Drug reactions are very diverse, and below are the types of drug reactions that cause
scaly rashes:
1. Allergic contact dermatitis (eczema)
• A localized allergic contact dermatitis is caused by topical preparations to which the
patient has been sensitized by previous contact. The reaction appears within 48h of
exposure.
• Skin is itchy, oedematous, erythematous, and scaly (may not be scaly if acute). It may
blister if the reaction is severe.
• Eczema may generalize if contact persists or if the patient takes a cross-reacting oral
preparation.
• Consider allergic contact dermatitis in chronic leg ulcers, hand dermatitis, facial
dermatitis, otitis externa, or pruritus ani. Causes include topical neomycin, benzocaine,
incipients in topical medicaments, and topical hydrocortisone.
• Allergy to the constituents of subcutaneous heparin presents with well-demarcated
infiltrated eczematous plaques at injection sites.
• Treat eczema with emollients and topical corticosteroid ointments.
• Investigate possible allergic contact dermatitis by patch testing.
2. SDRIFE: symmetrical drug-related intertriginous and flexural exanthema
• A distinctive erythematous flexural reaction to systemic drugs and iodinated contrast
medium. Commonest cause: aminopenicillins.
• Latency hours to days. occurs without prior exposure to the drug.
• Itchy, erythematous papules coalesce to produce a symmetrical, well- demarcated
erythema on buttocks (baboon’s bottom) and/or V-shaped erythema of the lower
abdomen, groins, and thighs. Involves at least one other skinfold. May become bullous.
• No systemic symptoms or signs.
• Encompasses ‘baboon syndrome’, a distinct form of systemic contact dermatitis
(sensitization by skin contact, and subsequently patient takes the agent by mouth).
3. Drug-induced photosensitivity
• Drugs are the commonest cause of photosensitivity, more often phototoxicity than
photoallergy.
• Look for an eczematous rash on exposed sites, sometimes with pigmentation.
• Drugs may also cause photosensitivity by triggering PCT, pseudoporphyria, or subacute
cutaneous LE.
4. Lichenoid (lichen planus-like) reaction
• Very itchy, flat-topped, purplish red papules that may coalesce into erythematous
plaques and become generalized. often also an eczematous scaly component.
• Examine the buccal mucosa for reticulated white areas and erosions—found more often
in LP than lichenoid drug reactions.
• Resolution is slow (months) after the drug is stopped.
• The papules fade to leave macular hyperpigmentation that persists for months,
particularly in dark skin. Pigmentation tends to be more marked in lichenoid drug
reactions than in LP.
• Very potent topical corticosteroid ointments may be required to relieve itch.
• Common causes: gold, antimalarials, penicillamine, β-blockers, thiazides, NSAIDs.
Also reported with TNF-α antagonists.
5. Erythroderma (exfoliative dermatitis)
• Generalized scaly erythema—an emergency.
• Common causes: sulfonamides, carbamazepine, antimalarials, phenytoin, and gold.
6. Chronic eczematous eruptions
• Common in the elderly, but it may be difficult to identify the drug responsible.
• Calcium channel blockers (commonest cause).
