Independent UK medicine guidance
Mounjaro injection site reactions: redness and itching
Injection site reactions are a common Mounjaro side effect. They can include itching, redness, mild swelling or irritation around the place where the medicine was injected. This guide explains what a typical reaction looks like, how to reduce irritation, and when a reaction needs checking.
How common are they?
Where to inject
Mounjaro can be injected under the skin of the abdomen, thigh or back of the upper arm. Another person should administer injections into the back of your upper arm. The pink areas below show the recommended sites.
What does a typical reaction look like?
A straightforward local reaction generally:
- Appears close to the injection point
- Causes mild redness, itching or tenderness
- Does not continue rapidly spreading
- Improves rather than worsening
- Does not affect breathing or cause facial swelling
Redness can be more difficult to see on brown or black skin. Warmth, swelling, itching and tenderness may be easier signs to recognise.
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How can you reduce irritation?
- Avoid injecting into skin that is bruised, sore, hard, scarred or already inflamed
- Use a new compatible pen needle for every injection and follow the instructions supplied with the KwikPen
- Do not scratch the area
- A wrapped cool compress may help with mild irritation. Avoid placing ice directly on the skin
Injection site reaction or allergy?
A small localised patch of itching or redness where the needle entered is usually an injection site reaction. It should remain limited to that area and generally settle. An allergic reaction may affect areas away from the injection site and can cause widespread hives, facial swelling or breathing difficulties.
Call 999 for swelling of the lips, mouth, tongue or throat, breathing difficulty, collapse or other signs of anaphylaxis. Anaphylaxis can progress quickly and should never be watched at home to see whether it gets worse.
Read the full guide to severe allergic reactions
When should it be checked?
Ask your pharmacist, prescriber or GP for advice if the reaction:
- Becomes increasingly painful, hot or swollen
- Continues to spread
- Produces pus or discharge
- Is accompanied by a fever
- Fails to improve
- Happens severely after every dose
Common questions
How common are Mounjaro injection site reactions?
In pooled tirzepatide weight-management studies, injection site reactions were reported by 8% of participants taking tirzepatide and 1.8% taking placebo. Across the phase 3 programme, 91% were mild and 9% were moderate; none was classed as serious.
Where can Mounjaro be injected?
Mounjaro can be injected under the skin of the abdomen, thigh or back of the upper arm. Another person should administer injections into the back of your upper arm. Choose a different point within the permitted area each week.
How do I tell an injection site reaction from an allergic reaction?
A small localised patch of itching or redness where the needle entered is usually an injection site reaction, and it should stay limited to that area. An allergic reaction may affect areas away from the injection site and can cause widespread hives, facial swelling or breathing difficulties, which needs emergency help.
Related guides
Sources: Mounjaro Patient Information Leaflet (eMC) • Mounjaro Summary of Product Characteristics (eMC)
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