Health condition
Drug hypersensitivity syndromes
Severe cutaneous adverse reactions - DRESS, Stevens-Johnson syndrome/toxic epidermal necrolysis and acute generalised exanthematous pustulosis - are rare, dangerous drug reactions with distinct patterns; recognising them early and stopping the culprit drug saves lives.
Why trust this guide
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Editorial team
Written by our editorial team from the UK sources listed on each page.
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Sourced from guidance
Checked against current NHS and UK specialist society guidance at the time of review.
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Current for 2026
Reviewed on 2026-09-27 with the next review scheduled for 2027-09-27.
Key facts
The essentials at a glance.
The symptoms to recognise, when to seek help, which specialist to see, and what treatment usually involves.
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Warning features that separate a dangerous reaction from a simple drug rash: facial swelling, skin pain, blistering, mucosal involvement, fever and malaise
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The big three: DRESS (rash, fever, organ involvement, 2-8 weeks after starting a drug), SJS/TEN (blistering and skin detachment), and AGEP (sheets of sterile pustules within days)
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Common culprits include anticonvulsants, allopurinol, sulfonamide antibiotics and antivirals - and reactions can start weeks after the first dose
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Immediate withdrawal of the suspected drug is the single most important intervention, followed by specialist supportive care
Understanding the condition
What it is and what happens next.
What causes it, how it is diagnosed, and the treatment routes available in the UK.
What it is and how it feels
Most drug rashes are benign, but a small subset reflect a systemic immune assault in which the skin is only the visible front: liver, kidneys, lungs and blood counts can all be involved. The timelines differ usefully - AGEP erupts within days, SJS/TEN typically at one to four weeks, DRESS at two to eight weeks - and each has HLA genetic associations that increasingly allow pre-prescription screening for high-risk drugs such as carbamazepine and allopurinol in relevant populations.
Diagnosis and treatment routes
Anyone with a suspected severe reaction needs same-day assessment: culprit identification using timelines and validated scores, immediate cessation, bloods for organ involvement, and admission for all but the mildest cases - SJS/TEN to a burns or specialist dermatology unit. Afterwards, formal allergy documentation, family counselling about cross-reactive drugs, and follow-up for late autoimmune sequelae of DRESS (notably thyroid disease) complete the pathway. In much of the UK, a routine NHS referral to the relevant specialist clinic can take several months, whereas an initial appointment with a private consultant is typically available within around two weeks.
Seeing a specialist
Who to see, and how quickly you can be seen.
The specialist for this condition is usually a consultant dermatologist. Your GP can refer you on the NHS, or you can arrange a private consultation directly.
A quiet reminder
This guide is for information, not medical advice.
Your GP knows your history and can tell you which parts apply to you. If in doubt, see them - and seek urgent care for any of the emergency symptoms described above.
Frequently asked
Common questions about Drug hypersensitivity syndromes.
Quick, plain-English answers to what people ask us most.
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What are drug hypersensitivity syndromes?
Drug hypersensitivity syndromes are severe cutaneous adverse drug reactions, including DRESS, Stevens-Johnson syndrome/toxic epidermal necrolysis, and acute generalised exanthematous pustulosis, all representing serious, potentially life-threatening reactions to medication.
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What causes drug hypersensitivity syndromes?
These reactions occur when the immune system mounts a severe, delayed hypersensitivity response to a medication, with certain drugs (such as anticonvulsants, allopurinol, and some antibiotics) more commonly implicated than others.
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What are the symptoms of severe drug hypersensitivity syndromes?
Symptoms vary by specific syndrome but can include extensive skin rash or blistering, mucous membrane involvement, fever, and organ dysfunction, all representing a more severe reaction than a typical mild drug rash.
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Are drug hypersensitivity syndromes dangerous?
Yes, these are serious, potentially life-threatening reactions, particularly Stevens-Johnson syndrome/toxic epidermal necrolysis, which carries significant mortality risk, so prompt recognition and hospital-based treatment are essential.
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How are severe drug hypersensitivity syndromes treated?
Treatment always starts with immediately stopping the causative medication, alongside supportive care, and for some syndromes, specific treatments such as corticosteroids, with severe cases needing specialist burns or intensive care unit level support.
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How do these different drug hypersensitivity syndromes differ?
DRESS involves systemic organ involvement with eosinophilia over weeks, Stevens-Johnson syndrome/toxic epidermal necrolysis involves severe skin and mucous membrane blistering and shedding, and acute generalised exanthematous pustulosis causes widespread pustules - each has a distinct pattern requiring different specific management.
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How long does recovery take from severe drug hypersensitivity syndromes?
Recovery varies considerably by the specific syndrome and severity, ranging from weeks for milder presentations to months for the most severe cases like extensive toxic epidermal necrolysis, which can also leave lasting skin or eye complications.
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Can these reactions happen again with the same drug?
Yes, and the causative drug (and sometimes related drugs) should be permanently avoided once identified, with this clearly documented in medical records and communicated to all future prescribers given the severity of these reactions.
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How are these severe reactions distinguished from a simple drug rash?
The presence of mucous membrane involvement, extensive skin blistering or shedding, systemic symptoms, or organ dysfunction distinguishes these severe syndromes from a typical mild drug rash, which is why any concerning features need urgent specialist assessment.
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When should I seek help for a suspected severe drug reaction?
Seek emergency medical attention immediately for a widespread rash with blistering, mucous membrane involvement, or fever after starting a new medication - this needs urgent hospital assessment given the potential severity.