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How has the Mounjaro Kwikpen changed?

If you’ve used it before, you may have noticed an important change to the Mounjaro pen. As of April 2026, the design was updated so that a lot less liquid remains inside after you’ve used all four intended doses. This is because, previously, some people would use the leftover medication as a ‘fifth dose’.

This update was introduced to help make the treatment safer and more consistent, as the leftover liquid didn’t provide an accurate dose. Below, we’ll walk you through what’s changed, why the pen was redesigned, and how this translates in practice for you.

Medically reviewed by

Sanjeda Chowdhury, Superintendent Pharmacist + Independent Prescriber

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Reviewed on Jun 18, 2026. by Ms Sanjeda Chowdhury Superintendent Pharmacist + Independent Prescriber Registered with GPhC (No. 2202465) Next review due on Jun 18, 2029.

Last updated on Jun 22, 2026.

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What parts of the Mounjaro Kwikpen have changed?

The changes made to the Mounjaro KwikPen aren’t necessarily obvious when you first look at it. In fact, many people won’t notice any difference until they begin using the pen or use their fourth dose.

And while the active ingredient, strength, mode of administration, and efficacy remain the same, the design changes to the pen are intended to make dosing clearer and help prevent leftover liquid (also called overfill) from being used as an extra dose. Here are the main changes you may notice:

What parts of the Mounjaro Kwikpen have changed?

No. Although some people referred to the leftover liquid as the ‘golden dose’ or a ‘fifth dose’, it was never intended to be used as an extra week’s treatment. The overfill in older Mounjaro pens was only there to allow for priming the pen before each dose and to ensure that there was plenty of liquid in the pen for the four intended doses.

While it might’ve looked like there was enough liquid left for another injection, the amount of medication remaining was never designed to be used as a full dose, and thus never accurately measured. Because of this, trying to use this leftover liquid could result in injecting an unpredictable amount of medication, which could make the treatment less effective.

Why was the ‘golden dose’ dangerous?

While it’s understandable why some people were tempted to use the leftover medication, especially given the considerable cost of the treatment, using the overfill as an extra dose could carry some risks:

Inaccurate dose

With the old Mounjaro pen design (which is still in use as the updated pens are rolling out across pharmacies in the UK), there was no guarantee how much medication remained in the pen after the fourth dose. This means you could’ve ended up injecting too much or too little medication, making it impossible to follow your prescribed treatment plan accurately.

The lifespan of the pen

Mounjaro pens should only be used for up to 30 days after the first injection. So, if you use Mounjaro once a week, using a ‘fifth dose’ could’ve meant using the pen close to or beyond the recommended 30-day limit, which could affect the safety and quality of the medication.

Risk of damaging the pen

Some people have tried to access the leftover liquid by forcing the pen mechanism or extracting the medication manually. But this can damage the pen and may lead to injury or incorrect dosing.

What should you do if you were using the ‘fifth dose’?

Since the cost of weight loss medication can be a significant consideration, some people were relying on the extra liquid to help make treatment more affordable.

If you’re concerned about the cost of the medication, it’s worth speaking with one of our clinicians. Depending on your suitability, there may be other options available. For example, you could choose a more affordable weight loss treatment, such as Wegovy, or step down to a lower Mounjaro maintenance dose, which may help reduce the overall cost of treatment but still help you reach your weight loss goals.

It’s also important to remember that the medication itself has not changed. The active ingredient dose, concentration, and formulation remain exactly the same. The update only affects the pen design and the amount of leftover liquid that remains after the four intended doses have been used up.

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