When Your Mounjaro Pen Isn't Working as Expected

The biggest misconception: a pen that seems broken is usually fine

Most people who contact a pharmacy because their Mounjaro injection isn't working discover the pen itself is perfectly intact. The KwikPen is a precision device, but it depends on a correctly attached needle to function. Needles are not included in the pen and must be applied before each of the four doses the pen contains. If the needle is cross-threaded, not tightened fully, or has been left on from a previous injection and become blocked, the medicine has nowhere to go, and the plunger meets resistance that can feel like a mechanical fault.

There is a real frustration that comes with holding an expensive pen that appears to do nothing, especially if this is one of your early doses and you are still getting used to the process. That feeling is completely understandable. The good news is that the fix is usually simple. Our guide to how the Mounjaro pen works walks through the full injection sequence in detail.

Remove the needle, discard it safely, and attach a fresh one. Screw it on until it stops, then prime: hold the pen with the needle pointing up, remove both caps, and press the injection button until a small drop appears at the tip. If a drop appears, the pen is working. Proceed with the injection as normal. If no drop appears after two attempts with a fresh needle, contact your prescriber or pharmacist before using the pen again.

Specific faults and what each one usually means

Different symptoms point to different causes, and the pen-not-clicking guide covers that specific fault in more depth. For a broader picture of device issues:

The pen clicks but nothing is injected. This is almost always a needle problem. The internal mechanism has moved, but the medicine could not travel through a blocked or absent needle. Replace the needle and reprime.

The pen won't click at all. Check the dose counter. If it reads zero, the pen is empty, four weekly doses have already been delivered. You'll need a new pen for your next injection. If you're unsure whether you have enough needles to last the month, a pack of compatible replacement needles is available separately.

The pen clicks and medicine comes out, but less than expected. Partial delivery can happen if the pen is removed from the skin before the full count (typically holding for around ten seconds, though always follow the exact timing in your Patient Information Leaflet). Keep the needle in place until the count is complete and hold for a further two seconds before withdrawing.

The medicine looks discoloured or cloudy. Tirzepatide solution should be clear to slightly yellow and particle-free. Do not use a pen whose contents look unusual; contact your pharmacy. The NHS tirzepatide medicines page confirms what the solution should look like and when to discard a pen.

Storage problems that can cause the pen to fail

Mounjaro must be stored in the fridge at 2–8°C until you are ready to use it. The exact duration for which the pen can be kept at room temperature is set out in the Patient Information Leaflet, and that is the figure to follow, not general guidance from the internet. If your pen has been left in a warm bag, in direct sunlight, or in a car on a warm day, the medicine may have degraded even if the device looks normal. A pen that has been stored incorrectly should not be used; contact your pharmacy for guidance on replacement.

If you have been questioning whether your Mounjaro injection is working in the sense of producing weight loss rather than a device fault, that is a different question. Early in treatment, effects on appetite can be subtle. How long after injection Mounjaro starts working explains what to expect and over what timeframe. The Mounjaro injection page also covers technique, sites and rotation in full.

When to contact your prescriber and what to tell them

Most device issues are resolved with a fresh needle and a reprime. Contact your prescriber or pharmacist if: the pen consistently fails to deliver medicine despite a new needle; the dose counter behaves unexpectedly; the medicine appears visually abnormal; or you injected but are uncertain whether a full dose was delivered.

Do not inject a second dose to compensate for a suspected missed or partial dose. Speak to your prescriber first. The Patient Information Leaflet contains the manufacturer's instructions for missed doses; your prescriber will advise based on where you are in your titration schedule. For questions about your treatment plan or whether your current dose is right for you, this deeper troubleshooting guide addresses the clinical side as well as the practical one. You can also read the general FAQs or reach the team via the contact page.

If you are thinking about starting Mounjaro or want to understand the cost of private treatment, the cost page sets out what is included in a legitimate private prescription. And if you would like to explore your options, check your eligibility with our prescribers, the consultation is free and reviewed the same day by a real clinician.