Mounjaro Dates: Use-By, Expiry and When to Start

Step 1: Finding and reading the expiry date on your pen

Every Mounjaro KwikPen has an expiry date printed on the label, usually in the format MM/YYYY. This is the last month in which the pen is considered safe and effective under the storage conditions Eli Lilly specifies. The date applies to an unopened pen kept correctly in the fridge. Once you have used the pen for the first time, a shorter, separate time limit begins, refer to the Mounjaro expiration date guide and the Patient Information Leaflet for the precise window, because stating it here without qualification could cause confusion if your leaflet version differs.

One thing worth knowing: the expiry date is printed on the outer carton as well as the pen itself. If you have removed the pen from its box, the date is still readable on the barrel. Check it before every injection, not just when you first open the pack.

If you receive a pen with an expiry date that feels uncomfortably close (say, only a few weeks away) contact the pharmacy straight away. At nume, you can reach the aftercare team seven days a week via our contact page. A pen approaching its date is not automatically unusable, but it is worth flagging so your prescriber can advise.

The NHS medicines page for tirzepatide (Mounjaro) confirms that correct storage is essential to maintaining the medicine's stability, something the expiry date is designed to reflect.

Step 2: Understanding the use-by date once a pen is in use

This is the question our prescribers hear most often. People assume the expiry date and the in-use deadline are the same thing. They are not. Once you have administered your first dose from a pen, the remaining doses in that same pen must be used within a specific period, even if the printed expiry date is months away. The detail is in the leaflet that comes with your prescription, and it is important enough that we have covered it in full in a separate guide on Mounjaro use-by dates.

The practical implication: if you travel, delay a dose, or your schedule shifts around a bank holiday, you need to know this in-use window before you assume the remaining doses are still good. It is worth getting into the habit of noting the date of your first injection on each new pen, either in a notebook or on your phone.

Storage during that in-use period matters too. A pen that has spent too long outside the fridge (even within the room-temperature allowance) should not be used past the point the leaflet specifies. If you are uncertain whether a pen has been stored correctly, the right call is to check with your prescriber rather than inject and hope. Questions about specific storage situations are exactly what our FAQs and aftercare team exist to help with.

Step 3: When Mounjaro became available, and what that means for access today

Mounjaro's UK history begins with its marketing authorisation from the MHRA for type 2 diabetes, followed by its November 2023 licence extension for weight management in adults with a BMI of 30 or above, or 27 or above with at least one weight-related condition. Private prescribing through regulated online pharmacies began shortly after that extension was granted.

On the NHS, access has been phasing in gradually under NICE guidance (TA1026, published December 2024). The first cohort (adults with a BMI of 40 or above and four or more qualifying conditions) became eligible from June 2025. A second cohort covering BMI 35–39.9 with four or more conditions was activated in June 2026. Each phase expands access, but NHS waiting lists remain a reality for many people.

The Mounjaro release date timeline covers the full UK licensing history if you want the precise regulatory sequence. For context on how private access compares to the NHS route, the Mounjaro overview page sets out both options clearly.

One timing note that comes up: some patients plan their first order around payday, a Monday start, or the week after a holiday. That is entirely reasonable, just be aware that orders placed by 12pm on a working day at nume are reviewed the same day, so a Monday order can often arrive Tuesday if all goes well clinically.

Step 4: Checking whether an older pen is still safe to use

This question sits between the expiry date and the use-by date, and it matters most for people who have had a pen sitting in the fridge longer than expected, perhaps after a period of illness, a missed prescription renewal, or a change in clinical plan. The short answer is: look at the date on the pen, compare it with today, and check the in-use record if the pen has been opened. The detailed answer depends on storage conditions, not just the calendar.

A pen that expired last month is different from one that expired last year. A pen stored outside the fridge for a week is different from one kept at 5°C throughout. These distinctions matter because tirzepatide, like all biological medicines, can degrade in ways that are not visible to the eye. An injected dose from a degraded pen may simply be less effective, or it may cause unexpected reactions.

We have written a dedicated page on whether Mounjaro goes out of date and another on whether it has a use-by date for anyone working through this step carefully. Both pages reference the SmPC and Patient Information Leaflet as the final authority on specific timeframes.

For general context on what tirzepatide is and how it works as a treatment, the tirzepatide information page provides a solid grounding. And if you are weighing up your options more broadly, our weight-loss treatment overview explains what is available through a regulated private pharmacy in the UK. There is also a transparent breakdown of what treatment costs on the treatment and pricing page, if that is helpful context before you decide next steps.