Mounjaro comes as a pre-filled KwikPen holding four weekly doses.1 You inject it just under the skin, into an area with a layer of soft fat. Knowing where to inject Mounjaro makes each dose easier, more comfortable and less likely to irritate your skin.
This guide covers the three injection sites, how to choose a spot each week, what's normal afterwards, and the common mistakes worth avoiding.
Three areas to choose from, one golden rule: use a different spot every week. Get that right and you've done most of the hard work.
Where should you inject Mounjaro?
There are three Mounjaro injection sites:1
- your stomach (abdomen)
- the front of your thighs
- the back of your upper arms, but only if someone else does the injection
These areas have enough fatty tissue for the medicine to go in properly, and most people can reach them easily.
You should change your injection site with every dose.1 You can keep using the same area, such as your stomach, as long as you pick a different spot within it each time.2 If you also inject insulin, give the two injections separately and never in the same spot.1
Before your first dose, read the instructions for use that come with your pen. They show each step clearly.
Doing it yourself? Your stomach and thighs are the easiest places to see and reach without help.
Choosing your Mounjaro injection site
Each site has its pros. After a few weeks, most people work out which feels most comfortable for them.
“You can inject Mounjaro under the skin of your tummy or thigh. If you prefer the back of your upper arm, someone else should give the injection. Pick a different spot each week, even if you're using the same body area, and follow the instructions that come with your pen.
Ana Carolina GoncalvesSuperintendent Pharmacist
Stomach (abdomen)
The stomach is a popular choice. It's easy to see, and there's plenty of room to move between spots from week to week.
Keep away from the area right around your belly button. Inject at least 5cm (2 inches) away from it.2
Thighs
The front of your thigh is another good option. It's easy to reach and has enough fat to take the injection.
Stick to the area shown in the instructions for use, on the front of your thigh, where you can see what you're doing.2
Back of the upper arm
The back of your upper arm is also a licensed site. But you can't see it properly or hold the pen at the right angle on your own, so someone else should do it for you.1
If you're getting help, make sure the person injecting has read the instructions for use first.
Do not try to inject into the back of your own arm. Ask someone who has read the instructions to do it for you.
Does where you inject Mounjaro affect your results?
Lots of people search for the best place to inject Mounjaro for weight loss. The honest answer? There isn't one.
All three sites are licensed for Mounjaro, and the official guidance doesn't favour one over the others.1 So the best place is the one you can reach comfortably and inject into correctly, every single week.
Good technique and a regular weekly routine matter far more than which of the three sites you pick.
Mounjaro injection site reactions
Redness, itching or swelling where you've injected is common. It can affect up to 1 in 10 people.3
In weight loss trials, 8.0% of people taking Mounjaro had an injection site reaction, compared with 1.8% on a placebo (dummy treatment).1
These reactions usually settle on their own. If they bother you, let your prescriber or pharmacist know. They might suggest:
- changing how you rotate your injection spots
- a soothing cream for irritated skin
- an antihistamine to help with itching or swelling
- reviewing your dose
Some discomfort is normal, but injecting shouldn't be very painful. Serious allergic reactions to Mounjaro are rare.1 Call 999 straightaway if you have swelling of your face, lips, tongue or throat, or difficulty breathing.
Recommended reading: Mounjaro side effects: what they are and how to manage them
A little redness is normal. Swelling of your face or throat, or struggling to breathe, is not. That is an emergency, so call 999.
Common mistakes when injecting Mounjaro
A few simple slip-ups can make injections more uncomfortable, or stop the medicine working as it should.
Not storing your pen properly
Before you start using a pen, keep it in the fridge at 2°C to 8°C.2 After first use, keep it at no more than 30°C, away from heat and direct light.2
Throw the pen away after four weekly doses or 30 days after first use, whichever comes first. Do not inject any leftover medicine.2
Never freeze Mounjaro. If your pen has been frozen, throw it away, even if it's thawed out.2
Using the same spot every week
Choose a different injection spot each week. You can use the same body area, but rotating the spot helps reduce soreness and bruising.4
Not following the instructions
The instructions for use explain how to attach a needle, prime the pen and inject your dose. Following them step by step means you get the full dose, with less risk of bruising or a damaged pen.2
Recommended reading: How to take Mounjaro injections
Reusing needles
Use a new needle for every injection and dispose of it safely afterwards.2 Old needles are blunter, which can hurt more and raise the risk of infection. Your pen is for you only, so never share it, even with a fresh needle.2
Never reuse a needle or share your pen with anyone else, even if you change the needle first.
Frequently asked questions
Does Mounjaro hurt to inject?
You may feel a brief sharp scratch when injecting Mounjaro. How much it hurts varies from person to person. Use a fresh needle each time and follow the instructions supplied with your pen. Seek medical advice promptly if the pain is severe, keeps going or is getting worse.
Can I inject Mounjaro in the same place every week?
You can use the same area each week, such as your stomach, but you should pick a different spot within that area every time. Rotating the spot helps reduce soreness and bruising. Switching between your stomach and thighs works well too.
What should I do if I miss a Mounjaro dose?
If you miss a dose, take it as soon as you can, as long as it's within 4 days of when it was due. If more than 4 days have passed, skip that dose and take the next one on your usual day. Do not take two doses to make up for a missed one. Always leave at least 3 days between doses.
Can I change the day I inject Mounjaro?
Yes, you can change your injection day if you need to. Just make sure there are at least 3 days between two doses. After that, carry on with your weekly routine on the new day. Setting a phone reminder can make it easier to stick to your new schedule.
Why do I have a bruise after my Mounjaro injection?
A small bruise can happen if the needle nicks a tiny blood vessel under the skin. It's usually harmless and fades within a few days. Pressing gently with a clean tissue after injecting, rather than rubbing, can help. If the area becomes hot, swollen or increasingly painful, speak to a pharmacist or GP.
References
- Eli Lilly and Company Limited. Mounjaro KwikPen 2.5mg solution for injection in pre-filled pen: Summary of Product Characteristics. Electronic Medicines Compendium; 2026. medicines.org.uk/emc
- Eli Lilly and Company Limited. Mounjaro KwikPen: Instructions for Use. Electronic Medicines Compendium; 2025. medicines.org.uk/emc
- Eli Lilly and Company Limited. Mounjaro KwikPen: Patient Information Leaflet. Electronic Medicines Compendium; 2026. medicines.org.uk/emc
- Cambridge University Hospitals NHS Foundation Trust. Your obesity treatment: tirzepatide (Mounjaro). Cambridge University Hospitals; 2026. cuh.nhs.uk
This article is intended for informational purposes only and does not replace professional medical advice. Always read the patient information leaflet supplied with your medication and speak to a healthcare professional if you have specific concerns.