Your Mounjaro pen has jammed — here's what to do

You're holding a pen that won't budge, start here

You've pressed the button. Nothing happened, or you felt resistance you didn't expect. It's frustrating, especially mid-injection, and most people's first instinct is to press harder. Don't. Forcing a jammed Mounjaro pen risks bending the internal mechanism, which makes the situation worse and rules out any chance of salvaging the dose.

The first thing to establish is whether the pen is cold. Mounjaro is stored in the fridge at 2–8°C, but the device is designed to be used at room temperature. A pen taken straight from the fridge can feel physically locked because the viscosity of the solution increases in the cold, and the spring mechanism in the injector needs the cartridge to be at ambient temperature to work properly. Set the pen down on a clean, flat surface and leave it for 20–30 minutes. Don't run it under warm water, don't hold it in a closed fist for five minutes, and definitely don't put it in a microwave. Gentle, passive warming is all that's needed. A large number of jammed-pen reports trace back to this single cause, and our full guide on what to do if your Mounjaro pen jams covers every step in detail.

While you wait, check the pen cap and needle haven't been disturbed. If you've already attached the needle, check that it's seated straight, a cross-threaded needle can create back-pressure that mimics a jammed plunger. More detailed guidance on checking and attaching the needle correctly is on our Mounjaro injection technique page, which also covers site rotation and common mistakes.

After warming: uncap, hold the pen against the injection site at 90 degrees, and apply a firm, steady push on the button. You should hear and feel a click at the start of the injection and a second click when it's complete. The full injection takes about 8–10 seconds. Hold the pen in place until both clicks have registered.

If the pen still jams after warming, and what counts as a fault

A pen that jams after proper warming, that produces only a partial dose before stopping, or that makes an unusual sound when pressed is likely faulty. This is different from a cold-pen issue, and the response is different too.

Do not attempt a second injection with the same pen into the same site. If you're unsure whether a full dose was delivered, the step-by-step Mounjaro pen jammed troubleshooting guide covers how to inspect the dose window (the yellow indicator visible through the pen body) to tell whether the dose completed. A dose window that hasn't moved is a strong sign the medicine didn't deliver. A partially-moved window is less clear and warrants a call to your prescriber before you do anything else.

Once you've confirmed the pen failed, contact Eli Lilly's UK customer support line. Lilly operates a device complaint and replacement process: they'll want the pen's lot number (printed on the label), the expiry date, and ideally the pen itself returned for investigation. Keep a jammed pen rather than throwing it away immediately. You can store it safely in a sealed sharps container if the needle is attached, and hand it to the Lilly representative or your pharmacy as directed.

Separately, report the device issue through the MHRA's Yellow Card scheme at yellowcard.mhra.gov.uk. This isn't bureaucratic box-ticking, device faults logged via Yellow Card feed into ongoing post-market surveillance for Mounjaro, which carries a Black Triangle (▼) designation requiring additional monitoring. A few minutes of your time genuinely contributes to that picture.

Missed dose or partial dose: the practical question

The question your prescriber will want to answer with you is whether you received a meaningful dose. The NHS patient information resource for tirzepatide on the NHS website advises that if you miss a dose entirely, you should take it as soon as possible provided your next scheduled injection is more than four days away. If it's within four days, skip it and continue from the next scheduled day. Never double up.

A partial dose sits in a grey area. Don't guess, and don't self-prescribe an extra injection. Speak to your prescriber, at nume, that means contacting our clinical team who reviewed your consultation, available seven days a week through the aftercare line. They can advise based on which dose you're on, how far into your treatment you are, and what the pen inspection shows.

If you're weighing up the broader cost picture of replacing a failed pen, our Mounjaro cost and pricing page explains what's typically included in a private prescription and what recourse options look like.

Preventing a jam on your next pen

Most device jams are preventable. The habits that matter: always take the pen out of the fridge at least 20–30 minutes before you plan to inject; store pens upright in the fridge door rather than on a cold metal shelf; check the expiry date before each use; and attach a fresh needle immediately before the injection rather than leaving one on from a previous attempt. Needles left attached allow air in and medicine out, both of which affect the mechanism.

If you're starting a new pen for the first time or moving to a higher dose, our guide to opening a new Mounjaro pen walks through the priming steps that apply to first use. And if you ever want to understand the full injection process from scratch, the Mounjaro pen overview covers how the KwikPen is designed to work, which makes troubleshooting a lot more intuitive.

If you're not yet on Mounjaro but want to understand whether it's clinically appropriate for you, our prescribers carry out a same-day clinical review for every new consultation. Check your eligibility by visiting our weight-loss treatments page.