Clinical QuickieDermatology
Viral vs Drug vs Allergic Eruptions
The timeline is often more useful than the color of the rash.
Viral exanthems follow an illness pattern
Systemic viral symptoms and exposure context can accompany a diffuse morbilliform eruption.
Drug eruptions require a medication timeline
New medications, dose changes, intermittent exposures, and prior sensitization matter; uncomplicated morbilliform drug eruptions are different from severe cutaneous reactions.
Urticaria behaves differently
Transient raised wheals that migrate over hours suggest histamine-mediated urticaria rather than a fixed exanthem.
Screen for severe reactions
Mucosal involvement, blistering, skin pain, facial edema, eosinophilia/systemic organ injury, or anaphylaxis features change the pathway.
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