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What to do if…
you start a new medication and develop a widespread rash or mouth sores

Short answer

Get urgent, same-day medical advice now, and call 999 immediately if you have any emergency symptoms (especially breathing trouble, swelling, blistering/peeling, or mouth/eye/genital sores).

Do not do these things

  • Do not “wait it out” if the rash is spreading quickly, you feel very unwell, or you have mouth/eye/genital sores, blisters, peeling skin, fever, or skin pain.
  • Do not take “one more dose” to see what happens.
  • Do not start leftover antibiotics, steroids, or strong creams to “cover it” without medical advice.
  • Do not take new over-the-counter medicines (including combination cold/flu products) unless a clinician/pharmacist advises it — it can muddy the picture.
  • Do not drive yourself for urgent care if you feel faint, drowsy, short of breath, or your lips/tongue/face are swelling.

What to do now

  1. Check for “call 999 now” red flags. Call 999 immediately if any apply:
    • Trouble breathing, wheeze, throat tightness, trouble swallowing/talking, or swelling of the lips/face/tongue.
    • A rash with blisters, peeling, or very painful skin.
    • Mouth sores, sore/burning eyes, or sores on genitals — especially with fever or feeling very unwell.
    • Dizziness, confusion, collapse, or you look blue/grey/pale.
    • If you cannot call 999 yourself, get someone to call for you. Only consider going to A&E if you can do so safely and you’re not delaying emergency help.
  2. If you’re not in immediate danger but it’s widespread or you have mouth sores: get urgent same-day advice.
    • Use NHS 111 (online or phone) right now, or contact your GP/out-of-hours service for urgent guidance.
    • Tell them: the medication name, dose, when you started it, when the rash/sores began, and whether it’s getting worse.
  3. Do not take the next dose until you’ve spoken to a clinician or pharmacist.
    • Exception: if you’ve been told a medicine must not be stopped suddenly (some do), still seek urgent advice immediately and explain what you’ve taken so far — let a professional tell you the safest next step.
  4. Get your “evidence for care” together (this helps the clinician act quickly).
    • The medication packaging and leaflet (or a photo of them).
    • A complete list of everything you’ve taken in the last week (prescribed, OTC, vitamins/herbals, patches/creams).
    • Clear photos of the rash in good light (wide shot + close-up), and note if you have a temperature.
  5. If you have an adrenaline auto-injector and you develop severe allergy symptoms, use it and call 999.
  6. If you’re seen by a clinician/hospital, ask for two practical safety steps before you leave:
    • That the suspected drug reaction is recorded clearly in your GP record/medication list (including the drug name and what happened).
    • Clear written advice on what to avoid next and what symptoms mean you should return urgently.

What can wait

  • Deciding whether you’re “definitely allergic” to the medication — that needs proper assessment.
  • Reporting the side effect (for example via the MHRA Yellow Card scheme).
  • Sorting work/school messages beyond a simple “I’m unwell and seeking urgent care”.
  • Deep research online about the condition or swapping medicines yourself.

Important reassurance

It’s reasonable to feel alarmed — a new widespread rash or mouth sores can be serious, and acting quickly is the right call. Getting urgent help is not “overreacting”; it’s how you prevent rare but dangerous reactions from escalating.

Scope note

These are first steps to keep you safe and get you assessed quickly. Follow-up decisions (restarting/stopping, allergy labels, future alternatives) should be made with a clinician who can review your history and the exact medication.

Important note

This is general information, not a diagnosis. If you’re worsening, unsure, or can’t get rapid advice, choose the safer option: urgent assessment (111/A&E/999 as appropriate).

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