Where to Inject Mounjaro: Best Injection Sites & Common Mistakes to Avoid

Mounjaro comes as a pre-filled pen containing four weekly doses, designed to be simple and convenient to self-administer at home. The medication is injected subcutaneously, which means just under the skin, into areas with a layer of soft fat.

Knowing where and how to inject Mounjaro is essential to getting the most from your treatment. Choosing the right injection site can make the process easier, reduce discomfort, and help the medication work as effectively as possible.

In this guide, we will take you through the best Mounjaro injection sites, step-by-step instructions for safe self-injection, and common mistakes to avoid so you can feel confident and in control of your treatment.

Where should you inject Mounjaro?

Where you inject Mounjaro is important to ensure the medication is delivered safely and effectively.

You can inject Mounjaro into your:

• stomach
• thighs
• upper arms (you may need someone else’s help to inject here)

These areas have more fatty tissue than other parts of the body and are easy to access for most patients.

You should choose an injection site that you can easily reach. This makes it easier to inject Mounjaro correctly and reduces the risk of pain or discomfort.

It is recommended to rotate the injection site (stomach, thighs, or upper arms) each time. You can choose the same injection site, but it should be at least one finger’s width from the previous injection spot. This helps to minimise irritation and skin damage.

Refer to the patient information leaflet as well as the instructions for use for more details.

Choosing Mounjaro injection sites

The three recommended injection sites are the stomach, the thighs, and the upper arms.

After injecting Mounjaro a few times, you will be able to identify which injection sites are most comfortable and work best for you.

Stomach (abdomen)

The stomach is a recommended injection site because it is easy to access for most patients and has a large surface area you can inject into.

Some patients find it less painful to inject into the stomach. This is because there is more fat tissue around the stomach than other injection sites, such as the arms.

You must inject at least 2 inches (5cm) away from your navel (belly button) if you choose this injection site.

Thighs

The thigh area is also recommended as it is easy to access and has enough fat tissue to effectively absorb the medication.

You should pick an area on the front of your thighs so you can see where you are injecting more easily.

You should avoid injecting into the inner thigh, as this area has more nerves and blood vessels. Injecting here increases the risk of injury and can cause significantly more pain compared to the recommended sites.

Upper arms

The back of the upper arm is another recommended injection site.

Someone else should deliver the injection if you choose this injection site. This is because the back of the arm can be difficult for you to see and reach. Injecting on your own can increase the risk of mistakes, which may cause injury or make Mounjaro less effective.

If you have asked someone else to inject for you, make sure they have fully read the instructions for use.

Mounjaro injection site reactions

It is common for redness, itching, or swelling to occur at the injection site. Up to 1 in 10 people taking Mounjaro have reported these side effects.

In weight loss clinical trials, 8.0% of patients experienced injection site reactions while taking Mounjaro. Out of those patients, 91% had mild reactions^[1].

These side effects are usually mild and should subside on their own as your body adjusts to the medication. They may return temporarily when you increase your dose, but this is normal.

Let your treatment provider know if you experience any of these symptoms. They might suggest:

• changing the injection site you use
• relieving any irritation with a topical over-the-counter cream
• taking an antihistamine to reduce any swelling
• trying a lower dose of Mounjaro

While some discomfort is normal, it should not be very painful when injecting Mounjaro. If you experience any severe swelling, irritation, or redness, you should get immediate medical assistance. This could indicate an allergic reaction to the medication.

Find out more in our guide: Mounjaro Side Effects: What They Are & How To Manage Them

Common mistakes when injecting Mounjaro

Not storing Mounjaro correctly

You must store Mounjaro at the correct temperature for the medication to be effective.

Mounjaro must be kept in the fridge between 2ºC and 8ºC if it has not been used yet. After first use, it can be stored for up to 30 days at room temperature, below 30ºC. You should keep your Mounjaro pen away from heat and light.

You must never freeze Mounjaro. If your pen has been frozen, you must dispose of it, even if it has thawed afterwards.

Not rotating injection sites

It is important to rotate your injection site each week as this helps maintain skin health and ensures Mounjaro is absorbed consistently.

If you prefer to use the same area of the body, such as the stomach, you should choose a different spot within that area each time.

Always inject at least one finger’s width (around 2cm) away from the previous injection spot. Injecting into the same spot repeatedly can cause irritation, inflammation, or hard lumps under the skin. It may also damage the skin and increase the risk of infection.

Not using correct injection technique

You must follow the injection instructions that come with your Mounjaro medication. These can be found in the patient information leaflet and the instructions for use.

Recommended Reading: How to Take Mounjaro® Injections

Following these instructions helps ensure you receive the correct dose of medication and reduces the risk of side effects or injury, especially to your skin.

Incorrect technique can also damage your Mounjaro pen or the needles, which may make the treatment less effective or unusable.

Not using a new needle for every injection

You must use a new needle for each Mounjaro injection. The needles are designed to be single-use and must be disposed of safely after use.

Reusing needles can increase the risk of infections and skin damage. It can also make the needle less effective, which may prevent you from receiving the full prescribed dose of medication.

Frequently asked questions about injecting Mounjaro

Is Mounjaro painful to inject?

You may feel slight pain or discomfort while injecting Mounjaro because the needle goes under your skin. Some people report feeling a ‘sharp scratch’, while others do not feel any pain.

If you feel severe pain that does not go away, you should seek medical attention.

Why does the injection site hurt?

Some pain, swelling, or bruising around the injection site is normal and often improves or stops altogether as your body adjusts to the medication.

Make sure you are rotating your injection site each time. You can consider taking an antihistamine to reduce any reactions to the injection. You can also use an ice pack to numb the injection site before injecting to reduce pain and swelling.

Why is the injection site red?

Redness around the injection site is also a common reaction. This could be a reaction to the medication or one of the ingredients in the pen. Your skin may also be sensitive to needles or being injected.

You should speak to your treatment provider if the redness does not go away or if it continues to be painful.

Do I have to change injection sites every time?

It is recommended to change the injection site (stomach, thighs, or upper arms) each time you inject. You can choose the same injection site, but you should avoid injecting into the same injection spot where you previously injected.

For example, you can inject into your stomach each week, but it should be in a different location on your stomach each time. You should leave at least a finger’s width (about 2cm) between injection spots.

Sources

1. Mounjaro KwikPen 2.5mg solution for injection in pre-filled pen, electronic medicines compendium, 2025. [Accessed 08/08/2025]

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