If your Mounjaro® pen is difficult to use, feels stiff, or does not release medication as expected, follow the steps below to ensure it is functioning correctly. Unlike Wegovy®, Mounjaro® pens must be primed before every injection to remove air and ensure the correct dose is delivered.
Use this article if you want to:
Understand common reasons a Mounjaro® pen may not work properly.
Follow step-by-step troubleshooting and priming guidance.
Identify physical pen faults like leaking, red seal damage, or contamination.
Learn about the MedExpress replacement and compensation policies for lost doses.
Step-by-Step Priming Guidance
Follow these steps using a new, disposable needle for every troubleshooting attempt:
Attach the needle: Remove the paper tab from the outer needle shield. Push the capped needle straight onto the pen and twist until secure. Remove the outer cap (keep for disposal) and the inner shield.
Set to Prime: Turn the dose knob back to “0” if necessary. Slowly turn the knob until you hear 2 clicks and the extended line appears in the dose window.
Remove Air: Hold the pen upright with the needle pointing up. Gently tap the cartridge holder to move air bubbles to the top.
Release Medication: Press the dose knob fully in and hold for 5 seconds. A small droplet at the needle tip confirms successful priming. A visible stream is not required.
Verify: Confirm that "0" appears in the dose window.
Important: Do not inject into the body during the priming or troubleshooting process.
Common Pen Issues and What to Do
Hard-to-Press Dose Knob
A stiff knob is often caused by a blocked, bent, or poorly attached needle. It can also occur if the knob was turned without a needle attached, causing internal pressure.
Action: Replace the current needle with a new one, reset the knob to "0," and repeat the priming process carefully.
Damaged Components or Leaking
If the red internal seal has burst or split, or if the pen is leaking significantly, it may be faulty.
Action: Stop using the pen immediately. Contact Customer Care with a video or clear photographs of the damage and confirm how many doses were successfully used.
Blood Inside the Pen
Occasionally, blood may enter the pen chamber during an injection.
Action: Stop using the pen and contact the team with photographs of the device.
Frozen, Discoloured, or Cloudy Medication
Mounjaro® solution should be colourless to slightly yellow.
Action: Do not use the pen if the medication has been frozen, the liquid contains particles, or it appears discoloured or cloudy, and contact the team with photographs of the device.
Lost Doses and Replacement Policy
If you believe a dose was lost due to a device fault, compensation or a replacement may be considered on a case-by-case basis.
The 3-Dose Rule: Pens that have already provided 3 successful doses do not qualify for a replacement pen.
Dosing Continuity: If you have not completed all 4 doses from your current pen, you should remain on your current dose strength until you have completed 4 successful doses before increasing strength.
Identifying Used Doses
Refer to the image below to identify the state of your pen based on the plunger position:
(Shows the plunger position moving down the barrel: prior to use, after 1st dose, after 2nd dose, after 3rd dose, and after the 4th/final dose).
Preventing Future Problems
Storage: Store the pen with the cap attached in a clean, dry place within the recommended temperature range.
Needle Safety: Never store the pen with a needle attached, as this causes leakage, blockages, and air entry.
Contamination: Keep the pen away from dust, lint, food, or liquids.
When to Contact MedExpress
Contact our Customer Care Team if the pen remains unusable after trying a new needle, if the medication is contaminated, or if you have a confirmed device fault.
Important: Do not dispose of a faulty pen until advised by the team, as we may need to arrange for its return for investigation by the manufacturer.
If you still need help after reviewing the information above, additional assistance is available.