Mounjaro Pen Malfunction: What To Do and How To Get a Replacement (October 2026)

01 · Core answer

What to do when your Mounjaro pen malfunctions

Stop what you are doing and do not force anything. Forcing a stuck dose knob or injection button can damage the mechanism, and a damaged pen is exactly the situation where you cannot trust the dose delivered. Put the pen down, keep the cap and any needle, and keep the box and the pharmacy label. These items let the pharmacy and the manufacturer trace the batch and investigate what went wrong.

Then work out which situation you are in, because the right next step differs. Anything you cannot explain should go to a pharmacist or prescriber rather than being solved by trial and error.

02 · Your next decisions

The two decisions you are actually facing

The first is whether the pen is faulty or behaving normally. Our guide on what to do if your pen goes wrong walks through the same checks in a shorter format, and the page on a pen reset button explains why the pen has no reset to press.

The second is what to do about your dose. If a dose may not have been delivered, the answer is not to inject again immediately. Missed-dose timings exist so that doses are never stacked too closely, and a prescriber can tell you how they apply to your situation. Contact us or your prescriber as soon as you can, so a replacement and a plan are arranged together.

Not yet a customer and researching treatment? You can see the Mounjaro pen prices and start a free consultation; the starting 2.5mg pen costs £169.99 at standard price, and genuine KwikPens come through official LillyDirect supply. If you are exploring the wider picture, our weight loss clinic page covers how treatment works in practice.

03 · Fault or normal behaviour

Is it a malfunction, or is the pen behaving normally?

Many pen worries turn out to be normal behaviour, so check that before assuming a fault. The Mounjaro KwikPen is set by turning the dose knob until the dose indicator shows the correct dose in the window. If you have been listening for a particular click and did not hear it, that alone is not evidence of a fault.

The most common false alarm is a dial that will not turn any further. You may still see a little liquid inside the pen at that point, and that does not mean you are being short-changed; it means the remainder is not a usable dose.

Another common worry is not feeling the injection. Many people feel very little when they inject, so a lack of sensation is not proof that nothing was delivered. Equally, it is not proof that everything was. If you genuinely cannot tell, treat it as an uncertain dose and speak to a prescriber rather than injecting again.

Genuine signs of a fault look different. They include a dial or button that is physically stuck partway, visible cracks or damage, a pen that has been dropped and now behaves differently, medicine leaking from the pen body, or a pen that has been exposed to freezing or heat. In any of these cases, stop using the pen.

Our guide to unjamming a stuck pen explains the sensible, non-forceful checks, such as making sure the needle has been attached the way the leaflet shows and that all caps have been removed as instructed. If those checks do not solve it, the pen should be treated as faulty.

A useful habit is to prepare each injection calmly, in good light, with the leaflet to hand if you are unsure. Our page on preparing your pen sets out that routine. Most problems that look like malfunctions are traced back to a rushed set-up, so slowing down is the cheapest prevention there is.

If you are unsure which category you are in, treat it as uncertain and ask. A pharmacist would rather answer a question that turns out to be nothing than miss a real fault.

04 · Step-by-step response

Step-by-step: what to do when the pen will not work

Work through the same sequence each time, because order matters more than speed. Begin by stopping. Do not keep pressing, twisting or shaking the pen in the hope that it will free up. Force is the most reliable way to turn a recoverable situation into a broken pen.

Next, make the pen safe. If a needle is attached and you are not about to inject, handle it carefully and use your sharps bin as you normally would. Needles, a sharps bin and injection guidance are included with every order from us, so you should already have what you need to dispose of a needle safely.

Then look at the pen with fresh eyes. Check the dose window and what it shows. Check that the pen has not been frozen or left somewhere hot. Check for cracks, leaks or damage. Check the needle is attached the way the patient information leaflet shows.¹³ These checks take moments and either resolve the issue or give you precise information to pass on.

If the problem persists, keep everything. Keep the pen, the cap, the needle you used, the outer box and the dispensing label. Take clear photos of the pen and the dose window if that helps you describe the fault. Write down the date, the day of the week, the dose strength and what happened, in plain words. A good description speeds up any replacement or investigation.

Now contact the people who can act. If your pen came from us, contact our support team by email, account chat or phone on weekdays. Our email is support@everydaymeds.co.uk. Tell us the dose strength, what you saw and whether you think a dose was delivered. Our prescribing team can then advise on your dose and arrange the next step.

Finally, think about reporting. Suspected defective medicines can be reported through the MHRA Yellow Card scheme, which is how regulators spot patterns that individual users cannot see.¹⁹ You do not need to be certain it is a fault to report; a reasonable suspicion is enough.

What you should not do is equally important. Do not try to take the pen apart, do not try to top it up, do not combine remaining medicine from different pens, and do not carry on injecting from a pen you no longer trust. Our page on what to do if a pen goes wrong repeats this sequence as a quick checklist you can keep to hand.

05 · Dose didn't go in

My dose didn't go in: shallow injections, leaks and uncertainty

If you suspect a dose did not go in, the safe response is to pause and ask, not to repeat it. Several things can make an injection feel as though it failed: a needle not attached properly, a pen with a fault, an injection that was not fully completed, or simply not feeling anything. Most of these cannot be distinguished from your armchair, which is why a prescriber's view matters.

Mounjaro is injected under the skin (subcutaneously) weekly, on the same day each week, at any time of day, with or without food.¹ Licensed injection sites are the stomach area (at least 5cm from the belly button), the front of the thigh, or the back of the upper arm if someone else gives the injection. Rotate the site with each dose.¹ A site that has been used repeatedly is a reasonable thing to vary, though it is not a proven cause of failed delivery.

A little moisture on the skin after an injection can happen and does not by itself mean the dose was lost. Equally, a wet patch with a strong sense that little was delivered is worth mentioning. The honest position is that you cannot measure how much reached the tissue, so the question becomes what is the safest course given the uncertainty.

The safe course is governed by the missed-dose rule. 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 That last rule is the reason not to inject again straight away: doing so could put two doses close together.

So the practical advice is to contact a prescriber and describe exactly what happened. They can consider whether the dose should be treated as delivered, partly delivered or missed, and what timing is safe.

If you are likely to have this happen again, ask yourself whether set-up is the issue. A calm, unhurried routine, as described in our guide to preparing your pen for injection, removes most avoidable causes.

06 · Dial, window, doses

How the dial, window and four doses are meant to work

Understanding the design makes it far easier to spot a real problem. The Mounjaro KwikPen is a pre-filled multi-dose pen in six strengths: 2.5mg, 5mg, 7.5mg, 10mg, 12.5mg and 15mg. Each pen contains four weekly doses of the strength printed on it.¹ You do not choose a dose by sliding between strengths on a single pen; the strength is fixed by the pen you were dispensed.

The dial exists so that the pen delivers its dose, and the window exists so that you can see that it is set correctly. If you rely on sound rather than the window, you may think a pen is faulty when it is simply quiet, or think it is fine when it is set wrongly.

Some people are tempted to combine the leftover with the next pen. That is not something the pen or the licence supports, and it removes the dosing accuracy you are paying for.

The same logic applies if you are counting weeks. A pen is intended to last a month of weekly injections, and the pen is built around that pattern. If you reach the end earlier than expected, or the dial stops sooner than it should, tell your pharmacy and keep the pen. It may mean a dose was dispensed incorrectly, a dose was taken in error, or the pen has a fault; only an investigation can say which.

There is no reset button because there is nothing to reset. The pen is a mechanical device, not an electronic one, and its dosing mechanism is set by the dial. Our page on whether there is a reset button on the pen covers this directly, and it is worth reading if you were hoping for a quick fix.

If you have been wondering about quantities, the page on Mounjaro measurements explains what the strengths mean and how they relate.

None of this requires you to understand the internals of the device. It requires only that you read the window, trust the number and stop when the pen tells you it cannot deliver a full dose.

07 · Storage damage

Heat, freezing and travel: when storage causes the problem

Some pen problems are not manufacturing faults at all; they come from how the pen has been kept. Keep Mounjaro pens in a fridge at 2–8°C and never freeze them. Once in use, a pen can be kept out of the fridge not above 30°C for up to 30 days — useful for short trips — after which it should be discarded. Keep the pen cap on to protect it from light, and never use a pen that has been frozen.¹

A frozen pen is the clearest example. If a pen has been at the back of a fridge against the cooling plate, left in a cold car, or put in hold luggage, you cannot see whether it has been harmed. The rule is simple: do not use it. Contact the pharmacy, explain what happened and ask for guidance.

Heat is the other risk. A pen left on a sunny windowsill, in a parked car or beside a radiator has been outside the conditions it was tested in. You do not need to guess how bad it was; you need to stop and ask. The in-use allowance is a specific licensed figure, not a general tolerance, so do not stretch it.

Travel is where many of these problems start. 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. 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.¹ Check destination rules for travelling with prescription injectables before you fly.

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 — figures circulating online often come from other countries' labels.¹³ This matters because the UK product information is the one that applies to your pen.

When you contact us about a pen that may have been stored badly, be honest about what happened. We would rather hear that a pen sat in a hot car than have you use it quietly. The answer might be to replace the pen, and the replacement conversation is simpler when the facts are clear.

Also check the basics before blaming storage: the pen cap should be on when you are not injecting, the pen should be out of direct light, and it should be somewhere that a child or pet cannot reach. Good habits remove most of the storage-related problems before they appear.

08 · Reporting and replacement

Reporting a defective pen and getting a replacement

Reporting and replacement are separate steps, and you should do both. Replacement gets you a working pen; reporting helps regulators and the manufacturer see whether a fault is isolated or part of a pattern. People often ask whether Lilly will replace a defective pen. The route is normally through the pharmacy that supplied it, which is why keeping the pen and packaging matters.

If your pen came from us, contact our support team promptly, because faulty, damaged or wrong-item claims on dispensed prescription medicines have a short window under our terms. Dispensed prescription medicines cannot be returned for change of mind, but a faulty, damaged or wrong item is the exception. Give us the dose strength, what you saw and, where possible, photos. Our prescribing team can also advise on your dose in the meantime. Support is by email, account portal chat or phone on weekdays, and our email is support@everydaymeds.co.uk.

For reporting, the MHRA Yellow Card scheme accepts reports of suspected side effects and of defective medicines.¹⁹ The MHRA also publishes guidance on counterfeit medicine alerts.⁷ A good report states the product, strength, what happened, when it happened and whether anyone was harmed. You can include batch details from the box if you have them, which is another reason to keep the packaging.

The patient information leaflet also describes how to report problems and who to contact.¹³ If your problem is a side effect rather than a device fault, report that too. The two are often confused: a pen that works perfectly can still be associated with nausea or other symptoms, and those are a different conversation with your prescriber.

What happens to the pen after you report? Keep it safe until the pharmacy tells you what they want. Do not throw it away, and do not return it by post without instructions. If it contains medicine, it should be handled with the same care as any sharps-related item.

Be realistic about timing. A replacement is dispensed by a registered pharmacy after a clinical decision, which means it is not instant. We ship by tracked, temperature-controlled delivery, and same-day dispatch applies when an order is placed by 3pm. A prescriber will also want to know whether your weekly dose has been delayed.

Our wider pen-fault checklist gives a quick summary to follow while you are mid-problem, and the page on freeing a stuck pen safely covers the checks that sit before a replacement request.

09 · Genuine pens only

Why unofficial supply and sharing make pen problems worse

A pen problem is much harder to resolve if you are not sure where the pen came from. Mounjaro is a prescription-only medicine, supplied only after a consultation reviewed by a GPhC-registered prescriber. A pen from a social media seller, an unregulated website or a friend has none of the traceability that makes a replacement or investigation possible.

The MHRA publishes counterfeit medicine alerts and runs the Yellow Card scheme precisely because fake or tampered injectables exist.⁷ A counterfeit pen can look convincing and still fail to deliver a dose, contain the wrong amount of medicine, or be unsafe. If a pen from an unusual source malfunctions, do not simply try again; treat it as suspect, keep it and report it.

Before buying anywhere, check the pharmacy is registered. You can look up any UK pharmacy on the public GPhC register; ours is GPhC 9012878. We supply genuine KwikPens through official LillyDirect supply, direct from Eli Lilly's UK distribution, and all six doses are in stock. Be wary of anyone offering to supply without a consultation or prescription, or promising specific results.

Sharing pens is a related risk that some people do not think of as one. Mounjaro pens are dispensed to a named patient at a dose chosen for them. A pen is not designed to be split between people, and sharing raises hygiene and dosing concerns on top of the legal position. Our page on why sharing a pen is unsafe sets out the reasoning in detail.

Use the pen as designed, then dispose of it.

For readers who want to understand what the genuine product is, our page on the authentic Eli Lilly KwikPen describes what a legitimately supplied pen looks like and how it reaches you.

The point is not to frighten anyone. It is that a malfunctioning pen from a regulated supplier is a solvable problem, with a named pharmacy, a prescriber and a paper trail. The same malfunction from an unknown seller leaves you on your own, with a medicine you cannot verify.

10 · Keeping treatment on track

Keeping your treatment on track while a pen is replaced

A faulty pen is an inconvenience, not a reason to abandon treatment, but it can disturb your schedule, so plan around it. Everyone starts Mounjaro on 2.5mg weekly. This starting dose is about letting your body adjust, not about weight loss itself. After at least 4 weeks the dose increases to 5mg, and can then step up by 2.5mg at a minimum of 4-week intervals up to a maximum of 15mg weekly, depending on how you respond and tolerate it.¹

If a replacement pen is delayed, the main question is what happens to your next dose. 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 may also want to move your injection day to suit a new delivery. 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 That flexibility can help if a replacement arrives mid-week, but ask the prescribing team before changing anything after a problem dose.

If your gap becomes long, tell the prescribing team. They may want to discuss whether you restart at a lower dose, and that decision is clinical, not something to work out yourself. Our ongoing monthly reviews are there for this kind of question, and repeat orders re-check suitability rather than shipping automatically.

For future planning, consider keeping an eye on the date your next pen is due and reordering in good time. If you want to track doses, the free GLPTrackr companion app is available for iOS and Android. Payment options include Visa, Mastercard, Klarna and pay-monthly plans, with no subscriptions.

Treatment through us is private; the NHS route is separate and being phased in. NICE recommends Mounjaro for adults with a BMI of at least 35 plus at least one weight-related condition, and access is being phased in over several years.⁴ Whichever route you use, a faulty pen should be reported to whoever supplied it.

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