Fixed Drug Eruption (FDE)

Fixed Drug Eruption (FDE) is a distinctive cutaneous adverse drug reaction characterized by recurrence of lesions at exactly the same anatomical site whenever the patient is re-exposed to the causative medication.

It is one of the most common drug eruptions seen in outpatient dermatology and typically leaves persistent post-inflammatory hyperpigmentation after healing.

NOTE: A solitary hyperpigmented lesion on genitalia should always raise suspicion for FDE.

Etiology

Common offending drugs

• Sulfonamides (e.g., cotrimoxazole)

• Tetracyclines

• Metronidazole

• NSAIDs (diclofenac, ibuprofen, piroxicam)

• Paracetamol

• Antimalarials

• Barbiturates

• Anticonvulsants

Pathophysiology

• Drug acts as a hapten binding keratinocytes

• Memory CD8+ T-cells remain in the affected skin

• Re-exposure → rapid localized cytotoxic response

• Epidermal damage → erythema → blister → pigmentation

Key concept:Lesion “remembers” the drug exposure site.

Risk Factors

• Previous drug reaction history

• Repeated self-medication

• HIV infection

• Polypharmacy

• Frequent antibiotic or analgesic use

Signs and Symptoms

• Well-defined red-brown patch or plaque

• Edematous papule initially

• Becomes darker over hours to days

• Often multiple lesions

• Usually 5–10 cm diameter but may enlarge

• Sometimes bullous

• Burning or itching sensation

• Heals with dark hyperpigmentation

Common sites

• Genitalia (most characteristic)

• Lips

• Palms and soles

• Oral mucosa

• Trunk and limbs

Diagnostic Criteria

Clinical diagnosis based on:

• Recurrent lesion at identical site

• Red-brown patch or plaque

• Possible bullous change

• Predilection for genitalia/palms/soles/mucosa

• Post-inflammatory hyperpigmentation after healing

Investigations

Usually clinical — tests only when uncertain.

Optional tests

• Skin biopsy → interface dermatitis with necrotic keratinocytes

• Patch testing (after healing) to identify culprit drug

• Drug challenge test (specialist supervision only)

Treatment

Non-Pharmacological Treatment

• Immediate discontinuation of offending drug

• Avoid re-exposure permanently

• Patient education and drug allergy card

• Cool compresses for symptom relief

Pharmacological Treatment

Topical therapy

• Topical corticosteroids (eczema-strength potency)

• Speeds healing and reduces inflammation

Systemic therapy (moderate/severe cases)

• Prednisolone OR Hydrocortisone short course

• Oral antihistamines for itching

Bullous lesions

• Treat like superficial burns

• Prevent secondary infection

Complications

• Persistent hyperpigmentation

• Extensive bullous eruption

• Secondary infection

• Rare generalized FDE (severe form)

Prevention

• Avoid culprit medication permanently

• Document allergy in medical records

• Provide written drug-allergy warning

• Avoid self-medication

• Cross-reacting drug caution