Fixed Drug Eruption
The skin lesion you are describing is a Fixed Drug Eruption (FDE), which characteristically presents as a singular nodule or localized lesion that recurs at the same anatomic site upon re-exposure to the offending antibiotic. 1, 2
Clinical Characteristics
FDEs are unique cutaneous adverse drug reactions that present with the following features:
Lesions appear as solitary or multiple well-demarcated erythematous macules, plaques, or nodules that can evolve to form vesicles or bullae 2
The average lag period between drug intake and appearance of FDE is approximately 2 days, though patients with prior FDE history may develop lesions within minutes to hours of re-exposure 2
Commonly affected sites include the extremities, lips, head and neck, and genitalia, with nearly half of patients (46%) presenting with a single lesion 2
Upon re-exposure to the same drug, the lesion recurs at the exact same anatomic location, which is the pathognomonic feature distinguishing FDE from other drug eruptions 2
Causative Antibiotics
Antimicrobials are the most frequent cause of FDEs, accounting for 80.6% of cases:
Beta-lactam antibiotics (penicillins and cephalosporins) are among the most common culprits 1, 3, 2
Sulfonamide antibiotics, particularly trimethoprim-sulfamethoxazole, are well-documented causes 1, 4
All causative drugs in FDE cases are administered via the oral route 2
Diagnostic Considerations
Delayed skin testing (intradermal testing and patch testing) has poor sensitivity for FDE, making clinical history the primary diagnostic tool 1
Key historical features that increase likelihood of true FDE include:
History of previous FDE (present in 50.2% of cases) - patients with prior FDE are significantly more likely to develop rapid-onset lesions (84% develop symptoms within minutes to hours) 2
Patients with multiple lesions are more likely to have a history of FDE (66.7%) compared to those with single lesions (34.8%) 2
Management Approach
Immediate discontinuation of the suspected antibiotic is essential 3, 5
Document this as a drug allergy in the medical record to prevent future re-exposure, as subsequent exposures typically result in more extensive and multiple lesions 1, 2
For alternative antibiotic selection in patients with beta-lactam-related FDE:
Structurally different antibiotics should be selected - patients with penicillin or cephalosporin-related severe cutaneous reactions typically tolerate quinolones, glycopeptides, and carbapenems 3
Cross-reactivity risk depends on molecular similarity of side chains rather than the beta-lactam ring alone 1
Critical Pitfall
Do not confuse FDE with other maculopapular drug eruptions or viral exanthems - the hallmark distinguishing feature is the fixed anatomic location upon re-challenge, which occurs in FDE but not in other drug-induced rashes 1, 2. This distinction is crucial because FDE represents a definitive contraindication to future use of the culprit drug, whereas many reported "allergies" from non-specific rashes are actually mislabeling of non-allergic reactions 1, 6.