What to Do When Your Mounjaro Pen Seems Faulty

Is the pen actually faulty, or is something else going on?

This is the first decision to make, and it is worth being honest with yourself about it. The Mounjaro KwikPen has a specific sequence of steps, attaching a new needle, doing the flow check, dialling, injecting slowly, holding for ten seconds, then counting the full ten before withdrawing. Miss any one of those steps and the pen can behave in ways that look like a malfunction but are not.

Common things that mimic a faulty pen: the dose counter not moving (often the button was not held down long enough, or the needle had not pierced the skin fully); liquid appearing at the injection site after withdrawal (the ten-second hold was too short, or the needle gauge was not quite right for the tissue); the pen feeling stiff or blocked (needle not fully tightened, or a bent needle tip from a previous use). Before deciding the pen itself is defective, check the full injection technique step by step and confirm the needle was changed for this dose.

The NHS guidance on tirzepatide (available on the NHS medicines page for tirzepatide) recommends reviewing the patient information leaflet if you are unsure whether a dose was delivered, and contacting your healthcare team rather than repeating the injection.

Signs that point to a genuine device defect

Some things genuinely do indicate a problem with the pen itself rather than the technique. A cracked or cloudy cartridge window (Mounjaro solution should be clear and colourless), a dose counter that is stuck or moves backward without any injection, a button that is permanently depressed or will not spring back, liquid leaking from the pen body rather than the needle tip, or a damaged end cap that has compromised the sterile seal, these are not user-error scenarios.

Eli Lilly takes product complaints seriously and has a UK medical information line for reporting suspected device defects. Keep the faulty pen and its outer packaging, note the batch number printed on the label, and contact Lilly UK directly. They can log a formal product complaint and advise on replacement. Your dispensing pharmacy (in nume's case, a GPhC-registered UK pharmacy) is also a first port of call and can liaise with Lilly on your behalf.

Do not try to salvage a pen you believe is structurally compromised. If the cartridge is cracked, there is a contamination risk regardless of how much solution remains.

What to do about a missed or uncertain dose

This is where the decision becomes clinical rather than technical. If the dose counter moved from the expected number to the next one and the pen held still during the injection, the dose was almost certainly delivered even if you did not feel it, subcutaneous injections into abdomen or thigh fat can be nearly sensation-free. If the counter did not change at all, the dose was not delivered and you can attempt the injection again with a fresh needle attached correctly.

Where it gets uncertain (the counter moved but you saw liquid at the injection site, or you are simply not sure) do not inject a second time on the assumption the first did not work. Double-dosing tirzepatide carries a meaningful risk of intensified side effects, particularly nausea and gastrointestinal discomfort. The right step is to contact your prescriber. At nume, our prescribers are available seven days a week for exactly this kind of aftercare question; there is a direct route through our support team if you need clinical guidance fast.

The Mounjaro injection guide also covers the flow-check step that confirms the pen is working before you inject, worth revisiting if this uncertainty has come up more than once.

Protecting your treatment if a pen needs replacing

A genuine faulty Mounjaro pen in the UK should be reported to Eli Lilly and also flagged to the MHRA via the Yellow Card scheme, which accepts reports of both suspected adverse reactions and device defects. That reporting matters: it helps the regulator track manufacturing quality across batches.

If replacement is needed, the practical timeline is worth thinking through. Monday orders placed before 12pm with next-working-day delivery are predictable; if your pen becomes faulty over a long weekend or during a bank holiday, the conversation with your prescriber and the pharmacy may add a day. Worth bearing that in mind when you are close to your last dose of the month.

On the cost side, replacement pens are a private prescription item. The Mounjaro pricing page covers what is included in the cost of treatment through a regulated UK pharmacy. Replacements from unofficial sellers online carry a genuine risk: the MHRA has confirmed seizures of counterfeit tirzepatide pens from illicit supply chains, and a pen purchased that way offers no verified quality assurance whatsoever. If you need a replacement urgently, the right route is through the pharmacy that holds your prescription.

If you are ever uncertain whether your pen has been stored correctly (Mounjaro must be refrigerated between 2°C and 8°C; you can read more about Mounjaro pens, including how storage requirements differ between unused and in-use devices, before using it) treat that as a separate question for your prescriber before using it. Storage and device integrity are both part of clinical safety, not just brand standards. If you have already used some of your dose and are not sure how much remains, our guide to recognising an empty Mounjaro pen can help you confirm whether the cartridge still contains medication before you decide on next steps.