Topical Antimicrobials: Treating on the Surface
Not every infection needs a pill. For skin, eyes and small wounds, putting the drug exactly where the bug is means high local concentrations, tiny doses, and few whole-body side effects. But topical treatment has its own rules — and some agents are deliberately kept for the skin so we never risk their resistance in the body.
A child has honey-coloured crusts around the nose — impetigo, a superficial skin infection. An adult has a red, sticky eye — bacterial conjunctivitis. A patient with a burn needs to keep the wound from becoming infected. None of these needs a systemic antibiotic coursing through the whole body; each can be treated by applying the drug directly to the surface where the infection lives. Topical antimicrobials are a distinct and clever part of the toolkit — and understanding why we use them, and where their limits lie, is more useful than memorising a long list of creams.
Why topical works — and when
Put the drug where the bug is, and you can use tiny amounts of powerful agents. The logic of topical treatment is direct delivery: applied to the skin or eye, the drug reaches a very high concentration right at the infection while almost none enters the bloodstream — so it works locally and spares the body from side effects. This lets us use certain agents topically that we'd never want to use inside the body. The classic example is mupirocin, an excellent cream for impetigo and for clearing MRSA from the nose, kept almost exclusively topical so that resistance doesn't emerge where it matters systemically. Fusidic acid is similar. For minor wounds and burns, agents like topical antiseptics and silver sulfadiazine reduce the bacterial load. And for the eye, drops of chloramphenicol or fluoroquinolones treat bacterial conjunctivitis — chloramphenicol, a drug considered too toxic for routine systemic use, is perfectly safe as an eye drop because so little is absorbed. That's the recurring theme: topical use can rehabilitate a drug whose systemic toxicity or resistance risk rules it out for pills.
The limits and the traps
Topical treatment only works when the infection really is superficial and localised. A spreading cellulitis, an abscess, or any infection with systemic signs (fever, feeling unwell) needs a systemic drug — a cream cannot reach deep or bloodborne bacteria. Two other traps are worth knowing. First, avoid using topical versions of antibiotics that are precious systemically (you won't see topical vancomycin or ciprofloxacin creams handed out casually) because widespread skin use breeds resistance. Second, topical neomycin (found in many over-the-counter antibiotic ointments) is a common cause of contact allergic dermatitis — an itchy rash that can be mistaken for worsening infection. And a special mention for the eye: never use a topical steroid-containing preparation for a red eye without knowing the cause, because if the problem is a herpes simplex infection of the cornea, a steroid can make it dramatically worse. Topical, in short, is powerful precisely because it's local — respect that boundary and it's one of the safest tools in medicine.
- Topical delivers high local drug levels with minimal systemic absorption — few body-wide effects.
- Mupirocin (impetigo, MRSA nasal clearance) and fusidic acid are kept largely topical to protect them.
- Chloramphenicol eye drops are safe because so little is absorbed (unlike systemic use).
- Topical fails for deep or systemic infection — cellulitis/abscess/fever need systemic drugs.
- Neomycin commonly causes contact dermatitis; never put a steroid in a red eye of unknown cause.
There's a beautiful pharmacokinetic idea hiding in topical treatment: route can rescue a drug. Chloramphenicol taken by mouth can cause a rare but fatal bone-marrow failure, which is why we almost never use it systemically — yet as an eye drop it's a cheap, safe first-line treatment, because the amount absorbed into the body is negligible. The drug didn't change; the route did. It's the mirror image of the first-pass and bioavailability lessons from the kinetics chapter — where a drug goes, and how much reaches the bloodstream, can matter more than the drug itself.
- Using a cream for cellulitis or an abscess — deep/spreading infection needs systemic treatment.
- Overusing topical antibiotics on minor wounds — breeds resistance and causes dermatitis.
- Putting a steroid-containing drop in a red eye without a diagnosis — dangerous if herpes keratitis.
- Assuming 'topical = harmless' — allergy, resistance, and misdiagnosis are real risks.
Why is mupirocin deliberately reserved for topical use?
- Topical = high local concentration, minimal systemic absorption, few body-wide effects.
- Mupirocin/fusidic acid kept topical to protect them; chloramphenicol eye drops are safe.
- Topical fails for deep/systemic infection — cellulitis, abscess, or fever need systemic drugs.
- Watch neomycin contact dermatitis; never steroid-drop a red eye of unknown cause.
- Katzung BG. Basic & Clinical Pharmacology — Dermatologic & topical antimicrobials.
- Brunton LL, et al. Goodman & Gilman's The Pharmacological Basis of Therapeutics — Topical antimicrobials.
- Rang HP, Dale MM, et al. Rang & Dale's Pharmacology — Antibacterial agents.
- BNF — Skin & eye anti-infective preparations.
