Allergy: what it actually means

A side effect is not an allergy

Feeling sick, vomiting, diarrhoea, headache or thrush are side effects. They are unpleasant but predictable, they are not dangerous in most cases, and they do not mean you are allergic. Many people are recorded as "penicillin allergic" because of exactly these experiences.

Timing matters

Immediate allergic reactions often start within an hour of a dose. Delayed allergic reactions can appear hours or days later, and some can be serious. A rash during an infection is not always an allergy, but ask a clinician to assess it. Do not remove an allergy label yourself or take the medicine again to test it. If a rash blisters or peels, or you get sores in the mouth or eyes, stop the medicine and call 999 or go to A&E.

Why the label matters — and can often be removed

Around 9 in 10 people recorded as penicillin allergic are not truly allergic when tested. Carrying an incorrect label means you may be given antibiotics that work less well or have more side effects. If your label came from a mild or vague childhood reaction, ask your doctor whether it could be reviewed — specialist allergy services can test and often remove the label.

What to do

• If you have a reaction now: stop the medicine and get advice — 999 for swelling or breathing problems, otherwise doctor or NHS 111.
• Before any prescription, tell the clinician exactly what happened and when.
• If you were told as a child that you are allergic but can't remember why, mention it at your next doctor's appointment.

Which antibiotics are penicillins?

In this guide, the penicillin family includes:

• Amoxicillin
• Phenoxymethylpenicillin (Penicillin V)
• Flucloxacillin
• Co-amoxiclav

When a true allergy exists, prescribers commonly use alternatives such as Doxycycline, Lymecycline, Clarithromycin, Erythromycin, Nitrofurantoin, and others — the choice depends on the infection. Some cephalosporins may also be unsuitable because reactions can overlap.