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Cutaneous adverse drug reactions (drug eruptions)
Immediate or delayed: recognise, date and prepare for assessment
In this sheet
Three essential points
The date is almost as important as the medicine name
For each treatment, record the start date, date of the last dose and date of the first symptom. The expected delay varies greatly with the type of reaction.
Immediate reaction and delayed drug eruption: two broad groups
Immediate reactions include hives, angioedema and erythema, sometimes as part of anaphylaxis. Delayed reactions include maculopapular exanthem, FDE, AGEP, DRESS, vasculitis and SJS/TEN.
Skin plus systemic signs means look for a severe form
Skin pain, blisters, skin detachment, mucosal involvement, marked facial swelling, high fever, purpura, reduced urine output, jaundice or breathlessness require prompt medical assessment.
Immediate and delayed reactions
Immediate reactions
- Type
- Hives / angioedema
- Appearance / symptoms
- Transient wheals and swelling; sometimes widespread itching.
- Approximate delay
- Minutes to about one hour, sometimes up to six hours.
- Type
- Systemic reaction / anaphylaxis
- Appearance / symptoms
- Skin signs with breathing difficulty, faintness, low blood pressure or several organs affected.
- Approximate delay
- Usually rapid after exposure.
| Type | Appearance / symptoms | Approximate delay |
|---|---|---|
| Hives / angioedema | Transient wheals and swelling; sometimes widespread itching. | Minutes to about one hour, sometimes up to six hours. |
| Systemic reaction / anaphylaxis | Skin signs with breathing difficulty, faintness, low blood pressure or several organs affected. | Usually rapid after exposure. |
Important
Anaphylaxis is not a simple skin drug eruption: it is a systemic reaction. The skin may be affected, but the emergency is caused by respiratory or cardiovascular signs.
Delayed reactions: main timing points
- Reaction
- Fixed drug eruption (FDE)
- Usual delay during a first episode
- Often within a few hours to 48 hours.
- Reaction
- Maculopapular exanthem
- Usual delay during a first episode
- About 4–14 days.
- Reaction
- Acute generalised exanthematous pustulosis (AGEP)
- Usual delay during a first episode
- Often 1–12 days, sometimes very rapidly with certain antibiotics.
- Reaction
- Drug-induced cutaneous vasculitis
- Usual delay during a first episode
- Often 7–21 days.
- Reaction
- SJS / TEN
- Usual delay during a first episode
- Typically 4–28 days.
- Reaction
- DRESS
- Usual delay during a first episode
- Often 2–8 weeks.
| Reaction | Usual delay during a first episode |
|---|---|
| Fixed drug eruption (FDE) | Often within a few hours to 48 hours. |
| Maculopapular exanthem | About 4–14 days. |
| Acute generalised exanthematous pustulosis (AGEP) | Often 1–12 days, sometimes very rapidly with certain antibiotics. |
| Drug-induced cutaneous vasculitis | Often 7–21 days. |
| SJS / TEN | Typically 4–28 days. |
| DRESS | Often 2–8 weeks. |
The main presentations to recognise
Maculopapular exanthem: the most common
This often appears as symmetrical small red macules and papules, beginning on the trunk and sometimes spreading to the limbs. It may itch. Most uncomplicated drug exanthems improve after the responsible medicine is stopped, but some signs should prompt investigation for a more severe form.
DRESS: the skin is only part of the problem
DRESS (Drug Reaction with Eosinophilia and Systemic Symptoms) often combines an extensive rash, fever, facial swelling, enlarged lymph nodes, blood abnormalities and involvement of one or more organs. The liver is the most commonly affected internal organ.
- Full blood count: eosinophilia, atypical lymphocytes or other haematological abnormalities.
- Liver: raised transaminases and sometimes bilirubin.
- Kidneys: raised creatinine or urine abnormalities; less often cardiac or pulmonary involvement.
AGEP: numerous, rapidly developing pustules
Acute generalised exanthematous pustulosis causes many small sterile pustules on red skin, sometimes with fever. Neutrophilia is common, sometimes with eosinophilia. Pustular psoriasis is an important differential diagnosis.
SJS / TEN: a dermatological emergency
Stevens-Johnson syndrome and toxic epidermal necrolysis cause painful skin, blisters or detachment and significant mucosal involvement, particularly of the mouth, eyes or genitals, with fever and deterioration in general condition.
Emergency
Painful skin plus blisters or detachment, or mucosal involvement after a medicine, requires urgent medical assessment. The suspected medicine must never be reintroduced at home.
Why are blood tests important?
- Test
- Full blood count
- What it may investigate
- Eosinophilia, neutrophilia, atypical lymphocytes or cytopenias.
- Test
- AST / ALT / bilirubin
- What it may investigate
- Liver involvement.
- Test
- Creatinine / eGFR
- What it may investigate
- Kidney involvement.
- Test
- Urine dipstick / urine tests
- What it may investigate
- Haematuria or proteinuria, particularly with vasculitis.
- Test
- Targeted tests
- What it may investigate
- According to cardiac, pulmonary or other organ involvement.
| Test | What it may investigate |
|---|---|
| Full blood count | Eosinophilia, neutrophilia, atypical lymphocytes or cytopenias. |
| AST / ALT / bilirubin | Liver involvement. |
| Creatinine / eGFR | Kidney involvement. |
| Urine dipstick / urine tests | Haematuria or proteinuria, particularly with vasculitis. |
| Targeted tests | According to cardiac, pulmonary or other organ involvement. |
Vasculitis, fixed drug eruption and differential diagnoses
Drug-induced vasculitis: do not overlook it
Cutaneous small-vessel vasculitis classically presents with palpable purpura, often mainly on the legs. Other appearances are possible: petechiae, haemorrhagic blisters, nodules or ulcers.
- Vasculitis may remain limited to the skin.
- Joint pain, abdominal pain, haematuria, proteinuria or kidney involvement require broader assessment.
- When medicine-induced, the delay is often 7–21 days after treatment begins.
A skin biopsy with histology, sometimes supplemented by immunofluorescence, often has a central role in diagnosing vasculitis.
Fixed drug eruption (FDE): ‘always in the same place’
FDE causes one or more well-demarcated red or violaceous patches, sometimes with blisters, which return at the same site after re-exposure. Brown pigmentation may persist after healing, but is not obligatory.
It may not be the medicine
Depending on appearance and context, differential diagnoses include viral or bacterial infection, spontaneous hives, infection-related erythema multiforme, pustular psoriasis, autoimmune or infectious vasculitis, blistering disease, eczema, and inflammatory or autoimmune disease.
Why is a skin biopsy sometimes needed?
A small biopsy may help distinguish several drug eruptions, investigate vasculitis, confirm epidermal necrosis or distinguish AGEP from pustular psoriasis. It mainly helps define the type of reaction; on its own it generally does not identify the responsible medicine.
Timeline and causality: reconstruct every exposure
Not every medicine taken during the previous two months should be considered in the same way. Each treatment is placed on a timeline, then its delay is compared with the observed reaction type.
Do not omit any injection or one-off exposure
- Contrast medium for CT or MRI.
- Injected antibiotic, infusion, chemotherapy or biological medicine.
- Anaesthetic or perioperative medicine.
- Vaccine, medicine given in the emergency department or a single-dose treatment.
- Over-the-counter medicine, herbal treatment or dietary supplement.
Contrast media: an often-forgotten exposure
Iodinated contrast media may cause immediate reactions as well as delayed reactions, often affecting the skin, beginning more than one hour and up to several days after injection. Reactions also occur with some gadolinium-based contrast media.
The ideal record before consultation
- EXACT name of every medicine, dose, indication, start date and end date.
- Date and time of the first lesions; ideally the time of the last dose.
- Photographs of the rash on different days, prescriptions and emergency or hospital reports.
- Blood results, any biopsy, injections, contrast media and anaesthetic treatments.
- Over-the-counter medicines, painkillers, NSAIDs, paracetamol, supplements and herbal treatments.
Management principle
When a medicine is sufficiently suspect, stopping it is often central. However, do not change essential or life-saving treatment on your own: if a significant drug eruption is suspected, seek medical advice promptly. Depending on severity, local care, symptomatic treatment, corticosteroids or specialist hospital admission may be needed.
After assessment: what the patient should receive
The responsible or suspected medicine name, level of certainty, related medicines to avoid if necessary, permitted alternatives, a written report and sometimes a drug-allergy card.
When and how is allergy assessment performed?
The right time to test
- Situation
- Immediate reaction
- Timing guide
- Ideally about 4–6 weeks after the reaction; where possible within six months when test sensitivity declines over time.
- Situation
- Non-severe delayed reaction
- Timing guide
- Often at least 4–6 weeks after the rash has completely resolved.
- Situation
- DRESS
- Timing guide
- Generally wait at least six months, with full recovery and no ongoing relapse.
| Situation | Timing guide |
|---|---|
| Immediate reaction | Ideally about 4–6 weeks after the reaction; where possible within six months when test sensitivity declines over time. |
| Non-severe delayed reaction | Often at least 4–6 weeks after the rash has completely resolved. |
| DRESS | Generally wait at least six months, with full recovery and no ongoing relapse. |
Which tests can be used?
- Test
- Skin prick tests
- Main use
- Some immediate reactions.
- Test
- Immediate intradermal testing (IDT)
- Main use
- Some immediate hypersensitivity reactions, according to the medicine and validated concentration.
- Test
- Delayed intradermal tests
- Main use
- Some delayed reactions.
- Test
- Drug patch tests
- Main use
- Delayed exanthems, AGEP, DRESS and certain other presentations.
- Test
- Lesional patch tests
- Main use
- Particularly useful in some FDE.
- Test
- Targeted laboratory tests
- Main use
- Available only for certain medicines or mechanisms; BAT and LTT/ELISpot in selected situations.
- Test
- Drug challenge test
- Main use
- Reference test for some low-risk reactions when necessary and sufficiently safe.
| Test | Main use |
|---|---|
| Skin prick tests | Some immediate reactions. |
| Immediate intradermal testing (IDT) | Some immediate hypersensitivity reactions, according to the medicine and validated concentration. |
| Delayed intradermal tests | Some delayed reactions. |
| Drug patch tests | Delayed exanthems, AGEP, DRESS and certain other presentations. |
| Lesional patch tests | Particularly useful in some FDE. |
| Targeted laboratory tests | Available only for certain medicines or mechanisms; BAT and LTT/ELISpot in selected situations. |
| Drug challenge test | Reference test for some low-risk reactions when necessary and sufficiently safe. |
A negative test does not always clear the medicine
Test sensitivity varies greatly according to the medicine and type of drug eruption. Results must always be interpreted with the timeline and clinical phenotype.
FDE: sometimes test on the previous lesion
In fixed drug eruption, a patch test may perform better when placed on the previously affected area rather than on skin that never had the lesion. These investigations are performed only by a specialist team.
What about a drug challenge?
A challenge may be the best way to demonstrate or exclude hypersensitivity in appropriate situations. It is not indicated after every drug eruption. Severe high-risk reactions require a much more cautious strategy, and the suspected medicine is generally not readministered.
Never test a suspected medicine on your own
Deliberate reintroduction at home is dangerous, especially after a severe reaction. The decision to perform a challenge belongs to a specialist team after risk stratification.
Common myths
- Myth
- ‘Every rash while taking an antibiotic is an allergy.’
- SOS Allergo message
- False: infections and other causes may mimic a drug eruption.
- Myth
- ‘A medicine taken for a month can no longer be responsible.’
- SOS Allergo message
- False: DRESS typically occurs after several weeks.
- Myth
- ‘If I stopped the medicine before the rash, it is innocent.’
- SOS Allergo message
- False: some reactions may appear or progress after stopping.
- Myth
- ‘Negative IgE means the medicine is innocent.’
- SOS Allergo message
- False: many delayed drug eruptions are not IgE-mediated.
- Myth
- ‘Every test must be done immediately.’
- SOS Allergo message
- False: it is generally necessary to wait for recovery.
- Myth
- ‘Contrast medium means iodine allergy.’
- SOS Allergo message
- This is inappropriate wording: the precise contrast molecule must be identified.
- Myth
- ‘Purpura while taking a medicine is a simple drug eruption.’
- SOS Allergo message
- No: vasculitis must be considered.
| Myth | SOS Allergo message |
|---|---|
| ‘Every rash while taking an antibiotic is an allergy.’ | False: infections and other causes may mimic a drug eruption. |
| ‘A medicine taken for a month can no longer be responsible.’ | False: DRESS typically occurs after several weeks. |
| ‘If I stopped the medicine before the rash, it is innocent.’ | False: some reactions may appear or progress after stopping. |
| ‘Negative IgE means the medicine is innocent.’ | False: many delayed drug eruptions are not IgE-mediated. |
| ‘Every test must be done immediately.’ | False: it is generally necessary to wait for recovery. |
| ‘Contrast medium means iodine allergy.’ | This is inappropriate wording: the precise contrast molecule must be identified. |
| ‘Purpura while taking a medicine is a simple drug eruption.’ | No: vasculitis must be considered. |
Before your consultation: the SOS Allergo timeline
- Exact name and indication
- Start date
- Date of last dose
- Dose
- First skin symptom
- Fever / other symptoms
- Date treatment was stopped
- Improvement from
Final message
With a cutaneous drug reaction, the question is not only ‘which medicine did I take?’ but ‘which treatment did I start, stop or receive by injection, on which date, and how long before the first symptom?’ A precise timeline, photographs and laboratory results can turn a difficult account into a genuine diagnosis.
You may also want to read
Keep the patient sheet
Download the validated patient document in French to read again or prepare for your consultation.
Main references
- EAACI position paper on classification of cutaneous manifestations of drug hypersensitivity. Allergy. 2019.
- Barbaud A, et al. EAACI/ENDA position paper on drug provocation testing. Allergy. 2024.
- EAACI nomenclature / classification update on immediate and delayed drug reactions. Allergy. 2026.
- Tetart F, et al. Acute generalized exanthematous pustulosis: European expert consensus. JEADV. 2024.
- Barbaud A, et al. Updated EAACI Statement on Drug Hypersensitivity Skin Testing. Allergy. 2026.
- Alpsoy E, et al. Recommendations for diagnostic work-up of cutaneous small vessel vasculitis. JEADV. 2026.
- Brockow K, et al. Guideline for allergological diagnosis of drug hypersensitivity reactions. Allergo J Int. 2023.
- Torres MJ, et al. Practice parameters for diagnosing and managing iodinated contrast media hypersensitivity. Allergy. 2021.
- Barbaud A, et al. Skin tests in the work-up of cutaneous adverse drug reactions. Contact Dermatitis. 2022.
This sheet provides information and helps prepare for a consultation. It does not replace medical advice or instructions from a specialist team.
Scientific review: September 2026 · sosallergo.fr