Morbilliform drug eruption

Also called exanthematous drug eruption, maculopapular drug rash

The most common drug rash: symmetric erythematous macules and papules starting on the trunk 4–14 days after a new drug, becoming confluent, itchy but benign, provided the danger checklist is negative.

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How it looks

Symmetric, blanching erythematous macules and papules coalescing over the trunk and spreading to the proximal limbs, sparing mucosae, in a patient started on amoxicillin nine days ago.

Macule Papule Erythema Confluent Truncal Generalized

Rose is the lesion, grey the pattern the lesions make, green where on the body it lives.

Across skin tones

In deeply pigmented skin the eruption reads violaceous-to-dusky rather than pink and may be easier to palpate than to see; post-inflammatory pigment change commonly follows.

Don't miss

The same early rash can be the opening scene of DRESS or SJS/TEN, mucosae, skin pain, and fever are the tripwires that change everything.

Forms & variants

Psoriasiform drug eruption
Drug-provoked scaly psoriasis-like plaques (beta-blockers, lithium, antimalarials, and paradoxically TNF inhibitors); suspect it when 'psoriasis' first appears on a new drug.

Symptoms & course

Itch
Timing: 4–14 days after a new drug (sooner on re-exposure)

Diagnostic approach

Run the danger checklist: face oedema, mucosae, skin pain, blistering, fever, lymphadenopathy, eosinophilia, liver/renal function
A careful drug timeline IS the investigation

Differential, and how to separate them

Urticaria
Favours it: Individual wheals migrate within hours
Against it: Morbilliform lesions are fixed for days
Stevens–Johnson syndrome / toxic epidermal necrolysis
Favours it: Skin pain, dusky centres, blisters, mucosal erosions, fever
Against it: A benign morbilliform rash has none of these, check every time

Management principles

Stop or substitute the culprit where feasible
Topical corticosteroids and antihistamines for symptoms
The rash may take 1–2 weeks to fade after stopping

Clinical pearl: Build the timeline drug-by-drug on paper: the culprit was usually started 1–2 weeks before onset, not the drug given yesterday.

Keep going

Try the case: Day eight of the antibiotic Compare with Urticaria Back to the Atlas