Fixed Drug Eruption

Exanthema Fixtum — Drug-induced cutaneous adverse reaction characterized by sharply demarcated erythematous or violaceous patches/plaques that recur at the same site every time the offending drug is re-administered

Aetiology

Many drugs may trigger FDE, commonly:
Most common
  • NSAIDs → aspirin, ibuprofen, naproxen, mefenamic acid
  • Antibiotics → sulfonamides, tetracyclines, cotrimoxazole, penicillin
  • Anticonvulsants → phenytoin, carbamazepine
  • Others → barbiturates, allopurinol
Non-drug triggers
  • Rarely foods, food additives, coloring agents

Pathogenesis

  • Type IV (delayed) hypersensitivity reaction
  • Memory CD8+ T cells remain in the epidermis at the lesional site after first exposure
  • Re-exposure to the drug → rapid activation of memory T cells → local inflammation → recurrent lesion in same spot

Key mechanism points

Phase
Event
Sensitization
First exposure → immune priming
Latency
Drug-specific T cells persist in skin
Re-exposure
Rapid response → localized tissue damage → FDE recurrence

Clinical presentation

  • Well-demarcated round/oval erythematous patches
  • May evolve into plaques, blisters, or bullae
  • Burning or itching sensation
  • Lesions recur at identical sites with re-exposure
  • Healing leaves persistent hyperpigmentation
  • Onset after exposure:
    • First exposure: 1–2 weeks
    • Re-exposure: minutes to hours (rapid)
 
notion image

Common Sites

  • Lips
  • Genitalia
  • Hands and feet
  • Face
  • Trunk
  • Mucosa: oral, genital

Variants

  • Generalized FDE (multiple lesions)
  • Bullous FDE (blisters, may mimic SJS/TEN)
  • Mucosal FDE

Investigations

Clinical diagnosis is usually sufficient

Management

Immediate discontinuation of the offending drug

Most important step

Symptomatic treatment

Treatment
Use
Topical corticosteroids
Reduce inflammation during acute phase
Oral antihistamines
Relieve pruritus
Systemic corticosteroids
For severe/bullous/generalized FDE

Hyperpigmentation care

  • Typically fades slowly over months
  • Sunscreen and depigmenting agents may help, but often not required

Patient education

  • Avoid causative drug permanently
  • Document drug allergy clearly
  • Warn patient about cross-reacting medications when relevant