The Mounjaro KwikPen: what you are actually holding
In the UK, Mounjaro is supplied as the Mounjaro KwikPen — a disposable, pre-filled, multi-dose pen that is used by one person only and contains four fixed weekly doses of 0.6mL each.[1] One pen therefore covers roughly a month of treatment at a single strength, and there is a separate pen for each of the six strengths, from 2.5mg up to 15mg.[1] Which strength you are on, and when it changes, is set by your prescriber on the step-up schedule we explain on our Mounjaro doses and strengths page.[2] A dose change means a new pen, not a new setting on the old one — and because the pen in your hand lasts a month, storage between doses matters.
A practical footnote: descriptions of Mounjaro devices you find online — especially older posts, or pages written for other countries — do not always match the pen dispensed in the UK today. If your device does not match something you have read, trust the patient information leaflet and the instructions for use in your own box: they are written for exactly the device you are holding.
Once a week, on a day you choose
Mounjaro is a once-weekly subcutaneous injection — under the skin, not into a vein or muscle.[1] The UK label leaves the timing open: it can be injected on any day of the week, with or without food.[1] Once weekly means exactly that, so in practice the dose lands on the same day each week; the freedom is in which day you pick. A day tied to something you already do weekly is easier to keep, and since food makes no difference, the time of day is yours.
If your regular day needs to move, or a dose has been missed, do not guess and do not double up: the exact rules are in the patient information leaflet in your pack, and your prescriber or pharmacist can advise on your situation. The one principle that always holds — never inject extra medicine to make up for a missed dose unless a healthcare professional tells you to.
Where to inject: abdomen, thigh or upper arm
The label names three injection areas: the abdomen, the thigh and the upper arm.[1] None is "better" than the others as far as the label is concerned — the choice is comfort and practicality. The abdomen and thigh are easy to reach yourself; the upper arm is awkward single-handed, so it often suits weeks when someone can help.
Why rotation matters
The instruction that comes with the sites is to rotate them — not to inject the same spot week after week.[1] Injection-site reactions are among the effects listed in the labelling, and giving each area a rest between injections is the routine way to look after the skin you are injecting into.[1] Rotation can mean moving between areas — abdomen this week, thigh next — or to a different spot within the same area; the leaflet shows the pattern, and a simple note of where the last dose went keeps track.
If a site becomes sore, changed or worrying rather than briefly irritated, speak to your prescriber or pharmacist — and our Mounjaro side effects page covers the wider picture of what the trials and the label report.
Needles: fresh on, straight off
The KwikPen does not come with a needle permanently fitted, and the manufacturer's storage guidance is blunt on this point: never store the pen with a needle attached.[3] In practice, each injection follows the same short arc — a new needle on just before the dose, then off and into a sharps container straight after.
The step-by-step of attaching the needle, checking the pen and giving the dose is what the instructions for use in your pack, and the person who trains you, are for. We deliberately do not reproduce it here: a website paraphrase is the wrong place to learn injection technique; the leaflet's version matches the device in your hand.
Storing Mounjaro: the fridge, then the 30-day window
Before first use: in the fridge, never frozen
Unopened pens live in the refrigerator at 2°C to 8°C.[3] Just as important is the other direction: do not freeze the pen.[3] Tirzepatide is a peptide medicine,[1] and freezing is not something a fridge mishap can undo — keep pens away from the coldest spots, such as the back wall or a freezer compartment.
After first use: up to 30°C, up to 30 days
Once a pen is in use it no longer needs the fridge: it may be kept at up to 30°C for up to 30 days.[3] After 30 days the pen must be discarded even if there is medicine left in it.[3] Since the pen holds four weekly doses, a pen used on schedule finishes inside the window[1] — the 30-day rule bites when a pen sits part-used, for example across a treatment break. Writing the date of first use on the carton removes the guesswork a month later.
Unopened: fridge, 2°C–8°C, never frozen. In use: up to 30°C for up to 30 days, then bin the pen even if medicine remains. And never store the pen with a needle attached.[3]
Travelling with Mounjaro
The same two numbers do the work when you travel. A pen already in use is the easy one: it tolerates up to 30°C for up to 30 days, so for most trips it can simply come with you in its carton — what it cannot tolerate is the extremes, like a parked car in summer or anywhere past 30°C.[3] Unopened spare pens are stricter: their place is a 2°C–8°C fridge and they must never freeze — worth remembering before checking medicine into an aircraft hold or improvising with ice packs.[3]
For longer trips, hot climates or extra pens, ask your pharmacy team to help you plan — also the right place for questions about carrying prescription medicines across borders. And since the injection can be given on any day, a schedule shaken by time zones has some built-in flexibility; if a shifted dose leaves you unsure, ask rather than improvise.[1]
Sharps: what happens to needles and finished pens
Every injection produces a used needle, and needles are sharps — they belong in a proper sharps container, never loose in household rubbish where they can injure whoever handles the bag next. The patient information leaflet covers disposal, and your pharmacy team or prescriber can explain how sharps containers and collection work where you live — arrangements vary across the UK. Sorting out a container at the start makes the whole routine tidier.
Is Mounjaro safe? Where injections fit in the picture
"Is Mounjaro safe" is a question about far more than the injection. The most common side effects in the trials were gastrointestinal — nausea, diarrhoea, constipation, vomiting — mostly mild to moderate and related to dose rather than to injection technique.[4] Injection-site reactions appear in the labelling, but they are a small part of the overall profile.[1] Honestly framed: the weekly injection is rarely the hard part; the medicine's effects on the gut, especially at each dose increase, are what most people notice. Our side effects page goes through the trial numbers, the warnings and who should not take tirzepatide at all.
If you experience side effects from any medicine — including injection-site problems — you can report them through the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk,[5] and speak to your GP or pharmacist.
A word on buying Mounjaro online
Because so many people search for Mounjaro online, it is worth being plain about access. Mounjaro is a prescription-only medicine, authorised by the MHRA for weight management in adults on 8 November 2023;[2] whether you can take it is a decision for a qualified prescriber after a proper consultation, through the NHS or privately via a prescriber working with a GPhC-registered pharmacy. No legitimate route skips the consultation. A genuine supply also arrives as the regulated product described on this page — a KwikPen with a patient information leaflet, needing the storage conditions above. Anything sold without a prescription by an unregistered seller sits outside the regulated supply chain; the MHRA's FakeMeds campaign explains the risks of buying medicines that way. We do not recommend, rank or link to any provider.
Frequently asked questions
Where on the body do you inject Mounjaro?
Under the skin of the abdomen, thigh or upper arm, rotating the site with each weekly dose. It can be given on any day of the week, with or without food.[1]
Does Mounjaro have to be kept in the fridge?
Before first use, yes — unopened KwikPens live at 2°C to 8°C and must never be frozen. In use, a pen can stay out of the fridge at up to 30°C for up to 30 days, then must be discarded even if medicine remains.[3]
How long can a Mounjaro pen stay out of the fridge?
Do you leave the needle on the pen between doses?
No. The manufacturer's guidance is never to store the KwikPen with a needle attached — a fresh needle goes on just before each injection and comes off, into a sharps container, straight afterwards.[3]
Can I inject Mounjaro on a different day of the week?
The label allows any day of the week, with or without food.[1] To move your regular day, or after a missed dose, follow the patient information leaflet or ask your prescriber or pharmacist — and never inject extra unless a healthcare professional tells you to.
What do I do with used needles and finished pens?
Used needles are sharps: they go in a proper sharps container, never loose into household rubbish. The leaflet in your pack covers disposal, and your pharmacy team can explain how sharps collection works in your area.
References
- electronic medicines compendium (emc). Mounjaro KwikPen — Summary of Product Characteristics. medicines.org.uk
- MHRA / GOV.UK. Medicines regulator authorises diabetes drug Mounjaro (tirzepatide) for weight management and weight loss. gov.uk
- Lilly Medical (UK). How should the Mounjaro (tirzepatide) KwikPen be stored? medical.lilly.com
- Jastreboff AM et al. Tirzepatide Once Weekly for the Treatment of Obesity (SURMOUNT-1). New England Journal of Medicine, 2022. nejm.org
- MHRA. Yellow Card scheme — report a side effect. yellowcard.mhra.gov.uk