My Mounjaro pen isn't working – a step-by-step guide to what's wrong

Step 1 (check the basics before you do anything else

A question our prescribers hear most weeks goes something like: "I pushed the button and nothing happened) is the pen broken?" Nine times out of ten, something simple is behind it. Start here.

First, look at the yellow indicator window on the side of the pen. If the window shows an orange colour or no liquid is visible, the pen is empty. Each KwikPen contains four weekly doses, so if you've been using the same pen for a month, this is the most likely explanation. You can read more about how the device is designed to work on our guide to how the Mounjaro pen works.

Next, check that a new needle has been attached. The needle must be screwed on firmly and straight, with both the outer and inner needle caps removed before you inject. A cross-threaded or loose needle won't create the seal needed to push the medicine through. If the needle was left on from a previous injection (which the SmPC advises against) it may have become blocked, or air may have entered the cartridge.

Finally, check the liquid in the window. It should be clear or faintly yellow with no particles. A cloudy or discoloured pen should not be used; contact your pharmacy. The Mounjaro injection overview covers the full preparation checklist if you want a reference to work through.

Step 2 — temperature, pressure and technique

Mounjaro pens are stored in the fridge at 2–8°C, and a pen used straight from the fridge can feel stiff and may not deliver smoothly. Give the pen time to reach room temperature, check your Patient Information Leaflet for the exact permitted window at room temperature, because the timing matters and the leaflet is the authoritative source for your specific batch. The storage and temperature guide has more detail on what happens if the cold chain is disrupted.

Technique is the other factor that trips people up. Press the pen firmly and flat against the skin, not at an angle. Hold it there and press the button fully. You'll hear a click when the dose starts. Keep holding the pen in place, the injection takes around ten seconds to complete, and lifting the pen too soon is one of the most reliable ways to lose part of a dose. A second click signals the end; confirm the dose indicator has moved before removing the pen.

If you felt almost nothing, that isn't necessarily a sign of failure. The needle is fine enough that many people feel very little at all, particularly in the abdomen. The absence of a sensation doesn't mean the dose wasn't delivered. Sites worth rotating between are the abdomen, the thigh, and the upper arm, per the Mounjaro SmPC on the electronic Medicines Compendium.

Step 3, if the pen still won't fire, or you're not sure the dose went in

If you've checked everything above and the pen still doesn't click, or the indicator hasn't moved, treat it as a missed dose. Do not attempt to inject a second dose to compensate. The guidance in your Patient Information Leaflet is clear: if you miss a dose and your next scheduled injection is more than four days away, take the missed dose as soon as you remember; if it's fewer than four days away, skip it and continue with your usual schedule. Your prescriber can confirm the right approach for your specific situation.

Keep the pen. A genuine device fault is rare, but your pharmacy needs the pen back to investigate and, where appropriate, arrange a replacement. If you find the pen repeatedly fails to fire or behaves unexpectedly, our dedicated guide to a Mounjaro pen not working walks through the most common device faults and what to do next. Contact your dispensing pharmacy or prescriber promptly rather than ordering a new pen immediately, our support team is available seven days a week if you dispensed through nume.

It's also worth considering whether the pen may have been left out of the fridge for longer than the permitted room-temperature window, or exposed to direct sunlight or heat. Either can damage the medicine visibly or invisibly. If there's any doubt about the pen's storage history, don't use it, and our guide to why your Mounjaro pen may not be working includes a more detailed look at storage-related failures alongside the other common causes worth ruling out.

Step 4, after the injection: what to watch for

Once you're confident the dose was delivered, a small amount of liquid at the injection site is normal and doesn't mean you've lost the dose. A brief sting or redness that fades within minutes is also typical. If you notice significant swelling, prolonged pain, or a lump that doesn't settle within a day or two, let your prescriber know.

Some people worry when they don't feel any different after their first few doses. Mounjaro works gradually, and the starting dose is designed to let your system adjust rather than to produce immediate appetite changes, full effects at any dose typically build over several weeks. The timeline guide sets realistic expectations for each stage. If you're thinking about cost or switching providers because of repeated pen issues, the treatment overview explains what a full private prescription service should include.

Mounjaro is a prescription-only medicine assessed and supplied through regulated clinical pathways. If pen problems are recurring, or if you have any doubt about whether a dose was received, the right step is always to speak to a prescriber before your next scheduled injection. You can start a free consultation with our clinical team if you'd like a second opinion on how your treatment is going.