How long does an open Mounjaro pen last?

How many doses are actually in one pen?

A single Mounjaro KwikPen contains four doses, each delivered once a week. That means one pen takes you through roughly a month of treatment, four injection days, seven days apart. This is true at every strength: 2.5mg, 5mg, 7.5mg, 10mg, 12.5mg and 15mg are all supplied in the same four-dose format.

People sometimes wonder whether a pen they've started counts differently to a sealed one. It doesn't change the dose count (you still have however many injections remain) but it does change how carefully you need to track storage. The pen you opened on Monday morning doesn't have a different number of doses; it has a stricter obligation on where you keep it between uses.

For a broader look at how a single pen fits into your treatment plan, our guide to what one Mounjaro pen covers explains the practicalities week by week. And if you're still at the point of weighing up treatment, our Mounjaro overview covers how the medicine works and who it's licensed for.

Where should an open pen be kept between injections?

An opened pen should go back into the fridge (kept between 2°C and 8°C) after each injection. Most people tuck it into a fridge shelf rather than the door, since door temperatures can fluctuate when you're in and out making tea. The pen doesn't need to be upright; lying flat is fine.

There is a permitted window during which the pen can be stored at room temperature, useful for travel or if you inject somewhere other than home. The exact duration of that window, and the maximum temperature allowed, are specified in your Patient Information Leaflet and the Mounjaro Summary of Product Characteristics on the Electronic Medicines Compendium. We deliberately don't print a fixed number here, because it's the kind of detail that should come from the official source rather than a web page, and it's the kind of thing that occasionally changes between batch updates to the leaflet.

Two things to avoid regardless: never freeze the pen, and never leave it in direct sunlight or a hot car. A pen that has been frozen is not safe to use, even if it looks normal after thawing. If you're unsure whether yours has been stored correctly, contact our team or speak to your prescriber.

For anything specific to fridge storage logistics, our guide on keeping Mounjaro in the fridge covers what to check.

Does an open pen expire sooner than a sealed one?

The printed expiry date on the pen applies whether it is sealed or in use, opening the pen does not reset or shorten the printed date. What does change once the pen is open is your obligation to follow the room-temperature guidance precisely, because exposure to temperature outside the recommended range can degrade the medicine faster than the expiry date assumes.

So the practical answer is: check the expiry date before every injection, regardless of where you are in the pen. A fourth dose sitting in an opened pen that is within its expiry date and has been stored correctly is fine to use. One sitting in a pen that has been left out of the fridge for longer than the leaflet permits may not be, even if the date hasn't passed.

If you've received a pen and aren't certain how it was handled in transit, our guide on unopened pen storage explains what to look for. Every pen dispatched by our pharmacy travels in temperature-controlled packaging via tracked DPD delivery, but it's worth knowing what to check when it arrives.

What about the needle, should it stay on between injections?

No. The needle should be removed immediately after each injection and disposed of safely in a sharps container. Leaving a needle on between doses allows air to enter the pen and medication to leak, which can affect the accuracy of the next dose. It also raises the risk of needle-stick injury and contamination.

Before your next injection, attach a fresh needle. Changing the needle every time is not optional admin, it's part of how the pen works correctly. Our pharmacy includes needles with your treatment, and if you need a standalone supply, compatible replacement needles are available from us directly. A 1-litre sharps container is the appropriate way to dispose of used needles at home; most local councils collect them through GP practices or pharmacies.

The NHS patient information for tirzepatide covers injection technique and aftercare clearly, and it's worth reading alongside your own leaflet if you're new to self-injection. If you have questions about technique that haven't been answered at consultation, our aftercare team is available seven days a week. The Mounjaro injection guide on our site also walks through the process step by step.