If you want the needle size answer straight away: a 4mm, 32G pen needle is a common choice for Mounjaro. It is short and fine, and is suitable for many adults. Your pharmacist or prescriber can advise if a different size is more appropriate for you.
The rest of this page covers the practical details that matter: the Mounjaro KwikPen does not come with needles, which pen needles are compatible, how to inject safely, and what to expect after an injection.
What Needle Size Do You Need for the Mounjaro KwikPen?
A common choice is a 4mm length and 32G gauge. Here is what those numbers mean and why this combination is often used.
- Length (millimetres): Compatible pen needles are available in different lengths, including 4mm to 8mm. Mounjaro is injected into the fatty tissue under the skin. A 4mm needle is commonly used for subcutaneous injections in adults and may be more comfortable for some people. Your pharmacist or prescriber can advise if you need a different size.
- Gauge (thickness): Gauge works in reverse, so a higher number means a thinner needle. A 32G needle is very fine, while 31G is slightly thicker. The difference between them is relatively small.
For many people, 4mm x 32G is a practical option. However, the most suitable needle can depend on your individual circumstances, so follow the advice provided with your treatment or given by your healthcare professional.
One useful thing to know is that needle length does not change the amount of Mounjaro you receive. The dose is determined by the pen and prescribed strength, not by the length of the needle.
Does the Mounjaro pen come with needles?
No. The Mounjaro KwikPen is supplied without needles, so you need to obtain compatible pen needles separately. Your prescription for Mounjaro may not automatically include them.
You can buy replacement Mounjaro needles from a pharmacy, subject to availability and compatibility with your pen. It is a good idea to have compatible needles ready before your first injection so you are not caught without them.
For more information, see our KwikPen needles FAQ.
Which brands fit?
The KwikPen is designed to be used with compatible screw on pen needles that meet the relevant international standard, including ISO 11608 2. You are not necessarily limited to one brand.
Common compatible pen needle brands available in the UK include BD (Becton Dickinson), NovoFine, Terumo, GlucoRx and Owen Mumford. Always check that the needle is suitable for your specific pen before using it, and ask a pharmacist if you are unsure.
For most people, the needle specification and compatibility matter more than the brand name. A pharmacist can also help you choose an appropriate option.
Where can you get Mounjaro needles?
You can buy compatible pen needles from a pharmacy, either in person or through a reputable online pharmacy. If you are ordering online, check that the pharmacy is properly registered and that the needles are suitable for the Mounjaro KwikPen.
A 100 pack may be convenient if you are using one new needle for each weekly injection, but the amount you need will depend on how long you are using the pen and the pack size available.
A practical tip: if you are given 8mm needles, do not switch to a different size without checking that it is suitable for you. A 4mm needle is commonly used and may be more comfortable, but your pharmacist or prescriber can advise on the right option.
Most importantly, use a new needle for every injection. Do not reuse needles, as this can affect hygiene and may make injections more uncomfortable. Used needles should be placed in an appropriate sharps container rather than thrown loose into household waste. You can ask your local pharmacy about obtaining a sharps bin and disposing of it safely.
Is Mounjaro Self Injected? Yes, and It Is Manageable
Mounjaro is designed to be self injected once a week at home. Many people find the injections manageable once they become familiar with the process. The injection uses a small pen needle to deliver the medicine into the fatty tissue just under the skin. It is a subcutaneous injection, rather than an injection into a muscle.
If you feel nervous about injecting yourself, that is completely understandable. Taking a little time to learn the technique and follow the instructions provided with your pen can make the process feel more straightforward. If you remain uncomfortable or unsure, speak to your prescriber or pharmacist for guidance.
Does the Injection Hurt? For People Who Dislike Needles
An honest answer, because "does it hurt?" and needle anxiety are common concerns when people are starting Mounjaro.
Many people find the injection causes little discomfort, although everyone's experience is different. A 4mm, 32G needle is short and fine, and some people describe the injection as a small scratch or pinch. Others may barely notice it. Pain can also vary depending on the injection site and your individual sensitivity.
If you find needles difficult, these practical injection tips may make the process easier:
- Choose a comfortable injection site. The abdomen, thigh or upper arm can be used according to the instructions provided with your Mounjaro pen.
- Follow the storage instructions. Mounjaro needs to be stored correctly. Before using the pen, follow the product instructions about when to remove it from the fridge and how long it can remain at room temperature.
- Relax the area. Try to keep the injection area relaxed rather than tensing your muscles.
- Follow the pen instructions carefully. Use the injection technique provided with your treatment rather than rushing or changing the method yourself.
- Distract yourself. Listening to music, taking a slow breath or choosing a regular time each week may make the process feel more routine.
- Look away if that helps. You do not need to watch the needle if doing so makes you more anxious.
If you have severe needle anxiety or a phobia, let your prescriber know. There is nothing embarrassing about finding injections difficult, and they may be able to suggest practical support or techniques to make treatment easier.
Where to Inject Mounjaro: Sites and Rotation
Mounjaro is injected into the fatty tissue under the skin. The recommended injection areas are the abdomen, thigh and upper arm, following the instructions supplied with your pen.
- Abdomen (stomach): You can inject into the abdomen, but avoid the area immediately around your belly button. Follow the product instructions for the recommended distance.
- Front or outer thigh: The thigh can be a convenient option when you are injecting yourself.
- Back of the upper arm: This area can be used, although you may find it difficult to reach yourself comfortably. You may need someone else to help if this is your chosen site.
Rotate your injection sites. Try not to inject into exactly the same spot each week. Moving to a different area can help reduce repeated irritation to the skin. Avoid injecting into areas that are sore, bruised, scarred, hard, irritated or otherwise damaged.
There is no single injection site that is considered the best for weight loss. The important thing is to use an appropriate site and follow the instructions for your Mounjaro pen. If you are unsure where or how to inject, ask your prescriber or pharmacist to demonstrate the technique.
Injection site reactions can occur, although they are not experienced by everyone. If you develop significant swelling, a severe skin reaction or symptoms that concern you after an injection, seek medical advice.
How to Inject Mounjaro, Step by Step
Your Mounjaro Instructions for Use leaflet is the most important guide for your specific pen, so always follow it. The basic process is:
- Prepare the pen and wash your hands. Follow the storage instructions for your pen before use.
- Check the medicine through the dose window. It should be clear and colourless to slightly yellow. Do not use it if it looks cloudy, has changed colour, contains particles, or has been frozen. Check that you are using the prescribed strength.
- Attach a new needle. Remove the paper tab, screw the needle onto the pen as instructed, and then remove the outer needle shield and inner cap.
- Prime the pen if required. Follow the priming instructions supplied with your KwikPen. This helps confirm that the pen is ready to deliver the dose. If you see air bubbles, follow the manufacturer's instructions rather than trying to remove them yourself in another way.
- Choose and clean your injection site. Use an approved injection area and allow the skin to dry before injecting.
- Inject under the skin. Follow the instructions supplied with your pen for positioning the needle and injecting into the fatty tissue under the skin.
- Deliver the full dose. Press and hold the dose knob as instructed and keep the pen in position for the required amount of time. This helps ensure the prescribed dose is delivered.
- Remove and dispose of the needle safely. Withdraw the pen as instructed, remove the used needle and place it immediately in an appropriate sharps container. Do not reuse the needle.
If you miss a dose, follow the instructions in your patient information leaflet or speak to your prescriber or pharmacist. Do not take an extra dose to make up for a missed injection.
What Is Normal After Injecting, and What Is Not?
It is understandable to wonder whether what you are seeing after an injection is normal. Here is a simple guide to what you may notice and when it is worth getting medical advice.
Usually mild and temporary
- A tiny amount of bleeding. A small spot of blood can happen after removing the needle. You can gently press the area with clean tissue if needed.
- A small bruise. A minor bruise can happen if the needle catches a small blood vessel and will usually fade on its own.
- Mild redness or tenderness. The skin around the injection site may feel slightly sore or look mildly red for a short time.
- A small amount of swelling or itching. Mild injection site reactions can occur and often settle without treatment.
These reactions do not necessarily mean that you have done anything wrong. However, if a reaction is severe, persistent or getting worse, speak to your prescriber or another healthcare professional.
If the area feels uncomfortable, a cool compress may provide some relief. Do not rub the injection site.
When to contact your prescriber
Speak to your prescriber, pharmacist or GP if you notice:
- A lump that becomes increasingly painful, hot, swollen or starts to leak fluid.
- Redness or swelling that continues to spread or gets worse rather than settling.
- An injection site reaction that is severe or does not improve.
- Persistent or unusual pain after an injection.
If you develop swelling of the face, lips, mouth or throat, difficulty breathing, or a widespread severe allergic reaction, seek emergency medical help immediately. In the UK, call 999 in an emergency.
"I think I injected into muscle" or did it wrong
If you think you may have injected at the wrong angle or into a deeper layer than intended, try not to panic. Mounjaro is intended to be given as a subcutaneous injection into the fatty tissue under the skin, rather than into muscle.
Do not automatically inject another dose if you are unsure whether the first injection was delivered correctly. Taking another dose could result in you receiving more medicine than prescribed.
Instead, contact your prescriber or pharmacist and explain what happened. They can advise you based on the circumstances.
For future injections, follow the technique in your Mounjaro Instructions for Use leaflet and use the needle size recommended for your pen and circumstances. If you are unsure about the injection technique, ask your healthcare professional to demonstrate it rather than trying to correct your technique through guesswork.
Disposal
Used needles and finished pens should go into a sharps bin, not the household bin. Do not try to recap a used needle by hand, and never reuse needles.
When your sharps bin is full, follow your pharmacy or local council's guidance for collection or return. Depending on where you live, a local pharmacy may also be able to advise you about sharps bin return or collection arrangements.
Key Takeaways
- Most people use 4mm x 32G pen needles, which are short and fine.
- The KwikPen comes without needles, so you need to buy and attach a compatible needle for each injection.
- Compatible pen needles that meet the relevant ISO standard can be used with the KwikPen.
- Needle length does not change the amount of Mounjaro you receive, so choosing a shorter needle is mainly about comfort and suitability.
- Many people find the injection manageable, although some may feel a small scratch or brief discomfort.
- The abdomen, thigh and back of the upper arm are suitable injection sites. Rotate your injection sites and avoid irritated or damaged skin.
- Use a new needle for every injection and dispose of used needles safely.
- Mild bruising, a small amount of bleeding, redness or a small lump can happen after an injection. A lump that becomes hot, increasingly painful, swollen or larger should be checked by a healthcare professional.
