Patient information · Allergy
Penicillin allergy
most people who have the label are not allergic
Around one in ten people in the UK is labelled as penicillin-allergic. On formal testing, more than nine in ten of them are not. Carrying an incorrect label is not harmless — it means second-choice antibiotics, which work less well, cause more side effects and drive resistance.
Call 999 if, after taking any medicine, there is
- swelling of the lips, tongue, mouth or throat
- difficulty breathing, wheeze or a hoarse voice
- dizziness, collapse or loss of consciousness
- a widespread rash with feeling unwell
- blistering or peeling skin, or ulcers in the mouth, eyes or genitals
Blistering rashes, skin peeling and mucosal involvement suggest a severe delayed reaction such as Stevens-Johnson syndrome or DRESS. These are medical emergencies, and anyone who has had one must never be re-exposed or tested. In an emergency call 999, or go to the Royal London Hospital Emergency Department, Whitechapel Road, London E1 1FR.
Why so many labels are wrong
- A rash during a childhood illness — frequently the virus rather than the antibiotic. This is the single commonest reason.
- Side effects mistaken for allergy — nausea, diarrhoea, thrush and headache are side effects, not allergic reactions.
- A family member's allergy — drug allergy is not meaningfully inherited.
- Genuine allergy that has faded — around 80% of people with a true penicillin allergy lose it after ten years.
- No recollection at all — a label added decades ago that nobody has revisited.
Why the label matters more than people think
Alternatives to penicillin are generally less effective, broader spectrum and more toxic. People labelled penicillin-allergic have measurably higher rates of treatment failure, longer hospital stays, more C. difficile and MRSA infection, and worse outcomes after surgery. It also complicates treatment of syphilis in pregnancy, endocarditis and serious infection, where penicillins are genuinely the best drug. Removing an incorrect label is a real clinical improvement, not an administrative tidy-up.
What your history tells us
- What exactly happened — rash, swelling, breathing difficulty, vomiting?
- How soon after the dose — within an hour suggests a true immediate allergy; days later suggests something different.
- How long ago, and how old you were.
- What treatment was needed — nothing, antihistamine, adrenaline, admission?
- Have you had a penicillin since, knowingly or not? Many people have, without incident.
- Photographs, if any exist, are genuinely useful.
Testing
Where the history suggests a low risk, the label can often be removed after assessment, sometimes with a supervised oral challenge. Where the history is less clear, skin testing followed by a challenge is used. Testing is done in a setting equipped to manage a reaction, and is safe when the right people are selected.
Who should not be tested
- anyone who has had a severe blistering or peeling skin reaction, or DRESS
- anyone with organ involvement — liver or kidney — attributed to the drug
- those with unstable asthma or uncontrolled heart disease at the time
- people on medicines that would interfere, such as beta-blockers, until reviewed
- For these groups the label stays, and the value lies in recording exactly what happened so that alternatives are chosen sensibly.
What to do meanwhile
Keep carrying the label until it is formally removed — do not experiment. Make sure your records state what the reaction actually was, not just the word 'allergy', since that alone changes prescribing decisions for the better. If you carry an adrenaline auto-injector for it, keep doing so until told otherwise.
Why come to us. Almost nobody gets their penicillin label reviewed, because NHS allergy services are stretched and it is never urgent — until it is. We offer an unhurried allergy consultation that takes a proper history, arranges specific IgE blood testing where useful, and identifies whether you are a candidate for de-labelling. Allergy testing is performed here, and formal challenge testing is arranged through our CQC-registered partners. No referral needed, seven days a week.
Get the label reviewed before you need the antibiotic
Allergy consultation and testing on site, no referral needed.
Tower Bridge Hospital London
97–99 Whitechapel Road, London E1 1DT
WhatsApp 07903 284 189
info@mhwclinic.co.uk
Open Mon–Sat, 9am–7pm (closed Sundays until September)
In an emergency
Call 999, or go to the Royal London Hospital Emergency Department, Whitechapel Road, London E1 1FR.
When we are closed and it is not an emergency
Call NHS 111 or visit 111.nhs.uk.