Mounjaro Pen Not Working? Fixes and What to Check (November 2026)

01 · Start here

What to do first when your pen will not work

The first step is to stop, put the pen down and avoid forcing the dial or the injection button. A pen that has been forced may deliver an inaccurate dose or stop working altogether, and there is no safe way to guess how much medicine went in.

Then work through the likely causes in order. Most pen problems turn out to be one of a short list: the pen has no full dose left, the dial has not been set the way the instructions describe, the needle or caps have not been handled as the leaflet shows, or the pen has been stored badly. The patient information leaflet that comes in every box shows each step with pictures, and it is the authoritative guide for your pen.¹³

Here is a quick way to match the symptom to the likely cause and the next action.

02 · Decide

Is it the pen, or is it the medicine?

If the pen dials, injects and you complete each weekly dose as instructed, the device is working, and the question is about results. If the pen cannot be set or will not inject, that is a device problem and you should not carry on until it is resolved.

The distinction matters because the advice is opposite. For a device fault, the answer is to stop and get the pen checked or replaced. For slow results, the answer is almost never to change the pen or the dose yourself: it is to give the treatment time at the right dose and to review progress with your prescriber. Mounjaro is a once-weekly injection made by Eli Lilly, and every pen is a pre-filled KwikPen.¹

If you are a customer and are unsure which side of the line you are on, tell us what the pen is doing and what you have already tried. Our prescribing team can help you decide whether this is a faulty item or a normal pattern. You can read more about the pen at the end of a course on the last dose of a pen.

03 · Safety

What not to do

Do not try to rescue a pen by twisting harder, pulling parts off or injecting twice to make up for a dose you doubt. Do not share pens or use someone else's. Do not use a pen that has been frozen.¹

Injecting twice, or altering how the pen delivers a dose, is not licensed, not supported by the pen's design and not endorsed by any UK regulator, and it means unknown dosing accuracy and no safety data.¹ If a dose is in doubt, the safe route is to ask the prescriber what to do about that week's injection. If you are ready to review your treatment or need a new supply, you can view the Mounjaro pen prices and consultation page, where the standard price of the 2.5mg starting pen is £169.99 and in stock status is shown live.

04 · Dial and clicks

Why the dial or clicks seem wrong

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

People often expect a loud, regular click at every step, the way some other injector pens behave. A KwikPen may feel and sound different from what you expected, and the sound alone tells you nothing about whether the dose is right. What matters is what the window shows when you finish dialling and again after the injection, as the leaflet describes. If the number in the window is correct, the click is not the test.

If the dial feels completely different from last week, treat that as information. A dial that has become stiff, gritty or loose, or a window that does not line up cleanly, can point to a damaged pen. Do not keep trying with force. Put the pen down, check that you are following the numbered steps in the patient information leaflet, and check whether the pen has been dropped, frozen or exposed to heat.¹³

It also helps to compare the pen with a fresh look at the instructions rather than relying on memory. Many people who inject every week do it on autopilot, and a small change in routine, such as a different surface, rushed timing or a step done in a different order, can make a working pen seem broken. Slow down, lay out the pen, a new needle if the leaflet calls for one and your sharps bin, and go through each step in order.

Write down what you saw: what the window showed, what the dial did and what you tried. That note is useful if you need to speak to the pharmacy or prescriber, because it lets them tell the difference between a user-technique issue and a faulty device. If you want our help, contact us through the account portal or by email at support@everydaymeds.co.uk, and phone and chat details are in your account.

If the pen simply will not turn, our guides on a pen that will not twist and a dial that will not reach the dose go through those specific symptoms in more detail. Whatever the pattern, the principle is the same: the window is the source of truth, and a pen that cannot show you a clear, correct dose should not be used until it has been checked.

05 · Empty or locked pen

When the pen has nothing left to give

The most common reason a dial will not turn to a full dose is that the pen has already delivered what it holds. That is a safety feature, not a fault.

This is worth stating plainly because the temptation is real: you can see liquid in the cartridge, the injection day has arrived and it seems wasteful to throw the pen away. Resist it. A leftover amount is not a usable dose, and combining it with part of the next pen means the dose you actually received is unknown.

So the first diagnostic question is simple: how many weekly injections have you taken from this pen? If you have lost count, think back through your injection days, check any tracker or notes you keep, and consider whether the pen is simply at the end of its course. Our free companion app, GLPTrackr, can help you keep a running record of doses so this is never a guessing game.

If you are sure the pen is not at the end of its course and it still will not dial to a full dose, treat it as a possible fault and contact us rather than injecting. Tell us the strength, how many injections you believe you have taken and what the window shows.

What if the pen runs out on injection day and your next pen has not arrived? Plan ahead for this by reordering in good time. Repeat orders re-check suitability and are not automatically shipped, so a repeat has to be placed by you. If you have already missed the day, the missed-dose rule applies: 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

Our related guide covers the situation in more depth: when the last dose of a pen will not work. If a pen did not deliver a dose at all, do not assume that week's injection was taken; tell the prescribing team so they can advise on what to do about it.

06 · Storage damage

Could storage be the problem?

Yes: a pen that has been stored badly may be unsafe to use, whether or not it still dials. 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

If your pen has been at the back of a fridge against a cold wall, in a car on a winter morning or in a bag next to an ice pack, ask yourself whether it could have frozen. A frozen pen should not be used, even if it looks normal once it warms up. If it has been somewhere hot, such as a parked car or a sunny windowsill, treat that as a reason to check with the pharmacy before injecting.

For unopened pens, the rule is that they stay in the fridge until first use 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.¹ That caution is deliberate. Different markets carry different labels, and what you read on a forum may not apply to a UK KwikPen.

If you are travelling, carry pens in hand luggage rather than hold luggage, because hold luggage can fall below freezing. A cool bag with the prescription label or a copy of your prescription is ideal. 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

When we deliver, pens travel in temperature-controlled, discreet packaging by tracked next-day delivery. If a parcel has clearly been left somewhere unsuitable for a long period, such as outside in cold or heat, do not use the pen until you have spoken to us. It is far better to ask than to inject a pen whose condition you doubt.

Finally, remember the in-use limit. Even a perfectly working pen should be thrown away after 30 days out of the fridge at a temperature not above 30°C, so if you are unsure when you started carrying it at room temperature, ask rather than guess. Storage problems often show up first as a pen that feels or behaves differently, so a change in feel is another reason to stop and check.

07 · Signs it is working

How to know if Mounjaro is working

Mounjaro is working if your appetite, fullness and weight trend are moving in the intended direction over weeks, not days. Tirzepatide activates two natural gut-hormone receptors at once, GIP and GLP-1. Together these help regulate appetite, slow how quickly the stomach empties, and improve the way the body responds to insulin. In practice, most people feel fuller sooner and less hungry between meals, which supports a reduced-calorie diet.¹

So the earliest signs are usually about hunger rather than the scales. You may notice that you feel satisfied sooner, think about food less between meals, or leave food on the plate without effort. Those are the effects of the medicine's mechanism, and they are what makes a reduced-calorie diet easier to follow.

The first weeks are different for everyone. The 2.5mg starting weeks are about adjustment, and experiences differ widely: some people notice appetite change and early loss in week one, others feel little until a higher dose — both are normal. Early loss includes water weight, so the trend over weeks matters more than any single weigh-in. Mild nausea, altered appetite and tiredness in the first days after each injection are the commonest early effects and usually settle.¹

Beyond hunger, there are other things to look at. In trials, weight loss on tirzepatide came with average improvements in waist size, blood pressure, cholesterol and blood-sugar measures, and most participants with prediabetes returned to normal blood-sugar levels. These are trial-average secondary outcomes, not promises, but they are a reminder that the scales are not the only measure of whether treatment is doing something useful.² Clothes fitting differently, a change in waist measurement and results from routine checks with your own clinician can all point the same way when the scale is slow.

A practical approach is to keep a simple weekly record: weight at the same time of day, a waist measurement now and then, and a short note on appetite and side effects. Judge the direction of the line across several weeks rather than reacting to any single entry. Our guides on how you know Mounjaro is working and whether it is working for you go further, and the GLPTrackr companion app can hold the record for you.

No appetite change at all, with a pen that you are confident delivered the dose, is not a reason to panic. It is a reason to talk to your prescriber, who can look at your dose, timing and experience together.

08 · Realistic pace

What results should look like, and when

Weight loss on Mounjaro is gradual by design, so a slow start rarely means the medicine has failed. The first weeks on 2.5mg are an adjustment phase and losses are usually modest; most measurable progress builds from 5mg onward, month by month. Trials measured results at 72 weeks — think in months, not days, and judge progress by trend, not individual weigh-ins.²

That is why the headline numbers on the internet can mislead. In the SURMOUNT-1 trial of 2,539 adults, people taking tirzepatide alongside diet and exercise lost on average 15% of their body weight on 5mg, 19.5% on 10mg and 20.9% on 15mg over 72 weeks, compared with 3.1% with lifestyle changes alone. These are trial averages — individual results vary, and treatment works alongside a reduced-calorie diet and increased physical activity, not instead of them.²

Those figures were measured at the end of a long trial, after people had climbed through the dose ladder. They do not describe what the scale shows in the early months. In SURMOUNT-1, average weight loss built month by month, with the full averages measured at 72 weeks, not at the early stage. A sensible expectation in the early months on a standard titration is mid single digits as a percentage of body weight, with wide individual variation. Slower early progress does not predict failure — the dose you respond to may still be ahead of you.²

To put an approved percentage in human terms, for someone starting at 100 kg, 20.9% is about 21 kg, but that is a trial average at the top dose over a long period, not a forecast for you or a target for any given month.

There is also a formal point at which your prescriber looks at response. Treatment response is reviewed by your prescriber: if you have lost less than 5% of your starting weight after 6 months on the highest dose you can tolerate, your prescriber will discuss whether continuing or stopping is right for you.² That review is tied to the highest dose you can tolerate, not to the starting dose, so a person who is still on the lower rungs is not yet at that checkpoint.

If you are worried about pace, bring your record to your review. It is easier to have a useful conversation about dose and routine with a few weeks of data than with a feeling that nothing is happening. The question to ask is not whether the medicine works, but whether the dose, the food routine and the timing are right for you at this stage.

09 · Appetite fading

When appetite suppression seems to fade

If your appetite seems to be returning, the usual response is to review the basics and talk to your prescriber, not to change anything yourself. Everyone starts Mounjaro on 2.5mg once a week. 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 (7.5mg, 10mg, 12.5mg) up to a maximum of 15mg weekly, depending on how you respond and tolerate it.¹

That ladder explains a lot of the stories about Mounjaro seeming to stop working. Someone who feels strongly full at the start may find that sensation less obvious later, as the body adapts and portions drift back up. It does not necessarily mean the medicine has stopped acting; it can mean your appetite and your habits have settled into a new pattern that needs a fresh look.

Some questions are worth asking honestly. Are portions creeping up? Are you eating more calorie-dense food because it is easy to eat when you are not very hungry? Has sleep suffered, or has alcohol increased? Are your injections on the same day each week, without gaps?

Dose is a genuine lever, but only with your prescriber. Many people settle at a maintenance dose of 5mg, 10mg or 15mg — higher is not automatically better, and your prescriber will guide the right pace.¹ Some people do well on a modest dose, and stepping up brings more side effects without a matching gain. Others benefit from the next step. The right answer depends on how you respond and tolerate it, which is why reviews exist.

Does Mounjaro burn fat, or does it just suppress appetite? Tirzepatide acts on appetite, stomach emptying and the way the body responds to insulin: most people feel fuller sooner and less hungry between meals, which supports a reduced-calorie diet.¹ The medicine makes the diet easier to follow; it works alongside a reduced-calorie diet and more activity, not instead of them.

Never respond to fading appetite effect by taking extra, shortening the gap between injections or splitting doses. Those practices are not licensed and carry unknown risks. If you are on a standard titration and have not yet reached the dose that suits you, the answer may simply be time, and a review.

10 · Plateaus

Plateaus and stalls

A stall on the scales is common and usually temporary. Weight loss on Mounjaro is not linear — plateaus of a few weeks are normal and expected as the body adapts to a lower weight.² The mistake is to read a flat fortnight as proof that the pen has stopped working.

The responses are practical. Review food intake honestly, because portions drift back up. Protect protein and activity. Check the basics, such as sleep and alcohol. And discuss with your prescriber whether a dose step-up is appropriate if you are not yet at maximum.² Each of these is something you can do without touching the pen.

Just as important is what not to do. A plateau is not a reason to double doses or add unlicensed products.² Low-cost supplements marketed with dramatic weight-loss claims are not a substitute for a licensed medicine, come with no clinical oversight and should not be mixed in as a fix for a stall.

It helps to separate three situations that look similar. In the first, you are in the early months at a low dose and progress is modest, which is expected. In the second, you have been losing and have hit a flat stretch, which is a plateau. In the third, you have been on the highest tolerated dose for a long period with little change, which is when the formal response review applies. The right next step differs in each.

For the plateau case, measure more than weight. Waist, how clothes fit and energy can show progress that a bathroom scale hides. Also weigh consistently: same scale, same time of day, ideally across a weekly average, because water and gut contents move the number from day to day.

If you have made honest changes and the trend is still flat across many weeks, ask for a review rather than waiting. Your prescriber can look at your dose history, side effects and routine and discuss whether to hold, step up or reassess. Our broader guide, help when Mounjaro is not working, covers the same ground, and our weight loss clinic page explains the wider service and what support is included.

11 · Faulty pen

Reporting a faulty pen and getting help

If you think your pen is faulty, stop using it, keep it and the box, and tell us straight away. Do not attempt to repair it, and do not inject from a pen you doubt. Contact the pharmacy through your account portal or by email at support@everydaymeds.co.uk; phone and chat details are in your account.

It helps to have the details ready: the strength, the batch information on the packaging, roughly how many injections you have taken from the pen, what the window showed and what happened when you tried to dial or inject. A photograph of the dose window and the pen's condition can speed things up.

Our returns policy is that dispensed prescription medicines cannot be returned except where an item is faulty, damaged or the wrong item, and you need to contact us promptly in those cases, within the window set out in our returns terms. That is why reporting early matters: the sooner we know, the sooner we can look into it and arrange the right next step. If a replacement is appropriate, the prescribing team will advise on timing for your next dose so that you do not guess.

If a pen delivered a dose you are not sure about, do not make up the difference with an extra injection. Tell the team what happened and ask what to do. The rule about not taking two doses close together exists for good reason, and an uncertain dose is a clinical question, not a technical one.

Beyond reporting to us, the Medicines and Healthcare products Regulatory Agency runs the Yellow Card scheme, where you can report side effects and also report suspected problems with medicines.¹⁹ If you have had any unexpected symptoms alongside a pen problem, report them there and tell your prescriber.

Always be wary of pens that do not come from a regulated pharmacy. Counterfeit medicine alerts are published by the MHRA, and a pen from an unregulated seller offering to supply without a consultation or prescription is a safety risk, not a bargain.⁷ Our pens come as genuine KwikPens through official LillyDirect supply, dispensed by a GPhC-registered pharmacy (registration 9012878). If you are still working through a stiff plunger, see our guide to a pen that will not push down.

Payment is taken when you place the order; if the prescriber decides the treatment is not suitable, the order is refunded automatically in full.

12 · 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