Your Mounjaro pen has malfunctioned — what the reset button does and what to do next

You pressed the injection button and nothing happened (or it stopped partway through

You've found a quiet moment, rotated your site, attached a fresh needle, and pressed the button) and the pen hasn't moved, or it clicked and stopped before the dose window showed grey. That's a frustrating place to be, especially if it's your first encounter with the Mounjaro KwikPen.

The KwikPen is a pre-filled, multi-dose pen: each pen holds four weekly doses, with the dose window showing a number that counts down with each successful injection. When a delivery is interrupted (say the needle clogged, or the pen was angled awkwardly against the skin) the mechanism can lock before completing the dose. The reset button, located on the base of the pen, is designed to reengage the internal spring mechanism so it is in the correct position for another attempt. It is a device-reset, not a dose-reset: it says nothing about how much tirzepatide actually entered your tissue.

Before pressing anything, check the dose window. If the number has changed (for example, dropped from 4 to 3), the dose was most likely delivered even if the injection felt incomplete. If the number is unchanged and you are confident no medication was administered, the full malfunction guidance covers the step-by-step check in detail. The NHS tirzepatide medicines page also directs patients with pen problems to their pharmacist or prescriber in the first instance, which is the safest default.

What the reset button does, and the one situation where it genuinely helps

The reset button on the KwikPen base is a small circular button that, when pressed firmly, resets the injection mechanism to its ready position. Eli Lilly includes it specifically to address an incomplete dose delivery where the button was released too early, for instance, if you lifted the pen from your skin before the full ten-second hold was completed and the plunger retracted mid-way.

In that specific situation: remove the needle, apply a new needle from your pack (attaching a used needle risks clogged delivery on the retry), press the reset button on the base until you feel a click, and then reattempt the injection at the same or a different site. The guide to Mounjaro KwikPen needles explains needle attachment and why a fresh needle matters for each attempt.

The reset button does not help if: the pen was dropped and the internal mechanism is damaged; the dose window shows grey (dose already delivered); the pen has reached its final dose and locked permanently; or there is visible damage to the cartridge window. In those cases, pressing the reset button will either do nothing or (worse) risk a double-dose attempt. When in doubt, set the pen aside and call your prescriber or pharmacy before the day is out. A single missed or partial dose is far less consequential than inadvertent double-dosing.

Needles that are blocked or bent are a common culprit for incomplete injections. Keeping a supply of compatible replacement needles, such as those available from our needle packs, means you can always attach a fresh one before reattempting.

Faulty pens: when to ask for a replacement rather than a reset

Some malfunctions go beyond what a reset resolves. If the injection button cannot be pressed at all on a pen that should still have doses remaining, if the dose window is cracked or unclear, or if the pen was stored outside the recommended temperature range and you suspect the medicine may be compromised, the right path is a replacement, not repeated attempts to override the mechanism.

For patients sourcing Mounjaro through a regulated private pharmacy, a documented malfunction is typically grounds for a replacement pen. The process for getting a replacement Mounjaro pen after a malfunction involves reporting the fault clearly (noting the batch number printed on the pen and packaging) so the pharmacy and Eli Lilly can investigate the device defect.

It's worth doing this the morning you notice the problem rather than leaving it until you're due the next dose. If you ordered through nume and ordered before 12pm on a weekday, a replacement, once approved by our prescriber, can be dispatched the same day. Our support team is available seven days a week specifically for situations like this.

There's also an injection-technique question worth raising at the same time. Malfunctions are sometimes triggered by technique rather than a faulty pen, holding the pen at the wrong angle, not pressing far enough into the skin, or injecting into a site with scar tissue. The guide to choosing the right injection site is a practical read if you've had repeated trouble with the abdomen.

What to record and who to tell

Device malfunctions with licensed medicines are taken seriously by regulators. If your Mounjaro pen failed to deliver a dose correctly, note the batch number (on the pen label and outer box), the date, and what you observed, including the state of the dose window before and after. Your prescriber can file a device defect report, and patients can also submit one directly via the MHRA Yellow Card scheme, which accepts reports of faulty medicine devices as well as suspected side effects.

If you're considering whether Mounjaro is right for you, or you want to understand the wider treatment picture before your next dose, the Mounjaro overview covers the clinical background, and our clinical team reviews every consultation personally. For context on what the private prescription route involves financially, the Mounjaro cost page sets out what a legitimate, fully-supported prescription typically includes. When you're ready to speak with a prescriber, you can check your eligibility through a free consultation.