Device guide
The Modified Mounjaro KwikPen: What Changed and How to Tell Which You Have
Eli Lilly began rolling out a modified Mounjaro KwikPen in the UK from April 2026. The medicine, the dose and the injection technique are unchanged. What changed is the pen’s internal mechanism, which leaves less liquid behind after the fourth dose and shows more clearly when the pen is finished. Both versions have been in circulation during the changeover.
Key facts
- The change is to the KwikPen device only. Tirzepatide itself, the concentration and the dose are unchanged.
- Both versions deliver four fixed 0.6ml doses, one a week, at every strength.
- The modified pen has a longer plunger that sits lower when unused and travels further, so less liquid remains after dose four and the finished pen is easier to identify.
- Rollout began in the UK from April 2026 as stock of the original pen ran down. There was no single switch-over date, so both versions have circulated.
- GTIN and PIP codes are unchanged, so your prescription and pharmacy ordering are unaffected.
- Injection technique, storage and disposal are the same. Follow the instructions for use supplied with your pen.
- Whatever remains after the fourth dose is residual waste, not a measured dose, in either version.
What changed
Eli Lilly modified the KwikPen to reduce leftover medicine and to make it easier to see that the final dose has been given. The manufacturer describes three differences, all internal or visual:
- Plunger position. In an unused modified pen the plunger sits further down the cartridge than in the original.
- Extended plunger. An added internal component behind the plunger extends its length. The “guide to parts” section of the instructions for use shows this.
- End-of-pen indication. After the fourth dose the plunger sits near the end of the pen, clearly showing it is finished and ready to be discarded.
Everything else works the same way: a disposable, multi-dose, single-patient pre-filled pen holding enough solution to prime before each injection and deliver four fixed 0.6ml doses.
What has not changed
| Item | Status |
|---|---|
| Active ingredient and concentration | Unchanged. It is the same tirzepatide. |
| Doses per pen | Four fixed 0.6ml doses, one weekly, in both versions. |
| Injection technique | Identical. No new method to learn. |
| Strengths available | 2.5mg to 15mg, unchanged. |
| Storage | Fridge at 2 to 8C, or below 30C for up to 30 days once out of the fridge. |
| GTIN and PIP codes | Unchanged, so prescriptions and pharmacy ordering are unaffected. |
| Disposal | Same. See our pen disposal guide. |
How to tell which pen you have
Because the codes and the labelling are unchanged, the difference is in the device. In an unused pen, look at the plunger through the cartridge window: in the modified version it sits noticeably further down than in the original. After the fourth dose, the modified pen’s plunger sits close to the end of the cartridge with little liquid visible, where the original often left a clear amount behind.
If you are unsure, the instructions for use in the carton is the reference; the guide to parts diagram matches the pen it was packed with. Your pharmacy can also tell you which version it is currently dispensing.
Practically, it does not matter which you receive. Both deliver the same four doses of the same medicine, and you may receive either while stock turns over.
Leftover liquid, and why the change was made
Both pen versions contain slightly more solution than four doses require. That overfill exists so the priming step before each injection does not eat into the prescribed amount and so each of the four doses is delivered accurately. It is not a spare dose.
In the original pen the leftover was visible enough that it acquired a nickname online, and some people tried to draw an extra injection from it. That is not something a pen is designed or tested to deliver: the residual volume is not measured, the pen has no way to give an accurate fifth dose, and the manufacturer’s position is that anything remaining is discarded with the pen. Our guide to running out between pens covers what to do if a pen finishes before your next order arrives, which is the underlying problem for most people.
The modified pen reduces that leftover volume and makes the finished state obvious. If you previously saw a visible amount left after dose four, expect less, and expect a clearer signal that the pen is done.
Our Mounjaro click calculator now defaults to the restricted pen model for pens from April 2026, and you can switch it to the original model if you still hold an older pen.
Common questions
Has the Mounjaro medicine changed?
No. The change is to the KwikPen device. The active ingredient, tirzepatide, and its concentration are the same.
Does my dose or schedule change with the new pen?
No. Both versions deliver four fixed 0.6ml doses, one a week, at the strength on your prescription.
Do I need to learn a new injection technique?
No. The modified pen works the same way as the original. Follow the instructions for use supplied with your pen.
When did the modified Mounjaro pen arrive in the UK?
Rollout began from April 2026, with supply transitioning as stock of the original pen ran down. There was no single national switch-over date, so both versions have been dispensed during the changeover.
Can I ask my pharmacy for a particular version?
You can ask which version it is dispensing, but pharmacies supply from the stock they hold and cannot usually choose. Both are the same medicine and both are fine to use.
Is the leftover liquid in the old pen a usable extra dose?
No. The overfill exists so priming does not reduce the four prescribed doses. It is not a measured dose, the pen cannot deliver it accurately, and the manufacturer’s guidance is to discard whatever remains with the pen.
Sources
- Eli Lilly UK, communication to UK healthcare professionals on the modified Mounjaro KwikPen, March 2026, describing the device change, the four fixed 0.6ml doses, the April 2026 UK availability and unchanged GTIN and PIP codes.
- Eli Lilly UK Medical Information. How to use the Mounjaro KwikPen.
- Eli Lilly and Company. Mounjaro KwikPen patient information leaflet, electronic medicines compendium.
- Community comparison material compiled by the r/mounjarouk moderators, used as patient-led context and clearly identified as unofficial.
Monj is an independent price information service operated by Medstack Ltd. It does not sell, prescribe or supply medicines and does not give medical advice. Always follow the instructions for use supplied with your pen, and speak to your pharmacist if anything about your device seems wrong.
