-inflammatory. It carries two extra advantages that make it especially valuable. First, it is generally well tolerated and considered acceptable in pregnancy, making it a go-to when retinoids are being avoided. Second, it lightens post-inflammatory hyperpigmentation — the brown marks acne leaves behind — by inhibiting tyrosinase in overactive melanocytes. That makes it particularly useful in skin of colour, where those dark marks often distress patients more than the active spots did. Azelaic acid also reappears in the Rosacea chapter, where the same anti-inflammatory action is exploited for a different disease.

Combination products and the rung above topicals

The best regimens pair drugs that attack different steps in one convenient tube. The whole strategy of topical acne care crystallises into fixed-dose combination gels: adapalene plus benzoyl peroxide (step 1 + steps 3/4, with BPO shielding nothing to shield but adding bactericidal cover), clindamycin plus benzoyl peroxide (antibiotic protected against resistance), and clindamycin plus a retinoid. Combining into a single once-daily product also solves the adherence problem that quietly sinks so many regimens. But topicals have a ceiling. When the acne is severe, nodulocystic, scarring, or simply refractory to a well-followed topical regimen, treatment steps up to systemic therapy — oral antibiotics, hormonal agents such as combined oral contraceptives or spironolactone, and ultimately oral isotretinoin. Those belong to the Systemic Acne chapter, and knowing when a topical regimen has genuinely failed — rather than merely been abandoned early — is the clinical judgement that decides when to climb that ladder.