Here is a thing that surprised me when I went and read the actual labels: Mounjaro and Wegovy have different storage rules, and almost every "GLP-1 storage guide" on the internet flattens them into one set of numbers. They are not the same. One runs on a 30-day clock, the other on six weeks.

Everything below is taken from the UK Summary of Product Characteristics for each medicine, which is the regulator-approved document your pharmacist works from, rather than from other people's blog posts. Where something widely repeated turns out not to be in the label, I have said so.

The rules, per medicine

Sources: the Mounjaro KwikPen SmPC and the Wegovy FlexTouch SmPC, sections 6.3 and 6.4 in each.

One correction worth making loudly. You will find plenty of pages saying an unopened Wegovy pen can sit at room temperature for 28 days. That is not in the SmPC. What the label actually supports before first use is controlled shipment below 30°C for up to 48 hours, which is about your pen surviving the journey from the pharmacy, not about storing it on a shelf for a month. If in doubt, the fridge is the answer.

The clock people get wrong

The in-use window starts at first use and it does not pause. This is the single most common misunderstanding I see.

If you take a Mounjaro pen out of the fridge, use it, and then put it back in the fridge, the 30 days keeps running. Lilly is explicit that you may return an in-use pen to the fridge during that period without harming the product, but the clock does not reset and it does not stop. At day 30 the pen goes in the sharps bin even if there is medicine left in it. The same logic applies to Wegovy at six weeks.

This matters more than it used to, because microdosing and dose-splitting mean a pen can easily still have liquid in it long after its window closes. Having medicine left is not evidence that the pen is still good. Write the first-use date on the pen with a marker, or log it, which is the sort of thing the titration and pen-life planner exists to keep track of.

One more from the Lilly guidance that is easy to miss: do not store a KwikPen with the needle attached. It leaks, it can block the needle, and it lets air into the pen.

The fridge mistake

The Wegovy label contains four words that most people skip: keep away from the cooling element. That is the back wall of a domestic fridge, and it is regularly below freezing even when the fridge is set correctly.

Freezing is the one failure you cannot come back from. Both labels say the same thing: do not use a pen that has been frozen. Semaglutide and tirzepatide are proteins, and freezing damages the structure irreversibly. A pen can look completely normal and be useless.

The practical fix is boring and effective: store pens in the middle of the fridge or in a door shelf, not shoved against the back, and know what your fridge is actually doing. Domestic fridges vary by several degrees between shelves, and most people have never measured theirs.

Flying and holidays

Pens go in hand luggage. Always, without exception. The hold is unheated and routinely drops below freezing at altitude, and a frozen pen is a wasted pen. This also means that if your hand luggage gets gate-checked on a full flight, you take the pens out first.

Injectable medicines are exempt from the 100ml liquid restriction, but the exemption depends on being able to show what they are. Travel with the pharmacy label with your name on it, or your prescription, and keep the medicine in its original box. Security scanners do not harm the medicine, so there is no need to ask for a hand search.

For the trip itself, the useful mental model is that you are not trying to keep the pen cold so much as trying to keep it under 30°C and above freezing. A car parked in the sun in August is well over that. So is a beach bag. So, sometimes, is a British bedroom in a heatwave, which catches people out at home rather than abroad.

Long trip, or somewhere hot? Ring the pharmacy before you go rather than after. They deal with this constantly and can tell you what they would do with your specific pen and dose schedule.

What is actually worth buying

Two things, honestly, and both are cheap. I am not going to pretend you need a kit list.

An insulated case. The kind sold for insulin works identically for GLP-1 pens. The gel-pack versions keep a pen in range for a day of travel; the evaporative cooling ones need no freezer, which matters if you are going somewhere without reliable refrigeration. What you want is something that fits your specific pen, because the KwikPen and the FlexTouch are different shapes, and something that will not freeze the pen against a hard ice block. You can browse insulated medication travel cases on Amazon(paid link).

A fridge thermometer. Unglamorous, a few pounds, and the only way to know whether the shelf your medicine lives on is actually between 2°C and 8°C. Get one with a minimum and maximum memory, leave it where the pens go for a day, and you will find out whether your fridge dips below zero overnight. Mine did. Digital min/max fridge thermometers on Amazon(paid link) all do the same job.

That is genuinely it. A marker pen for writing the first-use date on the label costs nothing and does more than most gadgets.

Needles and sharps bins

These come from the pharmacy that dispensed your pen, and I am not going to link you anywhere else for them.

Needle gauge and length are part of how the medicine is prescribed and administered, not a shopping decision, and the wrong length changes where the dose lands. Your prescriber and pharmacist choose them for a reason.

A sharps bin is a similar story for a different reason: the bin is the easy part, and the disposal route is the part that matters. Used needles cannot go in household waste. Your pharmacy or local council arranges collection or exchange, and many councils do it free. Buying a bin online without sorting the collection just moves the problem into a cupboard.

When something has gone wrong

If a pen has frozen, been left somewhere hot, or gone past its in-use window, the answer is not on the internet and it is not from me. Ring the pharmacy that dispensed it, tell them exactly what happened and for how long, and let them decide.

I know the instinct is to work out whether it is probably fine, because these medicines are expensive and nobody wants to bin one. That instinct is exactly why it is worth saying plainly: a pen that has been frozen is not fine, however it looks, and a pharmacist would far rather have the phone call than have you inject something that has been sitting in a hot car since Friday.

FAQ

How long can a Mounjaro pen stay out of the fridge?

Up to 30 days after first use, at a temperature not above 30°C, then it must be discarded. Before that it lives in the fridge at 2°C to 8°C. Never freeze it.

How long can a Wegovy pen stay out of the fridge?

Wegovy's in-use shelf life is six weeks, and after first use it can be stored below 30°C or preferably in the fridge. Different medicine, different clock. Keep the cap on to protect it from light.

Can I take my pen on a plane?

Yes, in hand luggage, never in the hold. Bring the pharmacy label or prescription so the liquid exemption applies. Scanners do not affect the medicine.

What if it froze?

Do not use it. Both labels say a pen that has been frozen must not be used, because freezing damages the protein permanently. Ring your pharmacy.

Where do I get needles and a sharps bin?

From your pharmacy. Gauge and length are part of the prescription, and a sharps bin needs a disposal route, which your pharmacy or council provides, often free.

This article is information, not medical advice, and the SmPC for your specific pen and strength is always the authority. Questions about your own medicine belong with your pharmacist or prescriber.

If you want the first-use date, the pen-life countdown and your dose history in one place instead of on a sticky note, Healthcount does that free. And if you have landed here from a dose calculator, the evidence on why writing things down works is the more useful read.