Mounjaro pen not clicking — here's how to work out what's happening

Did the pen make any sound at all? Starting with what you heard

The KwikPen is designed to produce two audible clicks during a normal injection: a first click as the dose begins to deliver, and a second click when the full dose has been dispensed. If you heard neither, the needle almost certainly didn't make adequate contact with the skin. The pen's mechanism needs steady, perpendicular pressure against the injection site (abdomen, outer thigh or upper arm) with the base of the pen held flat rather than at an angle.

If you heard only the first click and then nothing, or if you're unsure how long you held the pen in place, the dose may have stopped mid-delivery. Understanding what the Mounjaro click means and when it should occur can help you recognise whether the injection completed as expected. The full Mounjaro injection technique involves holding the pen firmly against the skin for a slow count of ten after the second click to make sure every drop has been absorbed. Pulling the pen away before that count can interrupt the delivery without triggering the completion click.

Worth checking: is there any liquid on the skin or on the base of the pen? Residue can suggest a partial dose was delivered. If you're uncertain whether any medication was injected, contact your prescriber before attempting another dose, getting an accidental double dose is a real concern with any once-weekly medicine.

The needle is the part most people overlook

A Mounjaro KwikPen won't fire correctly unless the needle is attached cleanly and completely. If the needle wasn't threaded on straight, if it was the wrong type, or if it was reused from a previous injection, the pen's mechanism can engage without pushing the dose through. The needle should always be attached immediately before use and removed immediately after.

Using a new needle for every injection isn't just about hygiene, though that matters too. Reused needles develop microscopic bends at the tip that can block the channel or prevent the pen from seating correctly. The right needles for Mounjaro are compatible BD Micro-Fine or equivalent pen needles; the pen is not supplied with needles, so these are ordered separately.

If you keep your pen in a handbag or gym bag between doses, it's worth checking that a needle wasn't accidentally left attached. A capped needle left on overnight can draw air back into the tip and affect the next injection. Attaching fresh each time takes thirty seconds and removes this variable entirely.

Cold pens and storage: a factor that catches people out

Mounjaro must be stored in the fridge between 2 and 8°C, many people keep it on the fridge door. The Patient Information Leaflet sets out a limited period during which the pen may be kept at room temperature once removed, so always check that document for the exact window rather than relying on a general figure.

What's less well known is that injecting with a pen that came straight out of the fridge can be uncomfortable and can also affect how smoothly the mechanism operates. Allowing the pen to sit at room temperature for a few minutes before injecting can make a real difference to how the dose feels and whether the click mechanism fires cleanly. Don't warm the pen in hot water or direct sunlight, room temperature on the counter is enough.

For a broader look at what can stop the pen from working as expected, the Mounjaro pen not working guide covers additional scenarios including a blocked needle channel and what to do if the pen appears damaged. Storage and technique together account for the majority of click problems reported to prescribers, and both are straightforward to address, including questions about how many clicks to expect and what each one signals during a normal injection. For context on the medicine itself, the Mounjaro overview covers how the treatment works and who it's licensed for.

When to contact your prescriber rather than try again

Most click problems are technique or equipment issues, resolved with a fresh needle and careful attention to the steps above. But there are situations where the right call is to stop and speak to a clinician. Contact your prescriber or pharmacist if: the pen looks damaged or the viewing window shows no liquid remaining even though you don't believe a dose was delivered; you're genuinely unsure whether a full dose was injected and don't want to risk taking more; or the pen has been stored outside recommended temperature ranges for an extended period.

Reporting a pen that behaved unexpectedly is also important for medicine safety. The MHRA's Yellow Card scheme allows patients to report suspected device or medicine problems, and this feedback directly informs regulatory oversight of the product. The NHS tirzepatide guidance also lists the side effects and device-use advice you'd find in the printed leaflet, in plain English.

If you're on treatment through nume, our prescribers are available for aftercare seven days a week, use the contact page to reach the clinical team directly. And if you're still at the point of considering starting treatment, you can check your eligibility without any obligation.