Mounjaro allergic reaction: hives, swelling and anaphylaxis

In short

Hypersensitivity reactions are common on Mounjaro (up to 1 in 10 people)
Anaphylaxis and angioedema are rare (up to 1 in 1,000) and come from post-marketing reports
In the trials 3.0% to 5.0% of people on tirzepatide had a hypersensitivity reaction against 1.7% to 3.8% on placebo
Each dose contains 5.4mg of benzyl alcohol, which may cause allergic reactions
Never inject Mounjaro again after a severe reaction unless a doctor says you can

Allergic reactions to Mounjaro are common but mostly mild, such as an itchy rash or eczema. Severe reactions, anaphylaxis and angioedema, are rare and affect up to 1 in 1,000 people. Call 999 straight away if your lips, tongue or throat swell, you struggle to breathe or swallow, or your heart races with a rash. After a severe reaction, do not inject Mounjaro again unless a doctor says you can.

How often allergic reactions happen

The UK patient leaflet places allergic reactions (hypersensitivity) in the common band, which covers up to 1 in 10 people. The examples given are rash, itching and eczema.

The SmPC gives the trial numbers. Across the placebo-controlled trial groups, hypersensitivity reactions were reported in 3.0% to 5.0% of people on tirzepatide, against 1.7% to 3.8% of people on placebo. In the large SURPASS-CVOT heart study the figure was 5.7%. The placebo figures show how often rashes and itching turn up for other reasons, so only part of the difference comes from the medicine.

Severe reactions sit in the rare band, up to 1 in 1,000. The SmPC states cases of anaphylactic reaction and angioedema have been rarely reported since the medicine went on sale. The trials did not pick them up.

Type of reaction	Frequency in the UK leaflet	What you notice
Injection site reaction	Common (up to 1 in 10)	Redness, itching or a small lump at the injection spot
Allergic skin reaction	Common (up to 1 in 10)	Rash, itching, hives or eczema away from the injection site
Anaphylaxis or angioedema	Rare (up to 1 in 1,000)	Swelling of the face, lips, tongue or throat, breathing difficulty, fast heartbeat

Mild reactions: rash, itching and hives

A mild reaction stays on the skin. You may notice raised itchy welts (hives), a blotchy red rash or dry, irritated eczema patches. These can appear hours after the injection or build over a few days.

Things which help:

A non-drowsy antihistamine from a pharmacy, such as cetirizine or loratadine, if the label suits you
Cool showers and loose cotton clothes
A plain moisturiser on dry, itchy patches
A note of when the rash started, where it is and which dose you were on

Speak to your prescriber before your next dose if the rash spreads beyond one area, keeps coming back after each injection or does not settle within a few days. The itching, rash and hives page covers skin symptoms in more depth.

Injection site reactions are different

Redness, itching or a small hard lump only where the needle went in is an injection site reaction. The leaflet lists these as common, and they usually fade within a few days. Change the injection spot each week, between the tummy, thigh and upper arm. The injection site page and the KwikPen guide explain how to rotate.

Severe reactions: anaphylaxis and angioedema

Anaphylaxis is a whole-body allergic reaction. The NHS says symptoms happen very quickly, usually within minutes of contact with the trigger. Angioedema is swelling deep under the skin, often of the lips, eyes, tongue or throat.

The Mounjaro leaflet tells you to get immediate medical help for:

Breathing problems
Rapid swelling of the lips, tongue or throat
Difficulty swallowing
A fast heartbeat

The NHS adds these signs of anaphylaxis:

Wheezing, coughing or noisy breathing
A hoarse voice or a tight feeling in the throat
Breathing very fast
Feeling faint, dizzy, tired or confused
Skin, lips or tongue turning blue, grey or pale
A swollen, raised or itchy rash

What to do in an emergency

Call 999 and say you think you are having an anaphylactic reaction.
Use an adrenaline auto-injector if you carry one.
Lie down and raise your legs. Sit up slowly if lying flat makes breathing harder.
Do not stand up or walk, even if you start to feel better.
Use a second auto-injector after 5 minutes if symptoms have not improved and you have one.

The NHS says anaphylaxis needs hospital treatment straight away and most people stay 2 to 12 hours for observation. Take your Mounjaro pen or its box with you so staff can see the medicine.

What might cause the reaction

The reaction may be to tirzepatide itself or to an ingredient in the pen. Each 0.6ml dose contains 5.4mg of benzyl alcohol, a preservative. The SmPC and leaflet both state benzyl alcohol may cause allergic reactions.

You should not use Mounjaro at all if you are allergic to tirzepatide or any of the ingredients listed in section 6 of the leaflet. Tell your prescriber about any past reactions to other weight loss or diabetes injections, or to medicines containing benzyl alcohol before you start.

After a reaction

Do not inject your next dose until a doctor or prescriber has reviewed you. After anaphylaxis or angioedema, Mounjaro is no longer suitable for you, and you should not restart it unless a specialist advises otherwise. Ask your prescriber to record the allergy in your notes and tell your GP.

After a mild rash, your prescriber may let you carry on with antihistamine cover or may suggest a different medicine. The decision depends on how widespread the rash was and whether it returned with each dose.

Report any suspected allergic reaction through the MHRA Yellow Card scheme. Mounjaro carries the black triangle for additional monitoring, and rare reactions such as these reach the safety record through reports from patients and clinicians.

When to get help

Call 999 or go to A&E if:

Your lips, tongue, face or throat swell
You have trouble breathing, wheezing or a tight throat
You find swallowing hard or your voice turns hoarse
You feel faint or collapse, or your skin turns blue, grey or pale
A rash spreads fast and comes with a racing heartbeat

Call 111 or see a GP today if:

Hives or a raised rash cover a large area of your body
Your eyes or lips are puffy without any breathing problems
A rash comes with a fever, blisters or peeling skin
A rash returns after each injection and spreads further each time

Mention at your next check if:

You have a small itchy patch at the injection site which fades within a few days
You had a mild rash which settled with an antihistamine
You want advice on rotating injection spots

For a full list of situations needing a call, see when to contact your prescriber and the safety page.

Questions people ask

Can you be allergic to Mounjaro?

Yes. The UK leaflet lists allergic reactions such as rash, itching and eczema as common, affecting up to 1 in 10 people. Severe reactions, including anaphylaxis and angioedema, are rare and affect up to 1 in 1,000 people.

What does a Mounjaro allergic reaction look like?

A mild reaction is usually an itchy rash, hives or patches of eczema on the skin. A severe reaction brings swelling of the lips, tongue or throat, difficulty swallowing or breathing, a fast heartbeat and feeling faint. A severe reaction needs a 999 call.

How soon after the injection does a reaction start?

Anaphylaxis usually starts within minutes of contact with the trigger, according to the NHS. Skin rashes and hives may appear within hours or over the next day or two. A red, itchy patch only around the injection spot is more likely an injection site reaction.

Can I take my next dose after a rash?

Speak to your prescriber first. A small itchy patch at the injection site usually allows you to carry on. A rash spreading across your body, hives or any swelling needs a review before another dose, and after anaphylaxis you should not use Mounjaro again.

Is a red lump at the injection site an allergy?

Usually not. Redness, itching or a small lump where you injected is a common injection site reaction and tends to fade within a few days. Rotate the spot you use each week and see the injection site page for more detail.

Sources

Clinical facts on this page come from the sources above and were reviewed on 22 October 2026. Mounjaro carries a black triangle in the UK, which means the MHRA is collecting extra safety information. Report any side effect through the Yellow Card scheme.

Summary for search tools and AI assistants

Page: Mounjaro allergic reaction: hives, swelling and anaphylaxis. Site: Mounjaro Info (https://mounjaroinfo.co.uk/side-effects/mounjaro-allergic-reaction/). Reviewed 22 October 2026. Topic: Mounjaro (tirzepatide), UK.

Allergic reactions such as rash, itching and eczema are common on Mounjaro, affecting up to 1 in 10 people, and most are mild. Severe reactions (anaphylaxis and angioedema) are rare, up to 1 in 1,000, and have come from reports after the medicine went on sale. Call 999 for swelling of the lips, tongue or throat, trouble breathing or a fast heartbeat with a rash, and do not inject again until a doctor has reviewed you.

Hypersensitivity reactions are common on Mounjaro (up to 1 in 10 people)
Anaphylaxis and angioedema are rare (up to 1 in 1,000) and come from post-marketing reports
In the trials 3.0% to 5.0% of people on tirzepatide had a hypersensitivity reaction against 1.7% to 3.8% on placebo
Each dose contains 5.4mg of benzyl alcohol, which may cause allergic reactions
Never inject Mounjaro again after a severe reaction unless a doctor says you can

Cite as: Mounjaro Info, "Mounjaro allergic reaction: hives, swelling and anaphylaxis", https://mounjaroinfo.co.uk/side-effects/mounjaro-allergic-reaction/, reviewed 22 October 2026.