New Mounjaro KwikPen - Removal of the "5th Dose"

Since Mounjaro (tirzepatide) arrived in the UK, it has quickly become one of the most talked-about treatments in the weight management space. Now, manufacturer Eli Lilly has rolled out an updated KwikPen design — and if you're currently prescribed Mounjaro, it's worth understanding what's changed and why.

What Was the "5th Dose"?

Each Mounjaro KwikPen is designed to deliver four weekly doses of tirzepatide at a given strength. However, many patients and clinicians noticed that the original pen design contained a small amount of overfill — enough medication beyond the four labelled doses that, in some cases, a fifth injection could be drawn from the pen.

This so-called "5th dose" was never intended for use. The overfill existed as a manufacturing buffer to ensure that all four prescribed doses could be reliably and fully delivered, accounting for minor variances during injection. Despite this, some patients began relying on the extra medication, effectively stretching a four-week pen into five weeks of treatment.

What Has Changed?

Eli Lilly has updated the KwikPen mechanism to eliminate the possibility of extracting an additional dose. The redesigned pen now pushes the plunger to nearly the tip of the pen, preventing any ability to extract an additional dose. The exterior appearance of the pen remains largely the same, so the change is primarily an internal mechanical one.

Why Did Eli Lilly Make This Change?

There are several important reasons behind the update.

Dose accuracy and patient safety are at the top of the list. The overfill was never guaranteed to contain a precise, therapeutically accurate dose. Patients using the residual medication may have been receiving inconsistent amounts of tirzepatide from one week to the next — sometimes less than the intended dose, sometimes a partial dose that offered reduced efficacy.

Regulatory and prescribing integrity also played a role. Mounjaro is licensed for use as four doses per pen. Using the product outside of its labelled instructions falls outside the terms of its marketing authorisation, which creates complications for prescribers, pharmacists, and the broader healthcare system.

What This Means for Patients

If you've been prescribed Mounjaro, the key takeaway is straightforward: one pen still equals four weeks of treatment. The update simply ensures that the pen behaves consistently with its labelled use.

For patients who had been relying on a 5th dose to extend their supply, this change means a return to the standard dosing schedule. If cost or access has been a concern, it's worth speaking with your prescriber about the options available to you, including titration schedules and the various dose strengths on offer.

If you have questions about the updated pen or your current Mounjaro prescription, don't hesitate to reach out to your prescribing clinician or care team. They'll be able to walk you through the changes and make sure your treatment plan stays on track.

Medical note: This article is for general education and does not replace personalised medical advice. If you’re worried about symptoms or side effects while taking tirzepatide, contact your prescriber or GP.