From the Guidelines
An amoxicillin rash is a benign, non-allergic skin reaction that typically appears as flat, red spots, usually developing 3-10 days after starting treatment, and is characterized by its non-itchy and non-painful nature. When describing an amoxicillin rash, it is essential to note its appearance, distribution, timing, and associated symptoms to differentiate it from a true allergic reaction 1. The rash often begins on the trunk before spreading to the face, arms, and legs, and its characteristics can be summarized as follows:
- Appearance: flat, red, non-raised spots
- Distribution: starts on the trunk and spreads to the face, arms, and legs
- Timing: develops 3-10 days after starting amoxicillin treatment
- Associated symptoms: lack of itching, fever, or other concerning symptoms
It is crucial to distinguish this benign reaction from a true allergic reaction, which would present with hives, itching, swelling, difficulty breathing, or other systemic symptoms 1. The amoxicillin rash occurs in about 5-10% of patients taking the medication and is more common in patients with certain viral infections, particularly Epstein-Barr virus (mononucleosis) 1. This reaction is caused by a temporary, non-allergic interaction between the drug and the immune system rather than a true allergy, though it should still be documented in the patient's medical record. According to the most recent study, there is no clear evidence of cross-reactivity between amoxicillin and other antibiotics, such as cephalosporins, in patients with a history of penicillin allergy 1.
From the FDA Drug Label
- 2 Severe Cutaneous Adverse Reactions Amoxicillin may cause severe cutaneous adverse reactions (SCAR), such as Stevens-Johnson syndrome (SJS), toxic epidermal necrolysis (TEN), drug reaction with eosinophilia and systemic symptoms (DRESS), and acute generalized exanthematous pustulosis (AGEP) If patients develop skin rash they should be monitored closely, and amoxicillin discontinued if lesions progress.
- 5 Skin Rash in Patients with Mononucleosis A high percentage of patients with mononucleosis who receive amoxicillin develop an erythematous skin rash. Skin and Appendages:Rashes, pruritus, urticaria, erythema multiforme, SJS, TEN, DRESS, AGEP, exfoliative dermatitis [see Warnings and Precautions (5. 2)].
The amoxicillin rash can be described as:
- Erythematous (red)
- Severe cutaneous adverse reactions (SCAR), including:
- Stevens-Johnson syndrome (SJS)
- Toxic epidermal necrolysis (TEN)
- Drug reaction with eosinophilia and systemic symptoms (DRESS)
- Acute generalized exanthematous pustulosis (AGEP)
- Erythema multiforme
- Exfoliative dermatitis 2
From the Research
Description of Amoxicillin Rash
- The amoxicillin rash is often a result of non-immediate hypersensitivity, which can occur in children after oral administration of the medication 3.
- The rash can be mild or moderate and may not have any significant distinguishing features, making diagnosis challenging 3.
- The characteristics of patients with a positive allergic workup for amoxicillin hypersensitivity do not differ significantly from those without hypersensitivity, including age, sex, history of allergic disease, and type of rash 3.
Types of Rashes Associated with Amoxicillin
- Maculopapular rashes, urticaria, angioedema, contact dermatitis, anaphylaxis, and fixed drug eruption (FDE) have been reported as potential reactions to medications, although these are not specific to amoxicillin 4, 5.
- The onset of reactions can vary from a few minutes to several hours or longer, depending on the type of reaction 5.
Diagnosis and Testing
- Diagnosis of amoxicillin hypersensitivity is based on clinical suspicion and may be verified by challenge testing, such as intradermal tests (IDTs) and drug provocation tests (DPTs) 3.
- Patch testing may be used to diagnose contact dermatitis or fixed drug eruption, but other tests have not shown optimal reliability 5.