Drug Eruptions

Overview – Drug Eruptions

Drug eruptions are adverse skin reactions to medications, most commonly presenting as morbilliform or urticarial rashes. They are among the most frequent cutaneous presentations seen in hospital inpatients. Recognising and managing drug eruptions is essential to avoid further systemic hypersensitivity reactions and to prevent progression to severe conditions like Stevens-Johnson syndrome.

Morbilliform Drug Reactions

Aetiology

• Type IV hypersensitivity reaction
• Commonly caused by:
  • Antibiotics (e.g. penicillins, sulfonamides)
  • Antiepileptics
  • Allopurinol

Epidemiology

• Most common type of drug eruption

Pathophysiology

• Cytotoxic T-cell-mediated immune response
• Leads to cytokine release → inflammation in dermis and epidermis

Clinical Features

• Maculopapular exanthem:
  • Begins on trunk, then spreads to limbs and neck
  • Symmetrical distribution
  • May be annular, targetoid, or urticarial-like
• Onset typically:
  • 1–2 weeks after initiating drug
  • Can appear up to 1 week after stopping the drug
  • Faster upon re-exposure

Management

• Cease the offending medication
• Symptom control:
  • Topical corticosteroids
  • Antihistamines (limited benefit)
  • Emollients
• Monitor for systemic involvement or signs of more severe reaction

Urticarial Drug Reactions

Aetiology

• Type I hypersensitivity reaction
• Common triggers:
  • Drugs (e.g. NSAIDs, penicillins)
  • Foods
  • Insect stings
  • Plants

Pathophysiology

• IgE-mediated mast cell degranulation upon re-exposure to antigen
• Release of histamine and inflammatory mediators
• Results in dermal oedema and vasodilation

Morphology

Gross

• Raised erythematous papules or plaques (wheals)

Microscopic

• Dermal oedema
• Dilated dermal blood vessels
• Perivascular inflammatory infiltrate:
  • Lymphocytes
  • Neutrophils
  • Eosinophils
• Normal epidermis (no spongiosis)

Clinical Features

• Affects trunk and extremities
• Lesions are transient (<24 hours each), but full eruption may last days
• Seen in all ages, more common in 20–40 years

Management

• Identify and stop causative agent
• Symptomatic relief:
  • Cold compresses
  • Oral antihistamines (e.g. cetirizine, loratadine)
• Severe or persistent:
  • Short course of oral corticosteroids
• If anaphylaxis or throat swelling:
  • IM adrenaline

Summary – Drug Eruptions

Drug eruptions are common hypersensitivity reactions triggered by medications. Morbilliform drug reactions are the most frequent, presenting as a maculopapular rash, whereas urticarial drug reactions involve transient wheals and may signal a more immediate allergic response. Prompt identification and cessation of the offending drug is crucial. For related topics, see our Skin & Dermatology Overview page.