Why your Mounjaro pen isn't working — and what to do about it

The biggest myth: 'the pen is broken, I need a replacement'

Most people who contact their pharmacy convinced their pen is faulty find, on closer inspection, that one small step went wrong. The pen itself is rarely defective. Before concluding that your KwikPen has failed, work through a quick physical check, it genuinely takes under a minute. Hold the pen up to a light source and look through the clear inspection window: the liquid inside should be colourless or very slightly yellow, and the dose counter should show the number that matches the dose you were about to take. If the counter has already moved on by one, the dose was likely delivered even if you didn't feel it, semaglutide and tirzepatide injections are designed to be almost painless, and many people experience no sensation at all.

If the counter hasn't moved, the dose was not fully delivered. The guide to a Mounjaro pen not working walks through every fault state the counter can show and what each one means practically. NHS guidance on tirzepatide also directs patients to the Patient Information Leaflet as the primary troubleshooting reference, the leaflet is the clinical authority on what to do next, not social media advice or forum posts.

The correction many people need is straightforward: they released the button before the 10-second hold was complete, or they angled the pen so the needle tip pulled away from the skin during injection. Both are easily fixed on the next attempt, and if you are still having trouble after checking those steps, the advice on what to do when your Mounjaro pen isn't working covers the additional checks worth running before contacting your pharmacy.

Needle problems account for most mechanical failures

The needle is the most variable part of the system, and small errors here cascade into what looks like a pen fault. Needles must be attached immediately before each injection and removed straight after, leaving one in place between doses allows medicine to leak out, air to enter, and the channel to block. If you're hearing no click when you press the dose button, or you can see liquid beading on the skin surface rather than being absorbed, the needle is almost always the place to start.

Check that the needle is screwed on straight and firmly. A cross-threaded needle creates a partial seal, which means the pen may click but deliver little or nothing. Once you've confirmed the needle is secure, make sure the inner needle cap (the smaller of the two) has been removed before inserting into the skin, this is the single most common reason for a completely non-functional injection, and it's easy to miss in the pre-injection routine. Replacing a faulty needle before your next dose is simple; compatible replacement needles are available if you need a fresh supply.

The pen's clicking mechanism is worth understanding separately: one audible click signals the start of dose delivery, but the dose is only complete after the button is fully depressed and held. A single click followed by releasing the button early is a partial dose, not a full one.

Storage and temperature: the fault you might not think of

Mounjaro pens must be stored in the fridge at 2–8°C. If a pen has been left out at room temperature for longer than the window specified in its Patient Information Leaflet, it should not be used, refer to that leaflet for the exact time limit, as it varies and is the clinical standard. Equally, a pen that has frozen must be discarded; it cannot be thawed and re-used safely.

Cold medicine is viscous. A pen taken directly from the fridge and immediately injected will feel stiffer and the plunger harder to press than one allowed to warm up. If the button is genuinely difficult to depress, temperature is the first thing to rule out. Wait 30 minutes at room temperature, then try again. This is also why injecting into cold or recently exercised skin adds resistance, the abdomen, outer thigh and upper arm are all suitable sites, and rotating between them helps.

If you're ever uncertain whether your pen has been stored correctly or whether a dose was fully delivered, the right move is to contact your prescriber rather than repeat the injection. More detail on the injection process itself is available on the Mounjaro injection guide, which covers site selection, rotation and technique step by step. For the broader picture on what Mounjaro is and how treatment is structured, the Mounjaro overview is a useful starting point. Questions about what treatment costs privately can be found on the Mounjaro price page.

When to get medical help, and what not to do

A missed or uncertain dose is not a medical emergency, but it does need a clinical decision. Your prescriber will advise whether to take the dose as soon as you notice the problem (if your next scheduled injection is still several days away) or to skip it and resume the usual schedule. The key rule: never inject twice in one week to compensate. Two doses in quick succession significantly increases the likelihood of nausea, vomiting and other gastrointestinal effects.

Seek prompt medical attention if you experience severe stomach pain (particularly pain that radiates toward the back) with or without vomiting, as this may indicate pancreatitis. The MHRA's Yellow Card scheme allows patients to report suspected side effects directly to the regulator, and you can also use it to flag problems with the pen or packaging itself if you suspect a faulty or counterfeit product.

If you're on treatment through nume and run into a pen problem, our aftercare team is available seven days a week. You can reach us via the contact page. And if you're not yet on treatment but want a clinician to assess your suitability, the right first step is a free consultation with our prescribers, reviewed the same day by a real clinician, not automated software. Further background on how our clinical team approaches prescribing decisions is on the clinical team profile.