How long can you keep Mounjaro — and how to tell when it's still good

What expiry dates and storage instructions are based on

The storage rules for Mounjaro are set through the Summary of Product Characteristics (SmPC), the technical document approved by the MHRA when the medicine is licensed. Those rules aren't arbitrary. Tirzepatide is a peptide (a large biological molecule) and peptides degrade when exposed to heat, light or freezing temperatures. The manufacturers run stability testing under controlled conditions to establish how long the medicine retains its potency and safety profile. The dates printed on the pen and on the outer carton are the result of that testing, and they apply specifically to the storage conditions stated alongside them.

For Mounjaro, the primary condition is refrigeration between 2°C and 8°C. Kept correctly within that range, an unopened pen remains good until its printed expiry date. For detail on what changes once you move a pen out of the fridge (and the specific time limits that apply) the storage rules for an unopened Mounjaro pen are worth reading in full, since that window is more limited than many people expect. The Mounjaro SmPC, listed on the eMC (electronic Medicines Compendium), is the definitive reference; your Patient Information Leaflet covers the same information in plain language and should be kept with your pen.

One useful habit: before each weekly injection, take the pen out and quickly check the expiry date and the solution's appearance through the window. It should be clear and colourless to slightly yellow, with no particles. That thirty-second check is the simplest way to catch a problem before it matters.

The rules that change once a pen has been opened

An opened pen (one from which at least one dose has already been injected) is subject to a shorter in-use period that is separate from the printed expiry date. The NHS tirzepatide page and the Patient Information Leaflet both address this clearly, because it is one of the most commonly misunderstood aspects of using the KwikPen. The in-use period runs from the date of first use, not from the day you received the pen or the printed expiry date, and it applies regardless of how many doses remain in the pen.

For full detail on exactly how long an opened pen can be kept and what conditions apply during that period, the dedicated page on keeping an opened Mounjaro pen covers the specifics. The short version: always check the leaflet before assuming you can hold onto a part-used pen beyond a certain point. If the in-use period has passed, the pen should be disposed of safely in a sharps container (not in household waste) even if doses remain.

Temperature matters here too. If a pen has spent time above 30°C (in a hot car, near a radiator, or in a bag left in the sun) it should not be used, regardless of where it is in its in-use period. There is no reliable visual test for heat damage; the concern is peptide degradation that won't show up when you look at the solution. When in doubt, contact your prescriber before injecting.

Freezing, sunlight and other situations that end a pen's life early

Mounjaro must not be frozen. If a pen has been stored in a freezer, even briefly, it should be discarded. The same applies if a pen has been left in direct sunlight for an extended period or kept consistently above the room-temperature limit set in the SmPC. Cold chain integrity matters throughout the supply chain too, which is part of why sourcing through a GPhC-registered pharmacy that draws from the licensed UK supply chain is not an administrative detail but a clinical one. Pens sourced through unverified online sellers or social-media channels may have spent time outside the required temperature range without any record of it. The MHRA has published warnings about counterfeit and illegally traded weight-loss pens, some of which have contained entirely different substances to the labelled medicine.

If you are ever unsure whether your pen has been stored correctly (perhaps it was left somewhere warm during a journey, or arrived in packaging that seemed warmer than expected) the right move is to contact your prescriber or pharmacist before using it. Treatment cost is a real consideration for many people, and the cost of Mounjaro treatment page covers what's typically included in a legitimate private prescription. But the cost of an unusable pen is smaller than the cost of injecting a degraded or counterfeit medicine.

The NHS tirzepatide medicines page provides a patient-level summary of storage guidance alongside the full side-effect profile and what to do if you notice a problem with your pen. It is worth bookmarking alongside your Patient Information Leaflet.

Duration of treatment is a separate question from storage

How long a single pen can be kept is distinct from how long you can stay on Mounjaro as a course of treatment. Those are two different questions, and they have different answers. Storage rules are set by the medicine's licence and the SmPC; duration of treatment is a clinical matter decided between you and your prescriber, based on how well the medicine is working and your overall health picture. The SURMOUNT-1 trial, published in the New England Journal of Medicine, ran to 72 weeks, and NICE's appraisal of tirzepatide (TA1026) does not set a fixed maximum duration in the way that the TA for semaglutide does. For more on what the evidence says about long-term use, the page on how long you can be on Mounjaro covers the clinical picture in detail.

How long Mounjaro takes to produce meaningful results is another common question, weight changes typically become noticeable over weeks to months rather than days, and the timeline for Mounjaro's effects explains what the trial data shows at different stages. And if you are thinking ahead to what happens when treatment ends, the evidence on maintaining weight loss after stopping is also worth reading before making any decisions.

If you have questions about your pens that fall outside storage basics (a missed dose, an unusual appearance, a travel situation) our prescribers are available seven days a week. You can also visit our FAQs for answers to the questions that come up most often.