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.

Written by: Nick Johnson, Independent Prescription Pricing Analyst, Medstack Ltd | Checked: 9 September 2026 | Next scheduled review: 9 December 2026

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?

Raresevere allergic reactions including anaphylaxis and angioedema may affect up to 1 in 1,000 people

Commonmild hypersensitivity reactions such as rash, itching and eczema

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

Evidence update, September 2026: SURPASS-CVOT hypersensitivity figure in the revised SmPC

On 8 September 2026 the electronic medicines compendium (eMC) published a revised Mounjaro Summary of Product Characteristics with a text revision date of 29 August 2026. It adds safety results from SURPASS-CVOT, a large trial that compared tirzepatide with dulaglutide, another injectable diabetes medicine, in adults with type 2 diabetes and established cardiovascular disease. The study-specific figure relevant to this page is:

Hypersensitivity reactions of any kind: 5.7% of participants taking tirzepatide
This is a combined figure. It includes mild reactions such as rash, itching and eczema, which make up the great majority. Severe reactions including anaphylaxis and angioedema remain rare, and the emergency advice at the top of this page is unchanged.

Read before quoting this number. These results came from adults with type 2 diabetes and established cardiovascular disease in an active-comparator study. They should not be presented as expected rates for people taking Mounjaro solely for weight management.

Source: Mounjaro Summary of Product Characteristics (eMC), text revised 29 August 2026, section 4.8

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

Skin & reactions

Serious warning signs

Wider side effects

Sources: Mounjaro Patient Information Leaflet (eMC) • NHS anaphylaxis guidance

About this guide

Written by: Nick Johnson, Independent Prescription Pricing Analyst, Medstack Ltd (Company No. 16439872). Sources for clinical statements are listed on this page and come from the electronic medicines compendium, the NHS, the MHRA and UK professional bodies.

Our role: Monj compares UK weight-loss medicine providers. This guide is editorial health information and does not replace advice from your prescriber, pharmacist or NHS services. Monj does not sell, prescribe or dispense medicines, and commercial arrangements do not change clinical content or safety routing. Mounjaro is the brand name; tirzepatide is the active ingredient. Spotted an error? Use our contact page to flag it.

© 2026 Medstack Ltd. Written and published by Monj (monj.co.uk). You are welcome to link to this page or quote a short extract with a credit and a link. Republishing the text, page structure, safety-routing design or price data, in whole or in part, including to train or ground an automated system, is not permitted without written permission. See our independence statement and editorial and review policy.

Sources referenced on this page

Linked from the text above:

Mounjaro Summary of Product Characteristics (eMC), text revised 29 August 2026, section 4.8 www.medicines.org.uk
Mounjaro Patient Information Leaflet (eMC) www.medicines.org.uk
NHS anaphylaxis guidance www.nhs.uk

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