Overview
If you're experiencing an issue with your Mounjaro® pen, this article explains common causes, troubleshooting steps, and what support may be available. If you use Wegovy® instead, see the separate article on faulty or compromised Wegovy® pens, since Mounjaro® and Wegovy® pens work differently and require different troubleshooting steps.
Use this article when you want to:
Understand common issues that can occur with a Mounjaro® pen
Learn how to prime your Mounjaro® pen correctly
Find out what to do if your pen is faulty, leaking, frozen, or damaged
Know when and how to contact MedExpress about a pen issue
What Common Issues Can Occur With a Mounjaro® Pen?
Patients may experience:
Hard-to-press dose knobs
No medication flow during priming
Bent or blocked needles
Medication leakage
Burst or damaged red seal components
Blood visible inside the pen
Frozen pens
Damaged or contaminated pens
Lost doses
Why and When Should I Prime My Mounjaro® Pen?
Unlike Wegovy®, Mounjaro® pens should be primed before every injection. Priming removes air from the pen and helps ensure the correct dose is delivered.
How Do I Prime My Mounjaro® Pen?
Attach a new needle securely.
Turn the dose knob until you hear 2 clicks and see the extended line in the dose window.
Hold the pen upright with the needle pointing upwards.
Tap the cartridge gently to move air bubbles upwards.
Press and hold the dose button for 5 seconds to release a small amount of medication into the air.
Confirm the dose window returns to "0".
A small droplet at the needle tip is normal and confirms successful priming. Do not inject during priming.
What Should I Do If My Mounjaro® Dose Knob Is Hard to Press?
A stiff dose knob is commonly caused by:
A blocked or bent needle
Incorrect priming
Turning the dose knob without a needle attached
Storage issues or contamination
What to do:
Replace the needle with a new one
Reset the dose knob back to "0"
Repeat the priming process carefully
Using a new needle resolves many issues.
What Should I Do If the Red Seal Is Damaged or the Pen Is Leaking?
If the red internal seal has burst, split, or the pen leaks significantly, the pen may no longer be usable. Stop using the pen and contact MedExpress's Customer Care with:
A video or clear photographs of the issue
Confirmation of how many doses were successfully used
What Should I Do If I See Blood Inside the Pen?
Blood may occasionally enter the pen chamber during injection. If this occurs:
Stop using the pen
Contact MedExpress's Customer Care Team with photographs of the pen
What Should I Do If My Pen Is Frozen, Damaged, or Contaminated?
Do not use your pen if:
The medication has been frozen
The pen is cracked or damaged
The liquid appears cloudy, contains particles, or is discoloured
Mounjaro® solution should appear colourless to slightly yellow.
What Should I Do If I Believe I've Lost a Mounjaro® Dose?
If you believe a dose was lost due to a pen fault, MedExpress's team may request photographs or video evidence to assess the issue. If a confirmed pen fault is identified, compensation may be considered by MedExpress's Customer Care Team.
You can check how many doses your pen has used by looking at the plunger position through the small window near the base of the pen:
Prior to use: The plunger sits at the very bottom of the window, with little to no piston rod visible.
After the 1st dose: A short section of the ridged piston rod becomes visible above the plunger.
After the 2nd dose: The piston rod extends further up, showing roughly the middle section of the rod.
After the 3rd dose: The piston rod is visible for most of its length, with the plunger positioned close to the top of the window.
After the 4th (final) dose: The plunger has moved to the very top of the window, with the full length of the ridged piston rod visible — this means the pen is empty.
Please note:
Replacement eligibility depends on the number of doses already used.
Pens with 3 successful doses already used would not qualify for a replacement pen.
Any compensation or credit is assessed on a case-by-case basis.
If you have not successfully completed all 4 doses from your pen, you should remain on your current dose strength until you have completed 4 successful doses before increasing strength.
How Can I Prevent Mounjaro® Pen Problems?
To help avoid issues:
Always use a new needle for each injection
Never store the pen with a needle attached
Always replace the pen cap after use
Store the pen within the recommended temperature range
Never press the dose button without a needle attached
Incorrect storage or use may damage the pen mechanism.
When Should I Contact MedExpress About My Mounjaro® Pen?
Contact MedExpress's Customer Care Team if:
Your pen is not functioning correctly.
You believe your pen is faulty.
You are unable to administer your medication.
Your medication appears damaged or contaminated.
You believe you have lost a dose due to a device issue.
What Happens If My Pen Needs to Be Returned for Investigation?
In some cases, MedExpress may need to arrange for the faulty or compromised pen to be returned for further investigation with the manufacturer. For this reason, please do not dispose of your pen unless you have been advised by MedExpress's Customer Care Team that it is safe to do so.
Where a return is required, MedExpress's team will provide full instructions on how to return the pen safely.