What To Do If Your Mounjaro Pen Malfunctions (October 2026)

01 · Act safely

What to do straight away if your Mounjaro pen is not working

The safest first move is to stop and put the pen down. A pen that sticks, will not dial, leaks or will not deliver is not something to push through. The Mounjaro KwikPen is a precision device built to deliver a measured weekly dose, and a forced pen can give you less than intended, or in some cases an uncertain amount.¹³

• Stop. Do not keep pressing, twisting or tapping the pen.
• Do not take the injection from a pen you are unsure about.
• Keep the pen, its box and any packaging so the batch details can be checked.
• Put the pen back where it is stored, with the cap on, until you have advice.
• Contact the pharmacy that dispensed it, and your prescriber if your dose day is close.

Read the instructions for use in the patient information leaflet again before doing anything else. They are the manufacturer's own steps for the pen, and many apparent faults turn out to be a missed step.¹³ If the leaflet does not resolve it, treat the pen as faulty.

Related situations: if a Mounjaro pen jams and how to unjam a Mounjaro pen safely.

02 · Diagnose

Is it really a malfunction, or is the pen working as designed?

Often the pen is behaving exactly as it was built to. The most common cause of a pen that seems stuck is that it has simply run out of doses. Each KwikPen contains four weekly doses.²¹

If you cannot tell which row applies, treat it as a fault. Using a pen when you are unsure of the dose is the one thing to avoid.

03 · Next step

When you need a replacement pen

If the pen is faulty, you need a replacement, and the supplying pharmacy is the right place to start. If you ordered from us, email support@everydaymeds.co.uk with a clear description of the problem and photographs of the pen and its label. Our team can also help through the account portal chat or by phone during support hours. Our returns terms cover faulty, damaged or wrong items, and we ask you to get in touch promptly so it can be sorted out.

If you are a customer who simply needs your next pen, our Mounjaro pen range is in stock across all six strengths, and the weight-loss section of our weight loss clinic explains the treatment pathway. A replacement for a faulty pen is a pharmacy and prescriber matter, so please contact them directly before ordering anything new.

04 · Dial and dose window

How the dial and dose window should work

The Mounjaro KwikPen is set by turning the dose knob until the dose indicator shows the correct dose in the window.

If the dial turns and the window shows the right number, the pen is behaving as expected at that step.

Do not try to work around a window that will not reach the right number. Splitting a dose across two pens, adding up smaller amounts, or injecting twice to make up a shortfall all fall outside the licence. Our guide to the pen reset question explains why the KwikPen has no reset button to press and why you should not attempt to disassemble it.

05 · Never force the pen

Why you should never force, dismantle or repair the pen

A KwikPen is sealed and is not designed to be opened or repaired.

Common impulses to resist include pressing harder on the button, twisting the dose knob against resistance, tapping the pen on a surface, warming it up, or running it under water.

The same caution applies to any plan to carry on with a questionable pen to avoid wasting medicine. A wasted pen is an inconvenience, while an unreliable dose is a clinical problem.

Doses should always stay within what the pen is designed to deliver. A malfunctioning pen is never a reason to start altering doses.

06 · Why pens go wrong

Why a pen can seem to stop working suddenly

A pen that works one week and fails the next usually has one of a few explanations: the pen is finished, a step was missed, storage damaged it, or there is a genuine manufacturing or handling fault. Genuine faults are uncommon, but they are possible, which is why pharmacies and regulators have a reporting route.

Finished pen: because each pen holds a set number of weekly doses, a dial that stops turning towards the end may mean the course is done. Check your dose log, your calendar, or the GLPTrackr companion app if you use it, to confirm how many doses you have taken from that pen.

Missed step: injection pens depend on following the sequence in the instructions for use. A step skipped in a hurry, or done differently from the last time, can make a working pen seem broken. The patient information leaflet is the reference, and it is worth rereading even if you have used the pen for weeks.¹³

Storage damage: Mounjaro pens should be kept in a fridge at 2–8°C and never frozen.¹³ Never use a pen that has been frozen. Keep the pen cap on to protect it from light, and remember that 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 A pen that has been left in a hot car, a bag in sunshine, or the freezer compartment is not one to troubleshoot, only to replace.

Manufacturing or handling fault: rarely, a pen arrives with a visible defect, or is damaged in the post. That is exactly what the faulty-item route is for.

07 · Storage and travel damage

Storage, heat and travel as causes of pen problems

Many apparent malfunctions begin with storage. Keep Mounjaro pens in a fridge at 2–8°C and never freeze them.¹ If you travel, 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.¹³ In-use pens tolerate up to 30 days not above 30°C, so most holidays need no special arrangements; for hot climates use an insulated pouch.

Unopened pens should be kept in the fridge and keep until the expiry date printed on the box. 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, because figures circulating online often come from other countries' labels.¹³

If a pen arrives warm, looks damaged, or you suspect it has been through extremes of temperature, contact the pharmacy before using it. Our deliveries use temperature-controlled packaging, but if anything about the pen on arrival concerns you, please say so and keep the packaging.

If you are travelling and a pen fails abroad, keep it, photograph it, and ask your supplying pharmacy for advice. Do not buy a replacement from an unregulated seller.

08 · Missed dose while waiting

What if the fault makes you miss your injection day?

A faulty pen can delay a dose, and the missed-dose rule applies to that situation. 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

In practice this gives you a window to get a replacement sorted. Contact your pharmacy on the day the fault appears, not days later, so you have time to receive a replacement and still inject inside the window.

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 If a replacement pen arrives on a different day, that can make a new schedule sensible. Talk to your prescriber before moving your day if you are unsure.

Do not double up to make up for a delayed injection. If you have been off treatment longer than usual because of a pen fault, or you are unsure how long it has been, ask your prescriber whether your dose should be reviewed, especially if you are on a higher strength. Our prescribing team handles dose follow-up as part of the ongoing monthly reviews.

09 · Reporting a defective pen

How to report a defective Mounjaro pen

Report a defective pen to the pharmacy that supplied it first, then, if you wish, to the MHRA through the Yellow Card scheme. The Yellow Card scheme accepts reports about side effects and about suspected faulty or counterfeit medicines and devices.¹⁹⁷ Reporting helps regulators spot patterns that a single pharmacy might miss.

When you contact the pharmacy, have the following ready:

• Your name and order details.
• The strength of the pen and the dose you were trying to take.
• The batch number and expiry date printed on the pen and box.
• What happened, in plain words, and what you did next.
• Photographs of the pen, the dose window, any leakage, and the packaging.

Do not throw the pen away, and do not return it by post until the pharmacy asks you to. If you used the pen and had any symptoms afterwards, tell your prescriber and include that in your Yellow Card report.

If you bought your pen from us and it is faulty, damaged, or the wrong item, contact us promptly. Our terms cover those cases, and our support team can tell you the next step.

10 · Counterfeits and unsafe pens

Could the fault be a sign of a counterfeit pen?

It can be, which is a reason to buy only from a regulated pharmacy. The MHRA publishes counterfeit medicine alerts, and its guidance is that GLP-1 and GIP products should only be obtained on prescription from a registered supplier.⁷ A pen bought from a social media seller, a beauty salon, or any website offering to supply without a consultation or prescription is not a pen you can trust, whatever it looks like.

You can check a pharmacy on the GPhC register. We are registered under GPhC 9012878, and our Mounjaro comes through official LillyDirect supply, direct from Eli Lilly's UK distribution. Our genuine Eli Lilly Mounjaro KwikPen page explains what an authentic supply chain looks like.

If you suspect your pen is not genuine, stop using it, keep it, and report it to the MHRA and to the pharmacy that supplied it.

11 · Sharing and borrowing pens

Why you should never borrow or share a pen to cover a fault

If your pen fails, you may be tempted to borrow a friend's or family member's pen. Do not. Mounjaro is a prescription-only medicine, prescribed for you at a dose reviewed for you, and pens are for single-person use.¹³ A borrowed pen may be a different strength, may have been stored incorrectly, and carries hygiene risks.

Our guide to sharing a Mounjaro pen explains these risks in more detail. The right route to a replacement is always via the pharmacy and your prescriber, who can confirm the correct strength for you.

Equally, do not switch to a different strength on your own to work around a faulty pen. Dose steps follow the licensed ladder, and your prescriber guides the pace.¹

12 · Talking to your prescriber

When to speak to your prescriber, not just the pharmacy

Contact your prescriber if you are unsure whether you received a dose, if you used a pen that you later suspected was faulty, or if a delay has lasted long enough that your dose may need reviewing. Everyone starts Mounjaro on 2.5mg once a week and the dose steps up depending on how you respond and tolerate it, so a longer gap may affect how your next dose is chosen.¹

Get medical help straight away if you have 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 the skin or eyes (possible gallbladder problems); or signs of a severe allergic reaction such as swelling of the face or throat and difficulty breathing. Report any new symptoms to your prescriber and consider a Yellow Card report.¹⁹

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