Mounjaro Pen Problems: What to Check and Do (October 2026)

01 · Start here

What to do first when your Mounjaro pen malfunctions

The safest first move is to stop, put the pen down and not force anything. A dose knob that will not turn, or an injection button that will not press, is a signal to check, not to push harder. Forcing a pen can damage it, and you may then be unsure how much medicine you actually received.

Work through these checks calmly, in this order. Each one takes moments, and most pen problems are solved by one of them.

• Look at the dose window.
• Check the needle. Make sure a fresh needle is fitted straight and that both needle caps have been removed, following the steps in your patient information leaflet.
• Think about how many doses you have used. A pen holds a limited number of weekly doses, and an empty pen behaves like a stuck one.
• Check the storage history. Do not use a pen that has been above 30°C or has been frozen.
• Stop and ask. If it still fails, do not improvise. Contact your pharmacy or prescriber.

For a step-by-step walk-through of a stubborn pen, our guide on how to unjam a Mounjaro pen covers the practical checks in more detail, and preparing your Mounjaro pen explains the set-up before each injection.

02 · Diagnose

Is it a real fault, or is the pen behaving normally?

Many reported pen problems turn out to be normal pen behaviour that looks like a fault.

If none of these fits, treat the pen as suspect and ask. A replacement conversation is far better than a dose of unknown size.

03 · Next step

Where to go from here

If your pen was supplied by us and you think it is faulty or damaged, contact our support team promptly through email or your account portal chat, and keep the pen and packaging. Our returns terms treat faulty or damaged items differently from ordinary dispensed medicines. If you are considering treatment and want to see how supply works, the Mounjaro pen prices and consultation page shows current availability, with all six doses in stock.

Related guide: Does mounjaro cause eyesight problems.

04 · Pen will not dial

Mounjaro pen will not dial or twist

A pen that will not dial is most often a pen that has run out of full doses. The KwikPen is set by turning the dose knob until the dose indicator shows the correct dose in the window. Each KwikPen contains four weekly doses.¹

So when the knob will not move, ask yourself how many weekly doses you have already taken from this pen. If you have used them all, the pen is finished and you need your next pen. If you are confident you have not used them all and the knob still will not turn, do not force it. The pen may be damaged, and your pharmacy should look at it.

If you are short of a dose because of a pen problem, speak to your prescriber or pharmacy.

05 · Needle and injection issues

Needle, button and injection problems

Resistance when you press the injection button is usually about how the needle is fitted, not about the medicine. Your patient information leaflet shows the correct way to attach a new needle, remove both caps and hold the pen for the injection.¹³ Follow the pen's instructions exactly, and do not rely on advice from forums or videos that conflicts with it.

If the button will not press, stop. Remove the needle as the leaflet describes, dispose of it safely in your sharps bin, and try again with a fresh needle only if the dose window still shows your full dose. If the window has changed, or you are not sure what has been delivered, do not take another injection to compensate.

Wondering how much you received is one of the commonest anxieties after a difficult injection. It is better to carry on with your normal schedule than to inject again. Tell your prescriber what happened, and they can advise on whether anything needs adjusting.

Needles, a sharps bin and injection guidance are included with every order from us, so you should not need to reuse a needle. Never reuse or share needles. For the related question of sharing a pen, our page on why you should never share a pen explains the safety position.

Choose a different injection site each week, as the leaflet advises. Licensed 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. Rotating sites helps to avoid sore or lumpy areas, which can make injections feel harder than they should.

06 · Priming and set-up

What happens if you do not prime your pen?

Whether a pen needs any pre-injection check is set out in the patient information leaflet for your pen, and that leaflet is the authority to follow.¹³ If the leaflet describes a check before your first use or before each injection, do it every time.

If you realise you skipped a step the leaflet describes, the sensible approach is calm and practical. Look at the dose window and check what it shows. Do not inject a second time to make up for it. Tell your prescriber or pharmacist what you did and what you saw, and let them advise.

Reading the leaflet instructions once at the start, and again if a pen ever behaves unexpectedly, is the most useful thing you can do. Our guide on preparing a Mounjaro pen gathers the practical points in one place.

Keep the pen cap on between uses to protect the pen from light. Take the pen out of the fridge only when you are ready to prepare it, and follow the leaflet about what to do with it before injecting.

07 · Storage-related faults

Storage problems that look like pen faults

Storage mistakes cause many apparent pen problems. Keep Mounjaro pens in a fridge at 2–8°C and never freeze them. Never use a pen that has been frozen.¹ A pen left against the back wall of a fridge, or beside a freezer compartment, can freeze without you noticing.

Once in use, a pen can be kept out of the fridge not above 30°C for up to 30 days, after which it should be discarded.¹ Discard it at that point even if some medicine appears to remain.

Heat is a risk too. A pen left in a hot car, a sunny window or a beach bag can end up outside the tested conditions. If that happens, do not use it. In hot climates, an insulated pouch helps, and when flying, carry pens in hand luggage, because hold luggage can fall below freezing.¹³

For unopened pens, store them in the fridge until first use, and they 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 you are unsure about a pen's storage history, the safe answer is to ask.

08 · Reporting a faulty pen

How to report a faulty Mounjaro pen

Report a faulty pen in two places: your pharmacy first, then the MHRA Yellow Card scheme. Keep the pen, the needle if still attached, the outer box and any dispensing label, because the batch details help the investigation. Do not throw the pen away until you have been told what to do, apart from used needles, which go in your sharps bin.

Contact the pharmacy that supplied the pen as soon as you notice the problem. If it came from us, email our support team or use the account portal chat, describe exactly what the pen did, and attach a clear photo if you can. Our returns terms allow faulty, damaged or wrong items to be returned, and the sooner you tell us, the easier it is to resolve. Support is available on weekdays during office hours.

You can report suspected side effects through the MHRA Yellow Card scheme, and you should tell your pharmacy about a suspected faulty pen.¹⁹

Describe what you were doing, what the pen did and whether you received any medicine. If you think you may have received the wrong amount, say so, and talk to your prescriber about next steps rather than injecting again.

If you bought a pen from a seller offering to supply without a consultation or prescription, treat it with extra caution. Pens from unregulated sources may be counterfeit, and a pen that behaves oddly from such a source should not be used. You can check any UK pharmacy on the GPhC register; ours is registration 9012878.

09 · Schedule problems

Missed, delayed or off-schedule doses

A pen problem sometimes causes a delayed dose, so it helps to know 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 means a pen failure on injection day does not automatically ruin your week. If your replacement arrives within the window, take the dose as the rule describes. If not, skip it and resume on your usual day, and tell your prescriber.

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 This can be useful if a pen problem has moved your schedule, but only within that limit.

What happens if you have time off Mounjaro? 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, and there is no way to speed this up.¹

Can you stop cold turkey? Mounjaro manages weight; it does not cure the biology behind it, and nothing needs doing while the medicine clears. Stopping should always be planned with your prescriber. In SURMOUNT-4, people who stopped tirzepatide after 36 weeks regained on average 14% of body weight over the following year, while those who continued lost a further 5.5% on average.¹⁴ Our wider guide to common Mounjaro problems covers other issues people raise.

10 · If it is not working

What to do if Mounjaro is not working

Before blaming the pen, check the results timeline. Weight loss on Mounjaro is gradual by design. 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.¹ Trials measured results at 72 weeks, so think in months, not days, and judge progress by trend, not individual weigh-ins.²

The starting weeks are about letting your body adjust, not about weight loss itself. Some people notice appetite change and early loss in the first week, 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.

If you suspect a pen is not delivering, the real signs are a window that behaves oddly, visible leaking, or a dose you could not confirm. A pen that delivers properly but gives a slower result is a different issue, and 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.¹

For perspective, in the SURMOUNT-1 trial, 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, and individual results vary.² Treatment works alongside a reduced-calorie diet and increased physical activity, not instead of them.

Do not raise your dose yourself or change the interval to chase results. Higher is not automatically better, and your prescriber will guide the right pace.¹

11 · Side effects and foods

Side effects, foods and long-term questions

People sometimes mistake early side effects for a pen problem. Mild nausea, altered appetite and tiredness in the first days after each injection are the commonest early effects and usually settle.¹ Many people choose a weekend morning to inject, so early side effects fall on days off. There is no best time of day, because Mounjaro works the same morning or evening, with or without food. What matters is the same day each week.

Which foods should you avoid? No food is banned on Mounjaro, including bananas. Many people feel worse after very fatty, fried or very sugary food, and smaller, slower meals work better. If particular meals leave you unwell after an injection, mention it at your review.

Long-term effects. Treatment is reviewed on an ongoing basis by your prescriber, and any new or worrying symptom should be raised with them. You can also report suspected side effects through the Yellow Card scheme.¹⁹

Mounjaro is licensed in the UK for weight management in adults and for type 2 diabetes, and is used alongside a reduced-calorie diet and increased physical activity. It is a prescription-only medicine, supplied after an online consultation reviewed by a GPhC-registered prescriber. This page is information, not a substitute for medical advice.

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.⁴ Many people who meet the licensed criteria choose regulated private treatment rather than waiting. Our weight loss clinic explains the options.

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