How Long Does Mounjaro Last When Opened? (October 2026)

01 · The core answer

How long does an opened Mounjaro pen last?

An opened Mounjaro pen lasts up to 30 days from the day you first use it. Once in use, a pen can be kept out of the fridge at a temperature not above 30°C for up to 30 days after first use, and must then be thrown away.¹ The 30 days is counted from first use, not from the day the parcel arrives.

The pen is built for once-weekly use. Each KwikPen contains four weekly doses.¹ If you inject on schedule, the pen is finished well inside its in-use window. The 30-day limit matters mainly when doses have been delayed, skipped or paused, which is when people start asking whether an old pen is still fine.

You can keep the in-use pen in the fridge or, if you prefer, out of it not above 30°C. Many people simply keep it in the fridge between doses, because that is where the pen is stored before first use anyway. Either way, the in-use limit stays the same. For the fridge side of this question, see our guide on keeping an in-use pen in the fridge.

02 · What to do

What should you do with the pen in your hand?

Work out which of these situations you are in, because each has a different answer.

• The pen is in use and inside 30 days (not above 30°C): carry on as normal, with the cap on.
• The pen has passed 30 days from first use: discard it, even if some liquid is visible. Do not carry on with it.
• The pen is unopened: keep it in the fridge and check the printed expiry date on the box.
• The pen has been frozen, or an unopened pen was left out of the fridge: do not use a frozen pen. For an unopened pen that was out of the fridge, check with the pharmacy.

If you are unsure whether a pen is still usable, ask the prescribing team before injecting. Our team can look at what you have and tell you whether to use it or replace it. Support is available Monday–Friday 9am–5pm on 0333 207 0413 or at support@everydaymeds.co.uk. Replacement is a clinical and practical decision, so do not guess.

If you are still deciding whether to start, the Mounjaro pen starts at £169.99 for the 2.5mg pen at standard price (all six doses are in stock), and the prescriber reviews your suitability first. You can also read about our wider weight loss clinic service.

03 · Storing an opened pen

How to store an opened pen

Store an opened pen with the cap on, away from light, and never let it freeze. The guidance is short: keep the pen cap on to protect it from light, and never use a pen that has been frozen.¹

The default is the fridge. Keep Mounjaro pens in a fridge at 2–8°C and never freeze them.¹ A frozen pen is not usable, however well it seems to have recovered.

Out of the fridge, the in-use pen tolerates temperatures not above 30°C for the full in-use window. That is why a pen kept in a bag on a day out, or on a bedside table in a cool room, is not a problem in itself. The risk is heat: a parked car, a sunny windowsill, a bag left by a radiator.

A few practical habits help:

• Put the cap back on straight after every injection.
• Keep the pen in its carton or a pouch in the fridge, so it is not knocked around.
• Write the first-use date on the carton, so you never have to remember it.
• Do not share the pen or its needles with anyone.

Writing the first-use date is the single most useful habit. People rarely forget to inject, but they often forget when they started a pen after a break in treatment, and that is when the in-use window quietly runs out. The patient information leaflet that comes with every pen repeats these storage rules, and it is worth reading once.¹³

04 · Why the 30-day limit exists

Why you cannot use Mounjaro after 30 days (not above 30°C)

The 30-day limit is the period the in-use pen is licensed for, so beyond it the pen is outside the tested window. After that point, sterility and potency are no longer assured, and the licence does not support further use.¹

This is why the question "has anyone used a Mounjaro pen after 30 days?" is the wrong one to ask. Other people's experience cannot tell you the dose is accurate or the pen is still sterile. Our advice is the same as the label's: discard it after 30 days and use a new pen.

Be careful with figures you see online. Storage numbers quoted on forums and social media often come from other countries' labels, which can differ from the UK one. For a UK pen, follow the UK leaflet and what your pharmacy tells you.

05 · Unopened pens

How long does Mounjaro last if it is not opened?

An unopened Mounjaro pen should be kept in the fridge and keeps until the expiry date printed on the box.¹ The in-use window only begins when you first use the pen. Until then, the expiry date is the one that counts.

That means you can safely hold a spare pen if your prescriber has dispensed your next dose in advance, as long as it stays in the fridge. Keep it in its carton, away from the freezer compartment, and check the date before you start it.

What if an unopened pen has been out of the fridge? This is where people often reach for a number of days they half remember from the internet. Please do not. If an unopened pen has been out of the fridge, check with the pharmacy before using it rather than relying on a remembered day limit — figures circulating online often come from other countries' labels.¹ We can ask how long, how warm and in what packaging, and give you a proper answer.

If you ordered from us, your pens arrive by free next-day tracked delivery in temperature-controlled packaging, so a pen should not be warming up in transit. Put it in the fridge as soon as you open the parcel, unless you are about to use it. If a delivery was delayed or left in the sun, tell us when you unpack it.

06 · Is my pen still good?

How to tell if Mounjaro has gone bad

The most reliable checks are about history, not appearance: has the pen passed 30 days from first use, passed its expiry date, been frozen, or been exposed to heat? If the answer to any is yes, do not use it.

Look at the pen's history first:

• Date: is it beyond the in-use window or the printed expiry date?
• Freezing: never use a pen that has been frozen.
• Heat and light: has it been left somewhere hot, or without its cap in bright light?
• Damage: is the pen cracked, or does the dose window or knob behave oddly?

Then check the pen itself against the patient information leaflet.¹³ A history problem is a reason to discard even if the pen looks perfectly normal.

If you have used a doubtful pen and feel unwell, get medical help straight away; for severe, persistent stomach pain, swelling of the face or throat or difficulty breathing, get medical help straight away. You can also report any suspected side effect or problem through the Yellow Card scheme.¹⁹

When in doubt, replacing the pen is a small inconvenience, and injecting from a compromised pen is not a risk worth taking for the sake of saving a dose.

07 · Doses left in a pen

How many doses are in a pen, and what is left after the last one?

Each KwikPen contains four weekly doses.¹ After the last of them, the pen is finished.

Some people notice liquid still visible in the cartridge and wonder whether there is a spare injection. Do not combine remnants from different pens, and do not try to draw the remainder out. The pen is made to deliver its listed doses accurately, not to supply bonus injections.

08 · Stretching or splitting a pen

Can you stretch a pen or split a 15mg pen into 5mg doses?

No: splitting a pen into smaller doses, or spreading doses out to make it last, is not licensed and not supported by how the pen is built.

The safety position is straightforward. Altering doses outside the licence means unknown dosing accuracy and no safety data.¹

If cost or side effects are the reason you are tempted, there are licensed ways to deal with both. Talk to us before changing anything.

09 · Delays, missed doses and timing

Can the same pen last past a month if doses are delayed?

Not safely: the in-use window runs from first use regardless of how many doses you have taken, so a pen used slowly still has to be discarded at 30 days (not above 30°C). If doses are delayed or missed, check the date before you inject from it.

The missed-dose rule keeps you on track. If you miss a dose, take it as soon as you can within 4 days (96 hours) of the scheduled time, then carry on with your usual day. If more than 4 days have passed, skip the missed dose entirely and take the next one on your normal day — never take two doses within 3 days of each other.¹

So, is it safe to inject Mounjaro more often than weekly? No. The medicine is licensed as a once-weekly injection, and the missed-dose rule exists precisely to stop doses bunching up. If you want to move your injection day, you can change your weekly injection day if needed, as long as there are at least 3 days between two doses.¹ That is a way of shifting the schedule, not of injecting more frequently.

If treatment was paused for a long time, a pen you opened before the break may be past its window. Treat it as expired, and speak to your prescriber before restarting. Do not decide that yourself. Our ongoing monthly clinical reviews exist for exactly this sort of question.

Why a half-life matters here too: tirzepatide has a half-life of about five days, so after a final dose it takes roughly four to five weeks to clear the system.¹ That is about the body, not the pen, but it explains why a short delay in injecting is not an emergency while a pen's age still is a hard limit. For more on that, see how long the medicine stays in your system.

10 · Travel and heat

Travelling with an opened pen

Most holidays need no special arrangements, because in-use pens tolerate up to 30 days not above 30°C.¹ The planning is mostly about avoiding freezing and heat.

You can fly with Mounjaro. Carry pens in hand luggage (hold luggage can fall below freezing), ideally in a cool bag with the prescription label or a copy of your prescription.¹ Keep your needles and pen cap together, and check destination rules for travelling with prescription injectables before you fly.

For hot climates use an insulated pouch.¹ Never leave a pen in a parked car, on a beach bag in direct sun, or in a hotel safe that heats up. When you arrive, put unopened spare pens straight into a fridge if there is one. A hotel mini-fridge works, as long as it does not freeze things at the back.

Trips and the 30-day window interact in a useful way. If your trip is short, the in-use pen can stay out of the fridge not above 30°C without trouble. If you are away for long enough that your pen would pass its window, take a fresh pen rather than hoping the old one will last. Unopened pens should stay in the fridge until you need them.

If something goes wrong, such as a pen left in a hot car or a freezer scare, it is better to call than to inject and hope. Our team can tell you whether to use it.

11 · Common mistakes

Common storage mistakes to avoid

The usual errors are small and easy to prevent once you know them.

• Losing track of the start date. Write it on the carton the day you first use the pen.
• Using the pen after a long gap. A pen opened before a break may be past its window.
• Leaving the cap off. The cap protects the pen from light.
• Freezing the pen. Do not keep it against the back of the fridge or near a freezer compartment, and never use a frozen pen.
• Hot storage. Cars, windowsills and radiators are all poor places.
• Trusting a number from an overseas label. Follow the UK leaflet.¹³

None of these needs a complicated routine. A fridge shelf, a pen carton, a written date and the cap on cover nearly all of it.

If you want a week-by-week view of the early weeks and where the pen should be kept, our week 1 storage guide walks through it. If you are curious about how long the pen's effects last as opposed to the pen itself, read how long the effects last.

12 · Pen life vs effect

Pen life is not the same as how long the medicine lasts in your body

The pen's 30-day in-use limit is about storage, while how long Mounjaro lasts in your body is about pharmacology.

In the body, tirzepatide has a half-life of about five days, so after a final dose it takes roughly four to five weeks to clear the system — there is no way to speed this up, and nothing needs doing while it clears.¹ It is why the medicine works as a once-weekly injection.

If your question is about the general picture, see how long Mounjaro lasts overall. If you are on 5mg and wondering what that dose means in practice, there is a page on how long 5mg lasts. And for side effects that follow each injection, how long symptoms last covers what is usual.

Looking after the pen properly is part of using the medicine as licensed. It means each dose is accurate, and each injection is from a pen inside its tested window.

13 · Serious side effects

Serious side effects: when to get help straight away

Serious side effects are uncommon but need urgent attention. Seek immediate medical help for: severe, persistent stomach pain that may spread to your back, with or without vomiting (possible pancreatitis); pain in the upper right abdomen, fever or yellowing of skin/eyes (possible gallbladder problems); or signs of a severe allergic reaction such as swelling of the face or throat and difficulty breathing. If you take insulin or a sulfonylurea alongside Mounjaro, watch for low blood sugar (shakiness, sweating, confusion) — your prescriber may need to adjust those medicines.¹

14 · NHS and private supply

NHS or private: does the storage advice change?

No: storage rules are the same whoever supplies the pen, because they come from the product information, not the supplier. If you are on the NHS or private, the in-use limit and expiry date work identically.

On access: Mounjaro is available on the NHS, but access is being phased in over several years. NICE recommends it for adults with a BMI of at least 35 plus at least one weight-related condition, and the NHS primary-care rollout began in June 2025 with the highest-need groups first (initially BMI 40+ with four or more qualifying conditions).⁴⁶ Full rollout is expected to take years, which is why many people who meet the licensed criteria choose regulated private treatment rather than waiting.

If you use us, a GPhC-registered prescriber reviews each order, and we supply genuine KwikPens through official LillyDirect supply. Repeat orders re-check suitability, and there is no auto-shipping, so a new pen arrives when you need it, not before. If the NHS guidance on storage interests you, we cover it in our NHS fridge storage page.

Mounjaro (tirzepatide) is a prescription-only medicine, supplied only after an online consultation reviewed by a GPhC-registered prescriber. It is used alongside a reduced-calorie diet and increased physical activity. Individual results vary. This page is information, not a substitute for medical advice.