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If you've been prescribed Mounjaro, you may have noticed that two versions of the KwikPen are currently in circulation. Both are the same medication, but they work slightly differently, and knowing which version you have matters when it comes to troubleshooting.
This guide explains the key differences, how to identify your pen, and what to do if something doesn't look right.
Eli Lilly, the manufacturer of Mounjaro, recently updated the KwikPen device. The change primarily affects how much medication the pen contains beyond your prescribed dose.
Both versions are designed to deliver the same effective dose. The update is a device change, not a formulation change, meaning the medication inside is identical. However, understanding the difference is important because it changes what you should expect to see at the end of your injection.
The most important visual clue is the thickness of the black component below the plunger: in the new version it matches the grey plunger in size; in the old version it is much smaller - approximately ¼ of the size.
The excess medication is only useful as an identifier if all doses have been used and the pen reads 0.
Old KwikPen
The legacy pen contained a small amount of extra medication beyond your prescribed dose. This is known as overfill. It was built into the device to ensure the full dose was always delivered, but it meant that when the pen finished injecting, a small amount of liquid was still visible in the inspection window.
This was completely normal and expected. The pen was empty, but the visible liquid was residual overfill, not a missed dose.
New KwikPen
The new pen has very little overfill. The dose is precisely calibrated, and the pen is designed to be nearly empty when the injection is complete.
This means that if the new pen stops injecting while medication is still visible in the inspection window, it is not overfill - it is a sign that the pen may have jammed mechanically. This requires a different response than the legacy pen would have.
You can identify your pen version by checking the label on the device. Here's what to look for:
Check the label on the pen body. The updated KwikPen will have revised labelling from Eli Lilly, typically indicating the new device version.
Check your delivery paperwork or the box insert as the version may be noted there.
If you're unsure, contact the Numan support team and we can confirm which version you have based on your current prescription batch.
The main visual difference (while the pen still has active doses) is the size of the black plunger — in the new pen it is much thicker than in the old one.
| Old | New | |
|---|---|---|
| Overfill present? | Yes | No |
| Thickness of section below plunger | Much smaller than the plunger | Equal in thickness to the plunger |
| Liquid visible after fourth dose? | Expected - pen is empty | Indicates a mechanical jam |
| Label design | Original Lilly labelling | Updated label with version indicator |
| What to do if pen stops early and liquid is visible | Check dose counter if shows 0 pen is complete, if any other number then stop injecting and follow troubleshooting guide | Stop injecting and follow troubleshooting guide |
| Troubleshooting | Legacy pen guides apply | New pen guides apply - check version first |
Regardless of which version you have, follow these steps before each injection:
Check the label to confirm your pen version (legacy or new).
Inspect the dose window. The pen should show the correct starting dose for your treatment stage.
After attaching the needle, perform the standard air shot before injecting.
Inject at the recommended pace - slow, steady, and continuous.
After the injection, check the dose counter.
For the legacy pen, the dose counter should read 0. Any liquid visible in the window is normal overfill.
For the new pen, the dose counter should read 0 and the inspection window should appear empty. Liquid remaining after the pen stops is a sign of a potential jam.
If you are using the new KwikPen and the pen stops before the dose counter reaches 0, or you see medication remaining in the window after the pen has stopped:
First, check the needle is attached properly. Sometimes the needle doesn't puncture the rubber covering properly which can prevent the pen from working.
Do not discard the pen immediately.
Contact Numan support via the app or website and we will advise on next steps, including whether a replacement pen is appropriate.
The Numan support team is available to help with device queries, dose concerns, and troubleshooting. You can reach us through the Numan app or at numan.com/support.
If your troubleshooting guide references pen-specific steps, make sure you're reading the guide for your version. Guides for the legacy pen and the new pen differ in places. When in doubt, check with us first.
Clinical Pharmacist and Copywriter, Master of Pharmacy (MPharm)
