Topical Corticosteroids

Inflammatory skin diseases — eczema, psoriasis, contact dermatitis — are driven by an overactive local immune response: immune cells pour in, blood vessels dilate, and the result is the familiar redness, swelling, itch, and scaling. A topical corticosteroid is a glucocorticoid applied right where the trouble is. It diffuses into skin cells, binds the glucocorticoid receptor, and travels to the nucleus to reprogram gene transcription — turning down the inflammatory signals (via lipocortin/phospholipase-A2 inhibition and NF-κB suppression) that produce prostaglandins, leukotrienes, and cytokines.

The effects are fourfold and explain both the benefit and the harm. It is anti-inflammatory (less redness/swelling), immunosuppressive (fewer active immune cells), vasoconstrictive (constricting dermal vessels — the very effect used to grade potency), and antiproliferative (slowing the growth of keratinocytes and fibroblasts). That last property is a double-edged sword: it helps thick, scaly plaques, but sustained it also thins the skin — the root of the local side effects.

Topical steroids are ranked into seven potency classes, from Class 1 (superpotent — e.g., clobetasol) down to Class 7 (least potent — e.g., hydrocortisone 1–2.5%). Potency is measured by how strongly the drug blanches (vasoconstricts) skin. Higher potency clears disease faster but carries more risk — so it is reserved for thick skin (palms, soles), resistant plaques, and short courses, while low potency is chosen for delicate skin and children.

The second dial is where you apply it, because absorption varies enormously by skin thickness. Thin-skinned areas — the face, intertriginous skin folds, and genitals — absorb far more drug (the scrotum absorbs ~40× more than the forearm), as does any area under occlusion (wraps, tight diapers) and a child’s skin (high surface-area-to-weight ratio). The practical rule that falls out of both dials: use only low-potency steroids, briefly, on the face, folds, genitals, and in children, and never seal a potent steroid under occlusion unless specifically directed. Put a superpotent steroid on the face and you invite atrophy, striae, and perioral dermatitis; put it under occlusion over a large area and you risk systemic absorption.

Most side effects are local and are the antiproliferative/vasoconstrictive effects overshooting. Prolonged or high-potency use thins the skin (atrophy), tears the dermis into permanent striae (stretch marks), and leaves vessels unsupported as telangiectasia and easy bruising; on the face it causes acne, perioral dermatitis, and rosacea. A crucial one is masking or worsening infection — the immunosuppression blunts the signs of a fungal or bacterial infection (tinea incognito), so steroids are not applied to untreated infected skin. Continuous use can also cause tachyphylaxis (a fading response). Reassuringly, atrophy and telangiectasia are largely reversible, though striae are not.

The systemic effects are the same story as oral steroids, but only appear when enough drug is absorbed — with high potency, large surface area, occlusion, prolonged use, and in children. Enough absorption can suppress the HPA axis (risking adrenal insufficiency if stopped abruptly), and in children slow growth. This is exactly why the potency-and-site rules exist. Topical corticosteroids carry no boxed warning — the safety story is entirely about using the least potency that works, on the smallest area, for the shortest time, and stepping down for chronic disease.