Unopened Mounjaro KwikPens must be stored in a fridge between 2°C and 8°C until the expiry date printed on the pen. Once you've used a pen for the first time, it can be kept either in the fridge or at room temperature below 30°C for up to 30 days — after that, it must be discarded even if medicine remains inside. The 30-day countdown starts from first use and cannot be reset or paused, including if the pen is returned to the fridge partway through. Mounjaro must never be frozen; if a pen has been frozen, it should not be used, and you should contact your pharmacist for a replacement. Getting these details right protects both the effectiveness of your treatment and your investment in it.
Unopened Mounjaro KwikPens should be kept refrigerated at 2°C to 8°C, ideally in their original carton to protect them from light. Stored this way, an unopened pen remains usable until the expiry date printed on the label and carton — the expiry date refers to the last day of that month. Avoid storing pens in the fridge door, where temperature fluctuates more than in the main body of the fridge, and never freeze them.
After First Use: The 30-Day RuleOnce you've injected from a KwikPen for the first time, its shelf life changes significantly. From that point, you have up to 30 days to use it — whether you keep it refrigerated, at room temperature, or move between the two — before it must be discarded, even if there's still medicine left inside.
| Before first use | After first use | |
|---|---|---|
| Storage | Refrigerate at 2–8°C only | Fridge (2–8°C) or room temperature, below 30°C |
| Maximum duration | Until printed expiry date | 30 days from first injection |
| Can you switch storage methods? | N/A | Yes — cycling between fridge and room temperature doesn't affect quality within the 30-day window |
| What resets the clock? | N/A | Nothing — the 30-day countdown starts at first use and cannot be paused or reset |
Worth knowing
A common misconception is that putting an opened pen back in the fridge "pauses" the 30-day countdown. It doesn't. Lilly's own guidance confirms that once a pen has been used for the first time, the 30-day window applies regardless of how you store it afterwards or how many times you move it between fridge and room temperature.
Each Mounjaro KwikPen is designed to deliver four weekly doses. In practice, this means the 30-day shelf life after first use lines up almost exactly with using all four doses on a normal weekly schedule — but if you miss doses or have gaps in your injection schedule, you may end up needing to discard a pen with unused medicine still inside once the 30-day window closes. Planning your injection schedule with this in mind helps avoid wasting doses.
It might seem overly cautious to discard a pen with medicine still inside, but the 30-day in-use limit reflects genuine stability data. Once a pen has been used, its seal has been compromised in a way that unopened, refrigerated storage doesn't involve, and the medicine's stability at room temperature over time has only been tested and confirmed up to this specific window. Beyond 30 days, there isn't reliable data to confirm the medicine remains at full strength and free from degradation, which is why the limit is firm rather than a rough guideline.
What If Mounjaro Has Been Left Out of the Fridge?If an unopened pen has been left at room temperature (below 30°C) for a period, it can generally still be used, but shouldn't then be treated as having an unlimited fridge-stored shelf life going forward — if in doubt, contact your pharmacist for specific guidance rather than guessing. If a pen has been exposed to temperatures above 30°C, left in direct sunlight, or in a hot car, don't use it — contact your pharmacist, who can advise whether it's still safe or arrange a replacement.
Important
If Mounjaro has been frozen at any point, do not use it under any circumstances, even if it appears to have thawed normally. Freezing damages the structure of tirzepatide, and there's no way to visually confirm this has happened. Contact your pharmacist immediately for a replacement.
Regardless of how carefully you've stored your pen, it's worth checking it before every single injection, since damage or degradation isn't always something you'd anticipate from storage history alone. This takes only a few seconds and is a genuinely important habit to build into your routine, not just a formality.
- Look through the inspection window — the liquid should be clear and colourless
- Never use a pen if the liquid looks cloudy, discoloured, or contains floating particles — this can also be a sign of a counterfeit product, so see our guide on how to spot fake weight loss injections if anything about your pen looks unfamiliar
- Check the expiry date printed on the carton — don't use a pen past this date
- Check the pen itself isn't visibly damaged or cracked
- Always use a new needle for each injection
Because the 30-day countdown is unforgiving once a pen is opened, it's worth thinking about timing before you start a new pen, not just after. If you know you have an upcoming schedule disruption — travel, a change in routine, or a period where you might delay a dose — opening a new pen right before that disruption means you're more likely to lose unused medicine when the 30 days runs out. Where practical, starting a fresh pen at a point where your schedule is more predictable for the following month helps you get full value from each pen.
Travelling With MounjaroFor short trips within the UK, where temperatures are moderate, you can generally carry an in-use pen at room temperature (below 30°C) without special equipment, provided you stay within the 30-day window. For warmer climates or longer trips, a few precautions help ensure your medication remains safe and effective for the duration of your travel:
- Use an insulated travel pouch or cool bag designed for medication, especially in hot weather
- Avoid direct contact between the pen and ice packs, which can freeze it
- Keep pens in your hand luggage when flying, not checked baggage, both to avoid extreme temperature swings in the hold and to keep your medication with you
- Store pens in their original packaging where possible, and keep them out of direct sunlight and away from dusty or high-friction areas like the bottom of a rucksack
- Never rest a pen upright on its dose knob or button, which can affect the mechanism
Once a pen reaches 30 days from first use, or you've taken all four weekly doses — whichever comes first — it must be discarded, even if medicine remains inside. Attempting to use leftover medicine beyond this point could mean an inaccurate or unreliable dose. Dispose of used pens in a proper sharps container; see our step-by-step injection guide for full disposal guidance.
Questions About Storing or Using Mounjaro?
Our GPhC-registered pharmacy team is available to advise on storage, travel, or any concerns about your medication — not just at the point of prescription.
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- Eli Lilly and Company. How should the Mounjaro® (tirzepatide) KwikPen® be stored? medical.lilly.com/uk.
- Mounjaro® 2.5mg–15mg KwikPen®, UK Patient Information Leaflet, medicines.org.uk (emc).
- Mounjaro® (tirzepatide) UK Summary of Product Characteristics.