Yes, erythema nodosum can absolutely occur as a drug-induced hypersensitivity reaction.
Erythema nodosum (EN) is a well-recognized manifestation of drug hypersensitivity, representing a delayed hypersensitivity reaction that can be triggered by various medications. While the 2022 Drug Allergy Practice Parameter 1 does not specifically list EN among the cutaneous drug reactions discussed (focusing instead on maculopapular exanthems, DRESS, SJS/TEN, and AGEP), multiple case reports and clinical reviews confirm drugs as an established cause of EN.
Key Clinical Points
Drug-Induced EN is Well-Documented
EN functions as a form of septal panniculitis believed to represent a delayed hypersensitivity reaction to various antigenic stimuli 2. Drugs account for approximately 3-10% of all EN cases 3, making medication exposure an important consideration in the differential diagnosis.
Commonly Implicated Medications
Several drug classes have been definitively linked to EN through case reports with positive rechallenge:
Azathioprine/6-mercaptopurine: Multiple documented cases with EN appearing 8-18 days after drug initiation, accompanied by high fever and elevated inflammatory markers. Rechallenge consistently reproduces lesions within hours 4, 5
Isotretinoin: EN can develop within 2 weeks of starting therapy, even in patients without acne (eliminating acne itself as the trigger) 6
Methimazole: EN reported 15 days after treatment initiation 3
Omeprazole: Confirmed by positive rechallenge 7
NSAIDs: While the 2022 guideline 1 extensively discusses NSAID-induced cutaneous reactions including FDE, DRESS, SJS/TEN, and AGEP, EN is not specifically mentioned in this context
Temporal Pattern
Drug-induced EN typically manifests 8-18 days after medication initiation 3, 4, consistent with a delayed hypersensitivity mechanism. This timing is crucial for establishing causality.
Diagnostic Approach
When evaluating EN, systematically assess for:
- Recent medication exposure (within 2-3 weeks)
- Infectious triggers (streptococcal infections, tuberculosis, histoplasmosis in endemic areas 8)
- Systemic diseases (sarcoidosis, Behçet disease, inflammatory bowel disease)
- Pregnancy
The diagnosis of drug-induced EN is strengthened by:
- Temporal relationship between drug initiation and EN onset
- Resolution upon drug discontinuation (typically within 2 weeks 5)
- Recurrence with rechallenge (though rechallenge is not recommended for diagnostic purposes given the morbidity)
Management
Immediate discontinuation of the suspected culprit drug is the primary intervention. EN lesions typically resolve within 2 weeks of drug withdrawal 5. Symptomatic treatment addresses the nodules themselves but does not treat the underlying trigger 2.
Clinical Pitfall
In patients with inflammatory bowel disease or other systemic conditions, drug-induced EN (particularly from azathioprine) can mimic disease flare 4. The key distinguishing feature is rapid recurrence within hours upon rechallenge, indicating hypersensitivity rather than disease activity.