Mounjaro injection site reactions: redness, lumps and itching

In short

The leaflet lists injection site reactions as common (up to 1 in 10) and injection site pain as uncommon (up to 1 in 100)
Placebo-controlled trials: 3.2% to 8.2% on tirzepatide reported a reaction, against 0.4% to 2.6% on placebo
Redness and itching were the most common signs (SmPC section 4.8)
The SmPC says to rotate injection sites with each dose
Heat, swelling, pus, spreading redness or a fever point to infection and need a same-day check

A red, itchy or slightly swollen patch where you injected Mounjaro is a common side effect and usually fades within a few days. The patient leaflet lists injection site reactions as common, affecting up to 1 in 10 people. Changing the spot each week and using a new needle every time keeps them to a minimum. Heat, swelling, pus or redness spreading out from the site point to infection and need a same-day check.

How common injection site reactions are

The Summary of Product Characteristics (SmPC) gives figures from four placebo-controlled trials. Between 3.2% and 8.2% of people on tirzepatide reported a reaction at the injection site, against 0.4% to 2.6% of people on placebo. The placebo figures show part of the problem comes from the injection itself rather than the medicine.

Reaction	Frequency in the UK leaflet
Injection site reaction (redness, itching)	Common, up to 1 in 10
Injection site pain	Uncommon, up to 1 in 100
Allergic reaction, including rash and itching elsewhere	Common, up to 1 in 10
Severe allergic reaction (anaphylaxis, angioedema)	Rare, up to 1 in 1,000

The SmPC names redness (erythema) and itching (pruritus) as the most common signs.

What a normal reaction looks like

A typical reaction stays within a few centimetres of the needle mark. You might notice:

A pink or red patch, or a darker patch on brown or black skin
Itching, sometimes starting hours after the injection
A small raised bump or firm area under the skin
A small bruise, especially if a blood vessel was nicked
Mild tenderness when you press the area

These usually start within a day of the injection and settle within 2 to 4 days. They do not need you to stop the medicine. The NHS lists itchy or red skin at the injection site among the common side effects of tirzepatide and notes redness can be harder to see on brown or black skin.

Why reactions happen

Several things play a part:

The skin reacting to a protein-based medicine placed under it
The needle itself, which is why people on placebo also had reactions
Injecting cold liquid straight from the fridge, which some people find stings more
Reusing a needle, which becomes blunt and tears the skin
Injecting into the same spot week after week

Reactions are more common in the early months for some people and fade as treatment goes on. The SmPC does not give a figure for this.

Rotating your injection sites

The SmPC says to rotate the injection site with each dose. The NHS says not to reuse the exact same spot. You have three areas to use:

Stomach area, at least 5cm from the belly button
Front of the thigh
Back of the upper arm, if someone else injects for you

A simple plan is to divide each area into four and work round them in order. Keep a note of where you injected each week. The KwikPen step by step guide covers the full technique.

Avoid skin which is bruised, scarred, tender, broken or has a mole or stretch mark. If you also inject insulin, use a different site for each medicine.

Easing a reaction at home

Hold a cool pack wrapped in a clean cloth on the area for 10 minutes. Never put ice straight on skin.
Do not rub or massage the site. The instructions for use say to press lightly with cotton wool if you see blood.
Leave the area alone and avoid tight waistbands over a stomach site.
For itching, a pharmacist can suggest an antihistamine tablet or a mild cream.
If the pen has been in the fridge, some people find leaving it out for a short time before injecting reduces the sting. The in-use pen can stay at up to 30C for up to 30 days.

Technique checks which reduce reactions

Run through these if reactions keep happening:

A new needle every time
A clean, dry site. If you use an alcohol swab, let the skin dry fully before injecting
The needle in straight, at a right angle to the skin
A slow count to 5 with the dose knob held in
No injecting through clothing

When a reaction is something else

Infection

Infection is uncommon but needs treatment. Signs include skin which becomes hot, more swollen and more painful over 1 to 2 days, redness spreading outwards, pus, and a fever or feeling shivery.

Allergic reaction

A rash or hives spreading beyond the injection site, itching all over, or swelling of the lips or eyes point to an allergy to the medicine rather than a local reaction. The allergic reaction page explains what to do. The itching, rash and hives page covers milder skin reactions elsewhere on the body.

Skin pain without a mark

Some people describe tender or sensitive skin with no visible reaction. The skin pain page covers this.

If anxiety about the needle is making injections harder, the fear of injecting page has practical steps.

When to get help

Call 999 or go to A&E if you have:

Swelling of the face, lips, tongue or throat
Difficulty breathing or wheezing
A widespread raised rash with feeling faint or dizzy

Call 111 or see a GP today if:

The site becomes hot, swollen and more painful instead of better
Redness spreads out from the injection site or forms a streak
The area leaks pus or opens up
You develop a fever, chills or feel generally unwell
A rash or hives appears away from the injection site

Mention at your next check if:

Reactions happen after most injections
A lump stays for more than 2 weeks
You keep bruising at the site
You want help with your injection technique

Report any reaction to the MHRA through the Yellow Card scheme. Mounjaro carries the black triangle for additional monitoring.

Questions people ask

Is it normal to get a red patch after a Mounjaro injection?

Yes. Redness and itching at the injection site are the most common reactions listed in the SmPC, and the leaflet rates them as common. A small pink or itchy patch which fades within a few days is not a reason to stop. On brown or black skin the area may look darker rather than red.

Why do I get a lump where I injected Mounjaro?

A small firm bump under the skin for a day or two is common after any injection under the skin. Bumps which keep appearing in the same place usually mean the same spot is being used too often. Move at least a few centimetres each week and tell your prescriber about any lump lasting more than 2 weeks.

Can I put ice on a Mounjaro injection site?

A cool pack wrapped in a clean cloth for 10 minutes can ease itching and swelling. Do not put ice straight on the skin, and do not rub or massage the site, which the instructions for use advise against.

Can I take an antihistamine for an itchy injection site?

Many people use an over-the-counter antihistamine or a mild steroid cream for a local itch. Ask a pharmacist which suits you, especially if you take other medicines. A widespread rash or itch all over the body is different and needs a call to your prescriber or 111.

Should I change where I inject if I keep reacting?

Yes. Try a different area, for example the thigh instead of the stomach. Check your technique too: a new needle each time, a clean dry site, and a slow count to 5 before removing the needle.

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 injection site reactions: redness, lumps and itching. Site: Mounjaro Info (https://mounjaroinfo.co.uk/side-effects/mounjaro-injection-site-reactions/). Reviewed 22 October 2026. Topic: Mounjaro (tirzepatide), UK.

Injection site reactions are a common Mounjaro side effect, affecting up to 1 in 10 people. In the placebo-controlled trials listed in the SmPC, 3.2% to 8.2% of people on tirzepatide reported one, against 0.4% to 2.6% on placebo, and the usual signs were redness and itching. Most reactions fade within a few days. Changing the spot each week helps, and a hot, swollen or spreading patch needs a same-day check.

The leaflet lists injection site reactions as common (up to 1 in 10) and injection site pain as uncommon (up to 1 in 100)
Placebo-controlled trials: 3.2% to 8.2% on tirzepatide reported a reaction, against 0.4% to 2.6% on placebo
Redness and itching were the most common signs (SmPC section 4.8)
The SmPC says to rotate injection sites with each dose
Heat, swelling, pus, spreading redness or a fever point to infection and need a same-day check

Cite as: Mounjaro Info, "Mounjaro injection site reactions: redness, lumps and itching", https://mounjaroinfo.co.uk/side-effects/mounjaro-injection-site-reactions/, reviewed 22 October 2026.