Mounjaro Pen Not Working: Troubleshooting Mounjaro Pen Malfunctions
Mounjaro is considered one of the best weight loss injections and is designed to deliver impressive results from your own home. It is administered via an easy-to-use pen that injects the solution subcutaneously (under the skin).
Although it’s a very rare occurrence, you may notice that your Mounjaro pen is not working as intended, and this may understandably cause some concern. The good news is that many issues can be easily resolved by using the KwikPen as instructed, without interrupting your weight loss journey.
In this article, we’ll discuss some common Mounjaro pen malfunctions and outline the necessary tips and tricks to overcome them. Read on to discover more.
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Mounjaro Pen Tips for Use
The Mounjaro KwikPen has been designed with simplicity in mind, as it is already pre-filled with the correct doses. It’s important that you take steps to ensure you are using Mounjaro correctly so it remains effective for weight loss.
Some of the top Mounjaro pen tips include:
- Wash your hands and the injection site before use.
- Inspect the pen for any visible signs of damage and check the expiry date.
- Always attach a new needle before each dose and remove it once your injection is complete.
- Prime the pen to remove air bubbles and make sure it is working correctly.
- Choose your injection site and rotate each week to prevent injury and irritation.
- Store the pen correctly.
- Always inject Mounjaro on the same day each week.
These tips will help prevent a Mounjaro pen malfunction, keeping it effective and preventing disruption to your weight loss progress. Our dedicated guide will show you how to safely inject Mounjaro.
Alternatively, watch this video to see how to correctly inject Mounjaro.
What are Mounjaro Pen Malfunctions?
A Mounjaro pen malfunction is anything that prevents the pen from working as it should, disrupting the injection process and potentially affecting your weight loss progress. Experiencing Mounjaro pen malfunctions can cause anxiety and confusion, but most can be managed quickly and easily.
Mounjaro pens have been designed to deliver an exact dose of the active ingredient, tirzepatide, but if your pen malfunctions, it can lead to a missed or incorrect dose. Some of the common reasons your Mounjaro pen is not working include:
- Bent or broken needle
- Jammed pen
- Frozen Mounjaro pen
- Leaking pen
It is important to understand how to manage common Mounjaro pen malfunctions to ensure you receive safe, effective doses each time you inject your medication, minimising the risk of side effects and limiting any disruption to your treatment.
9 Common Mounjaro Pen Malfunctions
Several malfunctions can occur when injecting Mounjaro, but it is important that you know what signs to look out for —and, importantly, how to deal with them. Here, we explore 9 common Mounjaro pen malfunctions and what to do if any occur during your treatment.
The Mounjaro Pen is Jammed
There are several reasons why the Mounjaro pen can become jammed. If you have followed all usage instructions for the Mounjaro pen and it is still stuck, there may be an issue with the activation mechanism. There could be many causes, and in some cases, you may be able to resolve the issue without starting a new pen. For example:
- The pen has not been primed: You need to prime the pen to remove air bubbles and enable the medication to flow properly. You should prime the pen a maximum of twice per injection.
- The pen was not stored correctly: When storing your Mounjaro pen, you must remove the needle and put the lid on to keep it protected from dust and debris.
- The pen needs a new needle: Use a new needle each time you use the pen. This will help prevent infections and injuries.
- The medication is being injected at the wrong angle: Position the pen at a 90-degree angle to your skin. An incorrect position can make the internal mechanism feel stiff.
- Your technique is wrong: Make sure you place your thumb firmly on the injection button and position your fingers comfortably around the pen shaft. Gripping the pen correctly can make the injection feel smoother and less resistant.
- The speed: If you press the plunger too quickly, it can feel like it is stuck, and you may not get the right dose. Press the plunger with steady pressure.
The Mounjaro Pen Won’t Twist
If the Mounjaro pen does not twist, it may be empty, jammed, or the last dose may not have been fully administered. Do not attempt to force the pen to twist. Instead, you should:
- Make sure the pen isn’t empty
- Remove and replace the needle
- Check for debris that may be clogging the mechanism
- Prime the pen
If you have tried all of the above and it still won’t twist, do not use the Mounjaro pen. You will need to open a new Mounjaro KwikPen.
The Mounjaro Pen is Stuck on the Last Dose
If your Mounjaro pen is stuck on the last dose, it is probably empty and locked as it should be. Each Mounjaro KwikPen is designed to deliver 4 doses. That means your pen will last four weeks before you will need to start a new one. Once all the doses have been used, the pen’s safety mechanism stops it from administering more doses after the fourth and final one, even if a small amount of liquid is left in the pen.
Mounjaro
- Approved Weight Loss Treatment
- Suitable for adults with a BMI of +30kg/m²
- Pre-filled injection pen
There is Liquid Left in the Mounjaro Pen
This one is not necessarily an indication that your Mounjaro pen is not working and may actually be completely normal. Each pen contains 2.4 ml (4 doses of 0.6 ml) of injectable solution, plus an additional amount as a safety measure to help with the priming process. It is important not to overprime your pens when using Mounjaro.
Even if you have injected Mounjaro correctly, there may still be a small amount of medication left over after all four doses have been administered. This is sometimes referred to as the “golden” or “fifth” dose, but should never be injected.
If you are concerned about the amount of liquid left in the Mounjaro pen after all four doses have been administered, contact your clinician for advice. They should be able to confirm whether everything is operating as intended.
There is an Air Bubble in the Mounjaro Pen
Small air bubbles in the Mounjaro pens are usually harmless and not a cause for concern. They can sometimes form within the cartridge if the needle is not removed after injection. Because the pen is designed to give a subcutaneous (under-the-skin) injection, rather than into a blood vessel, small air bubbles will not affect the dose or effectiveness or cause you harm.
Removing the needle after each injection will help prevent air bubbles from forming. Priming your pen correctly before use will also remove air bubbles, so you can inject with peace of mind. To deal with an air bubble in the Mounjaro pen, hold it with the needle pointing upward and gently tap on the cartridge to collect any bubbles at the top. You can then release some medication into the air by fully pushing in the dose knob and slowly counting to 5.
Should you notice an air bubble in the Mounjaro pen that seems unusually large, you may want to speak to your clinician for advice.
The Mounjaro Seal is Broken
The inner red seal of the Mounjaro pen should be flat and intact; if it is bulging or broken, do not use the pen, as this may indicate a problem. You should contact your clinician or the manufacturer to discuss getting a replacement.
When using the Mounjaro pen, press the dose knob only once a suitable needle is attached, as doing so incorrectly could impede the flow of the solution. If the flow of medication is blocked for any reason, the red seal may bulge. This could cause medicine to be wasted when a needle is attached, so a full dose may not be received.
The Mounjaro Pen is Leaking
If the Mounjaro pen looks damaged or is leaking, do not use it. Storing your pen with the needle on it can increase the chance of leakage, or dropping the pen can also cause it to leak. In addition, a leak may occur due to a bulging or broken seal.
If your pen is leaking, throw it away and start a new one, even if there is medication left in it.
The Mounjaro Needle is Bent After Injection
If the Mounjaro needles are bent after you have injected the medication, it isn’t usually a cause for concern. It may be that you applied too much pressure, did not replace the needle, held it at the wrong angle, or injected it into your arm. The best places to inject Mounjaro are your stomach or thigh, or upper arm if you have someone to help you.
Remove the bent needle and store your Mounjaro pen as you usually would. Then, replace the needle when it’s time for your next injection.
If you are concerned that the needle bent during the injection and are unsure whether you’ve received the full dose, you can check the dose window to see if the “0” icon is showing. Should you still not have a clear idea, it’s recommended to wait until your next scheduled dose rather than risk accidentally double-dosing.
Mounjaro is designed for long-term use, so missing one week won't derail your progress. You may experience fluctuations in appetite, so keeping to your diet and exercise regimen will be crucial for maintaining success. Getting back to your usual schedule with a new needle will also keep you on track.
The Mounjaro is Frozen
If your Mounjaro pen has frozen, you must throw it away, as it is unsafe to use and will not be effective. If tirzepatide is exposed to low temperatures, it can degrade the medication. You will need to start a new pen.
How to Prevent Mounjaro Pen Malfunctions
The best way to ensure your Mounjaro pen operates properly and does not malfunction is to follow all usage and storage instructions, as well as any advice your clinician gives you. Preventing malfunctions is an important step towards ensuring your treatment runs smoothly, without disruptions or delays, and that your medication remains effective.
Here’s how to prevent Mounjaro pen malfunctions:
1. Store The Mounjaro Pen Properly
Storage is crucial. Any unused Mounjaro pens must be stored in the fridge at 2-8 °C. You should make sure they are stored away from the freezer element and not in the door, as this is where the temperature fluctuates most. Opened pens can be stored at room temperature (below 30℃) for up to 30 days or in the fridge.
Keep all pens out of direct heat, sunlight, and humidity.
2. Handle Pens With Care
Mounjaro pens are breakable and fragile. It is important to inspect your pens to make sure they are still fully functional. Look for any visible signs of damage, such as cracks or leaks, before using the pen. If it is damaged, do not use it.
If you drop the pen, it can bend the needle, damage the mechanism, or lead to breakages. If there are any physical signs of damage, throw the pen away.
Mounjaro
- Approved Weight Loss Treatment
- Suitable for adults with a BMI of +30kg/m²
- Pre-filled injection pen
3. Follow The User Manual
The User Manual that comes with your pen is a great resource and will help ensure you use and inject the pen correctly. Some of the steps you should consider include:
- Pen assembly: Always use a new needle before injecting your Mounjaro dose and make sure it is properly attached.
- Prime the pen: You should prime the pen before the first use and after attaching a new needle. This will help make sure the pen is working as it should.
- Only inject the medication as you have been shown: Mounjaro must be injected into the thigh, stomach, or upper arm, at a 90-degree angle to your skin to avoid bending the needle.
If you are unsure about how to inject Mounjaro safely, check out our guide.
4. Check The Liquid
Before use, always check that the medication is clear and colourless or slightly yellow by looking through the medication window on the pen. If it is cloudy, frozen, or there are visible signs of debris, do not use the pen.
5. Check The Expiry Date
It is important that you do not use an expired pen. You can check the expiry date, which is printed on the pen label and on the original packaging.
Any open pens must be used within 30 days of the expiry date. If the pen goes out of date during use, you must throw it away and start a new one.
When to Stop Using a Mounjaro Pen
If you have tried all the troubleshooting steps or you’re not sure whether the pen is safe to use, contact your clinician for guidance. They will provide you with support and guidance to make sure your pen is safe and effective, or advise you to start a new one.
In some cases, a faulty pen must be sent back to the manufacturer for testing. Make sure you keep all the pen components, including the original packaging, just in case you do need to return Mounjaro.
Mounjaro Pen Malfunctions: Frequently Asked Questions
The Mounjaro KwikPen has been designed with simplicity in mind, making it a user-friendly self-injectable medication. However, there are some instances where things can go wrong, or you may have questions about what you should do if your pen is defective. Here, we answer some of the most common questions about Mounjaro pen malfunctions.
Can I Keep Using a Bent Mounjaro Needle?
No, you cannot use a bent Mounjaro needle, so you must replace it before injecting the medication. A bent needle can prevent the injection pen from working properly and may result in an incorrect dose, making Mounjaro less effective. A bent needle may also result in an injury.
Why is My Mounjaro Pen Not Working After Being in the Fridge?
Mounjaro should be stored in the fridge between 2 and 8℃. If the fridge was too cold or the pen was too close to the element, the pen may have frozen and is now unusable. Before use, check the liquid's colour to ensure it is clear, colourless, and free of debris. If not, you will need a new one.
How to Tell if Mounjaro Has Expired
The easiest way to tell if Mounjaro has expired is to check the expiry date printed on the pen label and on the original box. If the pen is past its expiry date, it must not be used, and you will need to start a new one.
You should also look at how the medication looks, even if it has not passed its expiry date. Do not use the pen if:
- The liquid has frozen
- There are particles in the liquid
- The liquid is cloudy or discoloured
- The pen has physical signs of damage
How to Report a Defective Mounjaro Pen
If your Mounjaro pen is not working and you have followed all the advice in this article, do not use it and contact our customer service team as soon as possible. Alternatively, you can contact Eli Lilly, the manufacturer of Mounjaro, directly about a defective pen.
A Weight Loss Service You Can Rely Upon
Using a weight loss injection like Mounjaro may seem daunting at first, but with the right level of advice and support, you should be able to navigate your journey with confidence. This is where Prescription Doctor comes in.
We’re proud to provide a holistic weight loss consultation service that extends far beyond simply offering effective medications. Our clinicians will be with you every step of the way, from recommending the right treatment for your circumstances to regularly checking in to monitor your progress. They are also on hand to answer any questions you may have.
In addition, we have a library of useful information on our website relating to Mounjaro and weight loss in general. So, whether your Mounjaro pen is not working, you’re curious about the best time of day to take the medication, want to know about the various injection sites, or don’t think you’re losing as much weight as you should, we have you covered. We can even help if you’re considering switching from Wegovy to Mounjaro, or vice versa.
Prescription Doctor is committed to taking the hassle out of healthcare, which is why all our treatments can be obtained following a short consultation, completed from the comfort of your own home at any time. Orders approved before 3pm on a weekday are eligible for next-day delivery, while everything is dispatched in plain packaging from our UK-based General Pharmaceutical Council-registered dispensing pharmacy, ensuring absolute discretion.
Ready to begin your Mounjaro journey with us? Start your consultation today.
Sources
Authored By
Leanne Edermaniger
Medical Content WriterPublished on: 30/10/2025
Reviewed By
Mohamed Imran Lakhi
MPharm - Lead PharmacistReviewed on: 30/10/2025
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