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Dr. Chrysoula I. Liakou MD, PhD
- Treated online: Partly. A doctor can assess a reaction, tell allergy from side effects and choose a safe antibiotic; testing to confirm or remove the allergy is done in person.
- What it is: an immune reaction to penicillin antibiotics, such as amoxicillin and flucloxacillin, from a rash or hives to, rarely, anaphylaxis.
- Mislabelled: about 10% of hospital patients have a penicillin allergy recorded, and 9 in 10 of them are not truly allergic.
- It can fade: about half of people lose a penicillin allergy within 5 years, and about 80% within 10 years.
- Emergency: call 112 for swelling of the mouth, face, lips, tongue or throat, trouble breathing, or a blistered or peeling rash.
Do not book a video appointment if, after a medicine, your mouth, face, lips, tongue or throat swells, you are wheezing or struggling to breathe or talk, your chest or throat feels tight, or your skin is blistering or peeling. Call 112 or go to an emergency department.
Can an online doctor help with a penicillin allergy?
Partly. A doctor by video can go through what happened when you reacted, tell an allergy from a side effect, judge whether your history is low or high risk, treat a mild reaction, and choose a safe antibiotic when you need one. Testing to confirm the allergy or remove a wrong label, usually a supervised dose of the antibiotic, is done in person, and the doctor can arrange it.
How an online consultation helps with a penicillin allergy
- Allergy or side effect: Feeling sick or diarrhoea on an antibiotic is a side effect, not an allergy.
- Your risk graded: Whether your history is low risk and the label may be wrong, or high risk and needs a specialist.
- A safe antibiotic: When you need treatment, one that suits your allergy history.
- A record that helps: What reacted, how and when, written down so that future doctors can act on it.
What the doctor checks in an online consultation for a penicillin allergy
The details of the original reaction decide almost everything, so the doctor asks about them carefully. The doctor asks about:
- The medicine: which antibiotic, why you were taking it, and how it was given.
- The reaction: what happened, such as a rash, hives, swelling, breathing difficulty, blistering or feeling sick.
- The timing: how soon after starting the medicine, and how many years ago it was.
- The treatment: whether you needed adrenaline, a hospital visit or nothing at all.
- Since then: whether you have taken any penicillin or a related antibiotic without problems.
- Other medicines: reactions to anti-inflammatory painkillers, anaesthetics or other medicines.
Before the call, look for any record of the reaction, such as a hospital letter or your health record, and the name of the antibiotic.
What is a penicillin allergy?
A penicillin allergy is a reaction of the immune system to a penicillin antibiotic. The penicillins include amoxicillin, co-amoxiclav, flucloxacillin and phenoxymethylpenicillin, so an allergy to one usually means avoiding them all. All medicines can cause side effects, but a drug allergy, caused by the immune system, is much less common. Penicillins and cephalosporins are the antibiotics most often linked to allergic reactions.
How common is a penicillin allergy label?
About 10% of hospital patients have a penicillin allergy recorded, and millions of people in the UK have one in their notes. Only about 1 in 10 of them are truly allergic.
What are the symptoms of a penicillin allergy?
An itchy, raised rash or hives, swelling of the lips and around the eyes, coughing, wheezing or a tight throat, usually soon after a dose. Some reactions are delayed by days or weeks and mostly affect the skin. Anaphylaxis is rare.
Swipe sideways to compare →
| Type | When | What you notice | What it means |
|---|---|---|---|
| Immediate allergy | In the first minutes to hours | Hives, swelling of the lips or around the eyes, wheezing, throat tightness | A true allergy is likely: needs assessment |
| Delayed allergy | Days or weeks | A rash, sometimes with problems in the blood, liver or kidneys | Needs assessment; severe forms need a specialist |
| Severe skin reaction | Days or weeks | Blistering or peeling skin, feeling very unwell | An emergency; a specialist decides what is safe afterwards |
| Anaphylaxis | Usually in the first minutes | Swelling of the throat or tongue, trouble breathing or talking, collapse | An emergency: call 112 |
| Side effect | Any time on the course | Diarrhoea, feeling or being sick, headache | Not an allergy |
How long does an allergic reaction to penicillin last?
Mild to moderate reactions usually respond to antihistamines, and hives usually get better within a few days. Any blistering, peeling or feeling very unwell needs a doctor urgently.
Side effect or allergy? Being sick after penicillin
Many penicillin allergy labels come from side effects, not allergy. Antibiotics commonly cause diarrhoea and feeling sick, which usually pass after the course ends. Being sick, an upset stomach, a headache or tiredness on penicillin are side effects, and do not stop you having penicillin again. Viral illnesses also cause rashes that can be wrongly blamed on the antibiotic taken at the same time, and amoxicillin can cause a rash in people with glandular fever, a viral infection.
A family history is not a reason to avoid penicillin either: penicillin allergy is not inherited.
Are you really allergic to penicillin?
Probably not: about 9 in 10 people with a penicillin allergy label can take penicillin safely. Labels often come from side effects, a rash during a viral illness, a childhood reaction nobody remembers clearly, or family history, and true allergies fade over time.
About half of people lose a penicillin allergy within 5 years, and about 80% within 10 years. A label that is wrong matters, because people with one are given second-choice, often broad-spectrum antibiotics, which are more likely to drive antibiotic resistance. The label is linked to treatment failure, infections after surgery, resistant infections such as MRSA and C. difficile, more antibiotic prescriptions and longer hospital stays.
How is a penicillin allergy tested and removed?
It starts with your history, which sorts people into lower and higher risk:
Swipe sideways to compare →
| Your history | Likely risk | Usual next step |
|---|---|---|
| Feeling sick, an upset stomach, headache, tiredness, or itching without a rash | A side effect, not an allergy | The label can be removed without a test |
| Only a family history of penicillin allergy | Not an allergy | The label can be removed without a test |
| You have taken a penicillin since without problems | Not allergic | The label can be removed without a test |
| A mild or uncertain reaction many years ago | Low | A supervised test dose of the antibiotic |
| Hives, swelling of the face or throat, breathing difficulty, or anaphylaxis | High | Specialist allergy assessment |
| Blistering, peeling skin, or a severe reaction with organ problems | Highest | Never retested; specialist advice |
The usual test is an oral challenge: a supervised dose of amoxicillin or the antibiotic you reacted to, with observation for about 2 hours and resuscitation equipment nearby. Doctors use scoring tools to estimate the risk: with the lowest score, the chance of a true allergy is under 1%. Specialists also use skin tests. If you pass, the label is removed from your record.
Can you test for a penicillin allergy at home?
No. Never test it yourself by taking penicillin. Testing is done where a reaction can be treated straight away.
Which antibiotics can you take with a penicillin allergy?
Avoid all penicillins, including amoxicillin, co-amoxiclav and flucloxacillin, until a test says otherwise. Antibiotics from other families are used instead, chosen for the infection and your history.
Can I take a cephalosporin, such as cefalexin?
Often, yes. The often-quoted 10% cross-reaction risk between penicillins and cephalosporins is a myth and a large overestimate: it depends on how similar the side chain of the molecule is. Most cephalosporins carry a cross-reaction risk under 1% and can be used for a non-severe penicillin allergy, such as a delayed rash without a severe skin reaction. Cefalexin is the exception, because it shares a side chain with the penicillins. After a severe reaction, the choice is made with a specialist.
Other medicine allergies: ibuprofen, aspirin and anaesthetics
Any medicine can cause an allergy, and the signs are the same: immediate rashes, swelling and breathing problems, or delayed skin reactions. A few need particular care:
- Ibuprofen, aspirin and other anti-inflammatory painkillers: after a severe reaction to one, avoid all of them, including products bought without a prescription. After a mild reaction, a doctor may discuss a different type of anti-inflammatory medicine. Aspirin sensitivity is linked to asthma and nasal polyps.
- Local anaesthetics: if you have reacted and need a procedure that uses one, a specialist referral is needed.
- General anaesthesia: anaphylaxis or another allergic reaction during or straight after an anaesthetic needs specialist investigation.
Living with a penicillin or drug allergy
- Make sure your record shows the medicine, what happened, how severe it was and when.
- Carry the information about your allergy at all times, for example on your phone or a card.
- Tell every doctor, dentist, nurse and pharmacist, every time you are offered a medicine.
- Check with a pharmacist before taking anything bought without a prescription.
- If your label may be wrong, ask about testing: removing it widens the antibiotics you can have.
What might the doctor recommend for a penicillin or drug allergy?
What happens next depends on the reaction, how long ago it was, and whether you need treatment now. This is the framework the doctors use:
Swipe sideways to compare →
| What you have | Likely first step | Suitable for online care? | When in-person care is needed |
|---|---|---|---|
| Swelling of the face or throat, breathing difficulty, or a blistering rash after a medicine | Emergency treatment | No | Call 112 now |
| Hives or an itchy rash on a new antibiotic, with normal breathing | Stopping it, an antihistamine, and an alternative if still needed | Yes | If swelling, breathing problems or blistering start |
| Sickness or diarrhoea on penicillin | Explaining it is a side effect, and how to manage it | Yes | If you get other side effects, or they do not pass |
| An old penicillin label you are unsure about | Grading the risk, and referral for testing | Yes, to assess | The supervised test dose |
| An infection that needs an antibiotic, and a penicillin label | Choosing a safe alternative | Yes | Specialist advice after a severe reaction |
| A reaction to ibuprofen or aspirin | Avoiding the group, and safe pain relief | Yes | Specialist referral after a severe reaction |
Not sure whether you are really allergic to penicillin? A doctor can go through your history and the next step in one consultation.
See available times →Special situations
Before surgery
A penicillin allergy label is linked to more infections after surgery, so if you have a planned operation and your label may be wrong, ask about testing well before it.
Asthma
A drug allergy can trigger an asthma attack in people with asthma. See asthma.
Children
Mobi Doctor consultations are for adults aged 18 and over. A child who is unwell needs a paediatrician or a local doctor in person, and emergency signs in a child mean 112.
When a penicillin allergy is treated online, and when you need in-person care
Treated online: assessing a reaction to a medicine, telling allergy from side effects, grading the risk of an old label, antihistamines for mild reactions, choosing a safe antibiotic, and referral for testing.
In-person care: the supervised test dose and skin tests, severe reactions, anaphylaxis, and severe skin reactions.
When to get in-person care
Call 112 or go to an emergency department now for:
- swelling of the mouth, face, lips, tongue or throat
- wheezing, chest or throat tightness, or trouble breathing or talking
- a skin rash that is blistered or peeling
- feeling faint, confused, or collapsing
Get medical advice the same day for:
- hives or an itchy rash after starting an antibiotic
- a rash with a high temperature or feeling unwell, days or weeks after a new medicine
- a reaction that does not improve with an antihistamine
How Mobi Doctor works
- Book a time that suits you: daily 7am–11pm CEST. You can see a doctor over video, usually within 15 minutes. No subscription, and the exact price is shown before you pay.
- Speak to a licensed doctor about the reaction: say which medicine, what happened, how long ago, and whether you have taken it since.
- Leave with a plan: whether the label is likely to be right, what to take instead, how to get tested, and which signs mean you should be seen in person. If the doctor prescribes, use the prescription at a pharmacy near you.
No sign-up needed
A reaction to a medicine can have more than one cause. Tick every symptom you have to see each condition page that mentions it. It finds the right page, not a diagnosis: a doctor gives you that on a video call. If you have an emergency sign listed on this page, call 112 instead.
Tap the body — an area or a point — to browse symptoms anywhere
Similar symptoms, different condition? Allergic Reaction can start the same way. Compare them, or look up any other symptom in the full symptom checker.
Frequently asked questions about penicillin allergy
Am I really allergic to penicillin?
Probably not: about 9 in 10 people with a penicillin allergy label can take penicillin safely. Labels often come from side effects, rashes during viral illnesses or reactions long ago.
What are the symptoms of a penicillin allergy?
The symptoms of a penicillin allergy are an itchy, raised rash or hives, swelling of the lips and around the eyes, coughing, wheezing or a tight throat, usually soon after a dose. Some reactions appear days or weeks later.
Is it normal to be sick after taking penicillin?
Yes. Feeling or being sick, tummy upset and diarrhoea are common side effects of penicillin and other antibiotics, not a penicillin allergy, and they usually pass once the course ends; taking the tablets with food often helps. An allergy shows as an itchy rash or hives, swelling of the lips or around the eyes, or wheeze, and a record of sickness alone should not stop you having penicillin in future.
If I am allergic to penicillin, can I take amoxicillin?
No. Amoxicillin is a penicillin, as are co-amoxiclav, flucloxacillin and phenoxymethylpenicillin, so a true penicillin allergy means avoiding them all until testing shows you are not allergic. Many people labelled allergic turn out not to be, and a specialist can remove the label with a supervised test dose; until then, a doctor chooses a safe antibiotic from a different family when you need one.
Can you be allergic to cefalexin if you are allergic to penicillin?
Cefalexin shares a side chain with the penicillins, so it is usually avoided. Most other cephalosporins carry a cross-reaction risk under 1% and can be used after a non-severe penicillin allergy.
How can I avoid a reaction if I have a penicillin allergy?
Make sure the allergy is recorded accurately, with what happened and when, in your health records and on any pharmacy profile, and tell every doctor, dentist and pharmacist; learn the names of the penicillins, amoxicillin, co-amoxiclav, flucloxacillin and phenoxymethylpenicillin, so you can check labels; and ask the doctor about cephalosporins, which most people with a penicillin allergy can take. If the reaction was mild or long ago, ask for testing, because removing a wrong penicillin allergy label is the best protection of all.
Is penicillin allergy hereditary?
No. Penicillin allergy is not inherited, and a family history alone is not a reason to avoid penicillin: a parent's reaction does not mean you will react, and you can take penicillin normally if you have never had an allergic reaction yourself. What matters is your own history, which is why a doctor asks what happened, when and how long after the dose.
Can a penicillin allergy go away?
Yes. About half of people lose a penicillin allergy within 5 years, and about 80% within 10 years, because the antibodies behind it fade without further exposure. That is why a reaction years ago is worth re-assessing: a specialist can test it with skin tests or a supervised dose, and removing a wrong label gives you back the best antibiotics for many common infections.
How is a penicillin allergy tested?
A penicillin allergy is usually tested with a supervised dose of amoxicillin or the antibiotic you reacted to, watched for about 2 hours. Specialists also use skin tests. Never test it yourself at home.
How long does an allergic reaction to penicillin last?
A mild allergic reaction to penicillin, such as hives, usually responds to antihistamines and gets better within a few days of stopping the antibiotic, though a rash that started late in a course can take a week or more to fade. Delayed rashes appear days or weeks after the first dose, and blistering, peeling skin, mouth ulcers, a high temperature or facial swelling need urgent care.
Why does a wrong penicillin label matter?
Because people with a penicillin allergy label are given second-choice antibiotics, which are linked to treatment failure, infections after surgery, resistant infections such as MRSA and Clostridioides difficile, and longer hospital stays. Most labels are wrong, often based on a childhood rash or a side effect, so having the label checked, and removed if it is wrong, improves the care you get for life.
Can you be allergic to ibuprofen?
Yes. Ibuprofen, aspirin and other anti-inflammatory painkillers can cause hives, swelling, wheeze or, rarely, anaphylaxis, and the reaction is separate from a penicillin allergy. After a severe reaction to one of them, avoid all of them, including products bought without a prescription and gels, and tell every doctor and pharmacist; people with asthma and nasal polyps are more likely to react.
Can an online doctor help with a penicillin allergy?
Partly. A doctor can assess a reaction, tell allergy from side effects, grade the risk and choose a safe antibiotic. Testing is done in person.
- Drug allergy: diagnosis and management (CG183), NICE, National Institute for Health and Care Excellence, published September 2014.
- Penicillin allergy and de-labelling guide, NHS Lothian, reviewed 7 July 2025.
- 5 common misperceptions about penicillin allergies, Mayo Clinic (US), 2018.
- Are you sure you are allergic to penicillin?, NIHR Evidence, National Institute for Health and Care Research (UK), 20 January 2022.
- Side effects of antibiotics, NHS, National Health Service (UK), reviewed 11 November 2022.
- Hives, NHS, National Health Service (UK), reviewed 26 April 2024.
- Nasal polyps, NHS, National Health Service (UK), reviewed 29 June 2023.
- Who can and cannot take amoxicillin, NHS, National Health Service (UK), reviewed 2 June 2025.
This page is for information only and is not a substitute for a medical consultation. Guideline links are provided for transparency; treatment decisions are made with your doctor.
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