My Mounjaro Pen Feels Broken — What's Actually Happening

The biggest misconception: a 'broken' pen is usually an incomplete technique

People often assume the pen has malfunctioned because nothing visible happened when they pressed the button. In reality, the Mounjaro KwikPen has a sequence that every step must follow, and missing one causes it to appear dead. The first thing to check is whether the needle was attached correctly. Needles must be screwed on until they stop; a loosely threaded needle can prevent the pen mechanism from engaging at all. You can read about attaching and changing Mounjaro pen needles for a step-by-step walk-through.

The second common culprit is insufficient skin contact. The pen will only release medication when the injector is pressed firmly and flatly against the skin, with the needle fully inserted. If you angle the pen, press too lightly, or pull back even slightly during the injection, the mechanism disengages. Hold the pen against your skin and keep steady pressure until the injection window turns yellow, that colour change confirms the dose was delivered. Letting go a moment too early is the single biggest cause of an apparently failed injection.

Third, check whether you removed the grey cap before pressing the button. It sounds obvious, but the cap and the needle cap are different things, and leaving either in place prevents the pen from firing. The full Mounjaro injection guide covers each cap removal in sequence. If you have followed every step correctly and the pen still did not work, our Mounjaro pen page explains exactly what the device should look, feel and sound like at each stage, which makes it easier to pinpoint where things went wrong. Look at the inspection window: liquid still visible means the dose was not delivered.

What to do if the inspection window still shows liquid after your injection attempt

Do not press the button again. Do not try a second pen from the same week's supply. A dose delivered twice in one week sits well outside the licensed schedule, and that decision belongs with your prescriber, not with a troubleshooting checklist. Call your prescriber or the prescribing pharmacy as soon as possible and describe exactly what happened: which step you were on, what the pen looked, felt and sounded like, and the current state of the inspection window.

If you sourced your Mounjaro through nume (sorry, through a different pharmacy) the process is the same: contact them directly. If you are a patient at nume, our aftercare team is available seven days a week and can connect you with a prescriber when clinical questions arise.

One practical storage note: pens kept in the fridge door are exposed to temperature fluctuations every time the door opens, and in rare cases very cold medication can affect injection viscosity. If you are ever unsure whether a pen has been stored correctly, our guide to Mounjaro pens covers handling, storage conditions, and what to look for before you inject. The SmPC on the Electronic Medicines Compendium gives the authoritative storage guidance for Mounjaro, the exact room-temperature window is stated there, and it is the document to follow, not informal advice. Never use a pen that has been frozen.

Pen arrived damaged or is leaking: what to do with a genuinely faulty pen

A pen that arrives with a cracked casing, is leaking medication, or makes an unexpected sound before use is a different situation from a technique issue. Do not attempt to use it. If you are wondering whether the casing can be inspected more closely, our page on whether you can open a Mounjaro pen explains what is and is not safe to do with the device. Place it in a sharps container if the needle is already attached, keep the outer packaging if possible, and contact your supplying pharmacy to report the fault and arrange a replacement.

You should also report genuine product defects to the MHRA via the Yellow Card scheme. This is not bureaucracy for its own sake, the Yellow Card database is how the MHRA identifies patterns in product quality issues across the country, and a single report from a patient can trigger an investigation. The reporting process takes only a few minutes.

Separately, if you have any doubt about whether a pen came from the licensed UK supply chain, that doubt matters. Counterfeit tirzepatide pens exist in the UK; the MHRA has conducted large-scale seizures and, in 2025, raided the first illicit UK site manufacturing fake pens. A genuine pen from a GPhC-registered pharmacy will have consistent, professional labelling, a batch number, and a UK patient information leaflet inside the box. If any of those are missing, report the seller and do not use the pen. For broader context on the Mounjaro injection and what to expect from a legitimate supply, that page covers the essentials.

When to seek urgent medical help

A pen that did not work correctly is frustrating, but it is rarely a medical emergency on its own. The scenarios that do require prompt medical attention are: if you believe you accidentally injected more than the prescribed dose (contact a prescriber or NHS 111 immediately), if you experience severe stomach pain that spreads to your back (a potential sign of pancreatitis, which the MHRA flagged in a January 2026 Drug Safety Update for GLP-1 medicines, get urgent help), or if you notice signs of an allergic reaction such as swelling of the face, lips or throat.

For questions about technique before your next dose, the nume FAQ page covers many common injection questions, or speak directly to our clinical team. If you are considering starting Mounjaro and have not yet had a clinical assessment, you can start your free consultation with our prescribers.