How Long Is Mounjaro Good for After Opening? (October 2026)

01 · The answer

How long an opened Mounjaro pen lasts

Once you have started using a pen, it can be kept out of the fridge not above 30°C for up to 30 days, after which it should be discarded.¹ That is the licensed in-use window for UK KwikPens. It applies whether the pen is kept in the fridge between doses, in a cupboard or in a bag on holiday, as long as it stays not above 30°C. The clock is about the time since you first used the pen, not about how much medicine is left.

Each KwikPen contains four weekly doses. Used on schedule, once a week, a pen is therefore finished well inside the in-use window. The limit mostly matters when doses are delayed or when someone finds a part-used pen at the back of a drawer.

02 · Your decision

What you are probably deciding

There are usually a few situations behind this. Either they have a pen with a dose or two left and the days are running out, or they are not sure when they first used it, or they have a pen that spent time somewhere warm or cold. Each has a clear, safe route.

Write the date you first use each pen on the carton. It is the simplest way to avoid doubt. A phone reminder set for your injection day does the same job, and the free GLPTrackr companion app can help you track a GLP-1 journey.

03 · Next step

If you need a fresh pen

If a pen has expired or has to be thrown away, you will need a replacement through your prescriber rather than a workaround. Our Mounjaro pen prices and consultation page shows current prices, with the 2.5mg pen at £169.99 and all six doses in stock. Repeat orders re-check suitability, and there is no auto-shipping. Orders placed by 3pm are dispatched the same day, with free next-day tracked delivery in temperature-controlled packaging. Our wider weight loss clinic page explains how treatment is reviewed.

Related guide: How long after opening mounjaro does it last.

04 · Why 30 days

Why an opened pen is limited to 30 days (not above 30°C)

Once a pen is in use, it can be kept out of the fridge not above 30°C for up to 30 days, after which it should be discarded.¹ Beyond that window, sterility and potency are no longer assured, so the sensible and licensed answer is to throw the pen away.

People often ask why a pen that still holds medicine must be binned. The reason is that the pen is a multi-dose device. It is not a sales tactic, and it is not a rule that exists to waste medicine.

The pen is designed for weekly use. Each KwikPen contains four weekly doses, so the licensed schedule uses it up inside the window.

Our guide to how long Mounjaro stays usable sets out the full picture across opened and unopened pens, and the patient information leaflet in every box repeats the in-use limit.¹³

Finally, remember that the same rule covers tirzepatide whatever the pen strength. A 2.5mg starter pen and a 15mg pen have the same in-use window. A stronger pen does not last longer, and a weaker pen is not more forgiving.

05 · Fridge or room temperature

Fridge or room temperature while in use

Between doses you can keep an in-use pen either in the fridge or out of it, as long as it stays not above 30°C. The fridge range is 2–8°C, and the medicine should never be frozen.¹ Neither choice extends the 30-day limit.

Many people ask whether they must refrigerate after opening. The licensed position is that refrigeration is not compulsory during the in-use window, provided the temperature stays not above 30°C. Our page on refrigerating after opening goes through the practicalities, and the fridge-specific view is in time in the fridge.

Practical rules that keep a pen safe

• Keep the cap on to protect the pen from light.¹
• Store it in its carton when you are not using it, so it stays clean and shaded.
• Avoid the back of the fridge, near the cooling element, where freezing is more likely. A pen that has frozen must not be used.¹
• Do not leave it in a car, on a sunny windowsill or beside a radiator, because heat above 30°C breaks the licensed conditions.
• Keep it away from children and pets, in a place you will remember.

Follow the instructions that come with the pen for how to prepare and use it.

06 · Unopened pens and expiry

Unopened pens and the printed expiry date

An unopened Mounjaro pen should be kept in the fridge and keeps until the expiry date printed on the box.¹ The 30-day in-use limit only starts once you begin using a pen, so a sealed pen in the fridge is not on a countdown. This matters if you order ahead or hold a spare pen while you finish the current one.

People sometimes ask about very old pens, for example whether a pen several months old can still be used. The answer depends on two facts: whether it has been opened, and what the printed date says. A pen that has never been opened and has always been in the fridge is fine until its expiry date. A pen that has been started is limited to the in-use window regardless of its printed date. Once the printed expiry has passed, the pen should not be used. Do not ask a friend or a forum whether it is probably fine, ask your pharmacy.

What if an unopened pen has been out of the fridge? 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.¹³ The UK label is the one that applies to a UK-dispensed KwikPen.

What an out-of-date pen means

Using medicine past its expiry means you cannot rely on its strength or sterility. The safe approach is not to use it and to ask your pharmacy how to dispose of it. Do not take a stronger or weaker dose to compensate, because dose changes belong to your prescriber, not to guesswork.

When you receive a delivery, check the expiry date on the carton and keep the pen in the fridge on arrival. Our delivery uses temperature-controlled packaging, but once the box is in your hands, storage is your responsibility. Our guide on storage after opening is a handy companion for the weeks that follow.

If you are building a small stock, label the cartons in the order you plan to use them. The pen with the earliest expiry goes first, and the in-use date goes on the pen you start.

07 · Leftover medicine

Can you use leftover Mounjaro?

You can use what is left in a pen only if the pen is still inside its 30-day window and it can still dial a full dose. After that, any remaining medicine should be discarded with the pen.¹ There is no licensed way to carry leftovers into a new pen or to combine pens.

Never try to top up a dose from a second pen, and never inject a smaller amount than your prescribed dose to use up the remainder.

If you notice the window will not reach the number you need, put the pen aside and speak to your pharmacy or prescriber rather than improvising.

Why people end up with leftovers

Leftovers typically arise when a dose was skipped, when a prescriber paused treatment, when side effects led to a dose being held, or when a pen was dialled incorrectly. In every case, the rule is the same.

If your prescriber moves you up or down the dose ladder, a part-used pen at the old strength is not something to carry over. Each pen strength is separate, and doses move up in steps that your prescriber sets. Many people settle at a maintenance dose of 5mg, 10mg or 15mg, and higher is not automatically better.¹

Dispose of used pens and needles using the sharps bin supplied with your order. Needles, a sharps bin and injection guidance are included with every order, so there is no need to put sharps in household waste.

08 · Signs a pen has gone wrong

How to tell if Mounjaro has gone bad

Judge a pen by its history and its appearance: the solution inside a genuine pen is clear and colourless. A pen is unsafe to use if it has been frozen, looks cloudy or discoloured, has particles in it, is past its in-use window, is past its printed expiry or has been kept outside the stated storage conditions.¹ Those are the checks to run, in that order.

A simple pre-injection checklist

• Do you know roughly when you first used this pen, and is it within 30 days?
• Has it stayed not above 30°C, with the cap on and away from light?
• Has it ever been frozen, for example by sitting against the fridge wall?
• Is the printed expiry date still ahead of you?
• Does the pen and carton look intact, with a readable label?

If you answer no to any of those, or you simply do not know, do not inject. Contact the pharmacy that dispensed the pen. If the pen came from a source other than a regulated pharmacy, treat it with real caution. The MHRA publishes counterfeit medicine alerts and runs the Yellow Card scheme for reporting problems.⁷ Genuine pens from a registered pharmacy come with a label and a clear supply chain. Ours are supplied through official LillyDirect supply, sourced directly from Eli Lilly's UK distribution.

A pen that has been frozen is a common cause of concern after a fridge mishap. The label is clear: never use a pen that has been frozen, even if it has thawed.¹ Do not try to judge whether it is probably fine.

If you have already injected from a doubtful pen

If you have already used a pen you now suspect was stored wrongly, do not double up on the next dose. Follow the usual dosing schedule and tell your prescriber. If you notice any unexpected symptoms, report them through the Yellow Card scheme.¹⁹ The prescribing team can advise on the next step, and ongoing monthly reviews are part of our aftercare.

Remember that being cautious costs you a pen, while being careless risks an unreliable dose. That trade makes the answer straightforward whenever the history of a pen is uncertain.

09 · Travel and heat

Travelling with an in-use pen

You can fly with Mounjaro, and the 30-day in-use window means most holidays need no special arrangements.¹³ Carry pens in hand luggage, because hold luggage can fall below freezing, ideally in a cool bag with the prescription label or a copy of your prescription. Check destination rules for travelling with prescription injectables before you fly.

Heat is the main enemy when you are away. In-use pens tolerate up to 30 days not above 30°C, so for hot climates use an insulated pouch.¹³ Do not leave a pen in a parked car, a beach bag in direct sun or a hot boot.

Packing tips

• Keep the pen in its carton and cap on, so it stays shaded.¹
• Pack needles and a travel sharps solution as well as the pen.
• Take a spare pen if your trip may be delayed, but keep it in the fridge until needed.
• Keep your prescription label with the pen, not in a separate bag.
• On arrival, put the pen in a hotel fridge if one is available, taking care it is not placed where it could freeze.

Think about the clock as well as the temperature. A pen you started before travelling is still limited to 30 days (not above 30°C) from first use, however long the trip. If your holiday will carry the pen past the window, start a fresh pen when appropriate and keep the older one at home, or speak to your prescriber about timing.

Time zones are a separate matter. If you cross time zones, the weekly rhythm matters more than the clock. Your usual day is what counts. If you need to change your injection day, at least 3 days (72 hours) must have passed since your last dose.

10 · Timing, missed doses and stretching

Dose timing, missed doses and not stretching a pen

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

You can also move your injection day if needed. You can change your weekly injection day if needed, as long as at least 3 days (72 hours) have passed since your last dose.¹ Those rules explain how to keep to schedule safely, and they have nothing to do with extending how long a pen lasts.

Why a pen should not be stretched past its window

Using a pen beyond 30 days (not above 30°C) breaches the in-use limit, so the medicine would be outside its tested window.

Timing of the injection itself

There is no best time of day. Mounjaro works the same morning or evening, with or without food, and what matters is the same day each week. Many people choose a weekend morning so early side effects fall on days off. A consistent day also makes it easy to count how long a pen has been in use, because each pen holds four weekly doses.

After a final dose, tirzepatide takes four to five weeks to clear the system, and nothing needs doing while it clears.

11 · 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

12 · Ordering and replacing pens

Ordering, storing and replacing pens sensibly

The easiest way to avoid storage worries is to plan your supply so that each pen is used on schedule and the next one is ready in the fridge. Each KwikPen contains four weekly doses, so a repeat order timed to arrive before your final dose keeps your routine unbroken.¹

How ordering works

• Free online consultation: a short health questionnaire, with no charge and no obligation.
• Prescriber review: reviewed by a GPhC-registered prescriber. Payment is taken when you place the order; if the prescriber decides the treatment is not suitable, the order is refunded automatically in full.
• Dispensing and dispatch: dispensed by our registered pharmacy (GPhC 9012878), with same-day dispatch when ordered by 3pm.
• Delivery: free next-day tracked delivery by DPD in discreet, temperature-controlled packaging. Saturday delivery is £14.99.

New customers save £40 on their first order with code FIRST40, and an automatic £25 reorder discount applies to every future order (code DOSE25). You can pay by Visa or Mastercard, or use Klarna or pay-monthly options, and there are no subscriptions. Prices are shown live on the Mounjaro pen page, checked 11 October 2026.

When a pen arrives

Open the box promptly and check the expiry date and your name on the label. Put unopened pens straight into the fridge at 2–8°C. Write the first-use date on the carton when you start a pen, and keep a note of the in-use window so you know when to discard it.¹

If a pen arrives damaged or is the wrong item, contact us within 3 days. Dispensed prescription medicines cannot otherwise be returned, so check the delivery on arrival. Our support team is available Monday to Friday, 9am to 5pm, by email at support@everydaymeds.co.uk, through the account portal chat or by phone on 0333 207 0413.

Availability on the NHS is a separate route. Mounjaro is available on the NHS, but access is being phased in over several years, and the primary-care rollout began in June 2025 with the highest-need groups first.⁶