Mounjaro Pen Malfunctions
Mounjaro is a popular and effective weight loss medication that can be conveniently self-administered at home. However, sometimes things can go wrong with your Mounjaro pen. In this detailed guide, we’ll show you how to identify and solve common Mounjaro pen malfunctions and issues to help you keep informed and stay on track with your medication.

Medically reviewed by
Dr Brenda IkejiLast reviewed: 16 Jan 2025
What is a Mounjaro pen malfunction?
A Mounjaro pen malfunction is any issue that stops the pen working as intended and disrupts the injection process. This can have a direct impact on your treatment and may cause some anxiety or confusion.
Mounjaro weight loss injection pens have been designed to deliver an exact dose of the active ingredient, tirzepatide. If the pen doesn’t work properly, it can lead to a missed dose.
Some of the common Mounjaro pen malfunctions include:
- a jammed pen
- bent needle
- needle not attached
- medication leaking from the pen
Understanding and identifying these potential malfunctions are important to ensure the safe and effective use of the Mounjaro pen.
Common Mounjaro pen malfunctions
There are several malfunctions you may encounter when using your Mounjaro pen. Here are some of the most common ones explained.
1. The Mounjaro pen is jammed
The Mounjaro pen can become jammed for many reasons, including:
- the pen has already been fully used
- the needle isn’t attached properly
- the pen hasn’t been primed before each use
- the pen hasn’t been stored properly
2. There's leftover liquid
Each Mounjaro pen contains more liquid than you will inject over the 4 doses, which means there will be some leftover liquid after your final dose. This is sometimes referred to as the “fifth dose” or “golden dose”. This isn’t anything to worry about and does not mean you have missed a dose. Keep an injection diary so you can check you have taken your correct doses and identify any missed ones.
3. The needle is bent or blocked
If the Mounjaro pen isn’t injecting properly or the needle looks bent or faulty, you should replace the needle with a new one.
Before using a Mounjaro pen, make sure the needle is straight, correctly attached to the pen, and that the inner and outer caps are removed. If the needle is loose, blocked, or bent, it will affect or prevent the medication flow and incorrectly deliver your dose.
4. The pen leaks
If the Mounjaro pen looks damaged or is leaking, do not use it. Instead you’ll need to use a new pen. If you store your pen with the needle left on, this can cause leakage. Equally, if you drop or damage your pen, it may leak.
5. The pen is frozen
You must store unused pens in the fridge until you are ready to use them. However, if you store them in a fridge which is cooler than 2°C or close to a freezer compartment, the medication may freeze.
How to fix a jammed Mounjaro pen
If your Mounjaro pen won’t inject or you suspect it may be jammed, try the following:
- Check the needle attachment – make sure the needle is attached securely to the pen.
- Remove both caps – ensure both the inner and outer caps are removed. If you are struggling to remove the cap, gently twist it back and forth and pull it off.
- Push the dose knob slowly – this should make it easier to release the medication.
- Replace the needle – if the needle is blocked, bent, or damaged, replace it with a new one then prime your pen before use.
How to prime the Mounjaro pen
Priming the Mounjaro pen before the first use is an important step and ensures that it delivers the correct dose and is working properly.
To prime you pen after you have replaced a needle or before using the pen for the first time, you should:
- Turn the dose selector button slowly until you hear 2 clicks and the extended line is shown in the dose window – this is the prime position.
- Holding the pen with the needle pointing up to the sky, gently tap the cartridge holder to collect any air bubbles at the top.
- Release some of the medication by pushing the dose button until it stops, then slowly count to 5 while holding the dose button in. The [0] icon should be visible in the dose window.
- Your pen is now primed if a small amount of medication is released from the tip of the pen needle.
- Do not inject the priming dose into your body.
Should I stop using my Mounjaro pen?
If you’ve followed all of the above steps (checked and replaced the needle, removed both the inner and outer caps, checked you’ve not injected all 4 doses) and your pen is still not working, it’s likely there is a malfunction you can’t fix.
You should stop using your Mounjaro pen if it:
- does not inject the medication, even after you have tried to troubleshoot the problem
- is frozen
- has physical signs of damage
- is leaking
What should I do if my Mounjaro pen is faulty?
If your Mounjaro pen is faulty and you have tried all the suggested fixes and have not been successful, you should not use the pen.
Contact our Patient Care team as soon as possible, so they can advise you on what you need to do next. The best way to get in touch with us is through your patient account or you can call our patient care line.
Alternatively, you can contact Mounjaro’s manufacturer, Eli Lilly, for advice about a defective pen.
How to prevent Mounjaro pen malfunctions
The best way to prevent malfunctions with the Mounjaro pen is to follow all the usage and storage suggestions correctly. By preventing malfunctions, you’ll ensure the pen delivers the correct medication dose and you’ll avoid any disruptions or delays in your treatment schedule.
By taking a few simple precautions, you can reduce the likelihood of some common Mounjaro pen malfunctions.
1. Properly store the Mounjaro pen
You should store any unused Mounjaro pens in the fridge at a temperature between 2 and 8°C. However, you must prevent the medication from becoming frozen. Used pens can be stored at room temperature (up to 30°C) for no more than 30 days.
All Mounjaro pens, whether used or unused, should be kept away from direct sunlight, extreme heat, and humidity.
2. Handle the pens with care
- Inspect the pen before use – Check your pen before each use for any visible signs of damage such as cracks or leaks. If the pen looks damaged, do not use it.
- Avoid dropping the pen – Dropping the pen can bend the needle, damage the internal mechanism, or break the pen. Handle the pen gently to keep it in good working order.
3. Follow the user instructions carefully
- Assemble the pen correctly – Make sure the needle is properly attached to the pen before attempting to inject the medication. If the needle is loose or not attached properly, it can cause leaks or incorrect dosing.
- Prime the pen – Follow the instructions to prime your pen before its first use or after you have attached a new needle to make sure the Mounjaro pen works as it should.
- Inject the medication exactly as you have been shown – You must only inject Mounjaro in the recommended injection sites, such as the abdomen, thigh, or upper arm. You should also insert the needle at the correct angle to avoid bending.
4. Inspect the medication
Before injecting, you should check that the medication is clear and colourless or slightly yellow. If it is cloudy, discoloured, contains debris, or is frozen, do not use the pen.
5. Use within the correct time period
- Check the expiry date – Before using the pen, make sure it is within the date. Do not use a pen which has passed its expiration date.
- Open pens – Any opened pens should be used within 30 days.
6. Seek advice if you’re unsure
If you are not sure about anything, speak to your doctor or pharmacist for advice. They can provide support and guidance to make sure you are using the Mounjaro weight loss injection pen safely and effectively.
Frequently asked questions
Can I still use my Mounjaro pen if the needle is bent?
No, if you notice the needle is bent before your injection, you should replace it with a new one.
What happens if the Mounjaro pen does not inject the full dose?
If the Mounjaro pen fails to deliver the full dose, so you do not see the [-0] icon in the dose window, you’ll need to reinsert the needle back into your skin and finish the injection. Do not redial the dose.
Why is my Mounjaro pen not working after being stored in the fridge?
If you have removed the Mounjaro pen from the fridge and it isn’t working, it could be that your fridge was too cold or the pen was stored too close to the freezer compartment and the medication has frozen.
If the pen has frozen, you must not use it and you should replace it with a new one.
Before injecting Mounjaro, we recommend waiting for the medication to reach room temperature before use after you have removed it from the fridge. This will make the injection more comfortable.
Why is there no click sound during the injection?
The Mounjaro Kwikpen doesn’t have a click sound. However, you can see if you have injected the full dose if the [-0] icon is shown in the dose window.
How can I tell if my Mounjaro pen is expired or defective?
If it is past the expiry date printed on the Mounjaro pen and packaging, you should not use the pen. You must also avoid using the pen if:
- the medication is frozen
- the liquid is cloudy, discoloured, or not its usual colour
- the pen has signs of damage or has been dropped
What happens if I miss a dose of Mounjaro?
If you miss a dose because your pen is defective and you do not have a replacement to hand, you should follow the instructions below:
- if it has been 4 days or less since you missed your last dose and you have received a replacement pen, you can inject the missed dose and then inject your next dose as usual on your scheduled day
- if it has been more than 4 days since you should have taken Mounjaro and you have not received a replacement pen, skip the missed dose and inject your next dose as usual on the scheduled day
If you are unsure, contact your doctor for further instructions.
Brenda studied medicine at St Georges University of London and has most recently worked in John Radcliffe Oxford University Hospitals, covering acute and general medicine. Before her medical career, Brenda was a medical scientist in microbiology working in several NHS diagnostic laboratories across England. She has also completed her Masters in Public Health and MSc in Genomic Medicine. She is currently a GP registrar.
Last reviewed: 15 Jan 2025
Sources
- Mounjaro KwikPen 10mg solution for injection in pre-filled pen, emc [accessed 02 January 2025]
- Mounjaro KwikPen 10mg solution for injection in pre-filled pen user manual, emc [accessed 02 January 2025]
- Tirzepatide, NICE [accessed 02 January 2025]
- Tirzepatide Once Weekly for the Treatment of Obesity, NEJM [accessed 02 January 2025]
- Troubleshooting common Mounjaro® (tirzepatide) KwikPen® questions, Lilly [accessed 02 January 2025]
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