Independent UK medicine guidance
Mounjaro allergic reaction: when to call 999
Severe allergic reactions to Mounjaro, including anaphylaxis and angioedema, are rare and may affect up to 1 in 1,000 people. A mild itchy or red injection site is much more common and does not necessarily mean you are allergic to tirzepatide. A rapidly developing reaction involving your breathing, throat, tongue or circulation is an emergency.
Call 999 immediately if: your lips, mouth, tongue or throat suddenly swell; your throat feels tight; you are struggling to breathe or swallow; you are wheezing, choking or gasping; your skin, lips or tongue turn blue, grey or unusually pale; you suddenly become extremely dizzy, confused, drowsy or faint; or you collapse or become unresponsive.
If you have been prescribed an adrenaline auto-injector for a previous allergy, use it as instructed and call 999. Do not drive yourself to hospital.
How rare is it?
What about a mild rash?
Mild hypersensitivity reactions such as rash, itching or eczema are listed as common. Contact your prescriber or pharmacist for advice if you develop a new rash after an injection.
- Take photographs if the appearance changes or fades before you can be assessed
- Redness may be less noticeable on brown or black skin, so also pay attention to swelling, heat, tenderness and itching
- Do not take another dose until you have received clinical advice if a healthcare professional suspects a medicine allergy
Injection site reaction or allergy?
Usually an injection site reaction
A small localised patch of itching or redness where the needle entered. It should remain limited to that area and generally settle.
Possible allergic reaction
May affect areas away from the injection site and can cause widespread hives, facial swelling or breathing difficulties.
If you are unsure whether a reaction is becoming serious, seek urgent help. Anaphylaxis can progress quickly and should never be watched at home to see whether it gets worse.
Read the full guide to injection site reactions
Common questions
How common are severe allergic reactions to Mounjaro?
Severe allergic reactions to Mounjaro, including anaphylaxis and angioedema, are rare and may affect up to 1 in 1,000 people. Mild reactions such as rash, itching or eczema are listed as common.
What are the signs of anaphylaxis?
Sudden swelling of the lips, mouth, tongue or throat, a tight throat, struggling to breathe or swallow, wheezing, choking or gasping, skin, lips or tongue turning blue, grey or unusually pale, sudden extreme dizziness, confusion, drowsiness or faintness, or collapse. Call 999 immediately and do not drive yourself to hospital.
Should I take my next dose after a rash?
Contact your prescriber or pharmacist for advice if you develop a new rash after an injection. Do not take another dose until you have received clinical advice if a healthcare professional suspects a medicine allergy.
Related guides
Sources: Mounjaro Patient Information Leaflet (eMC) • NHS anaphylaxis guidance
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