Using the last bit in a Mounjaro pen: what the instructions don't spell out

The myth: the last bit is leftover and shouldn't be injected

This is the most common concern our prescribers hear, and it gets things backwards. The Mounjaro KwikPen is manufactured to hold enough tirzepatide for exactly four injections. The tiny amount of liquid you might notice (visible at the tip of the needle or left inside the cartridge after pressing the button fully) is deliberate manufacturing overfill. It exists so that every injection, right through to the fourth, delivers the correct therapeutic amount. It is not a partial dose. It is not a residual you are expected to use by pressing harder or angling the pen differently.

Where the confusion usually comes from: after the fourth injection the plunger has reached the end of its travel, and some people see a drop of fluid on the needle tip and conclude they missed something. You didn't. That droplet is the standard tail-end of a subcutaneous injection, the same thing happens after the first, second and third doses too, you just notice it more at the end. The full injection technique guide on this site covers the correct steps for each dose, including how long to hold the pen in place before withdrawing it.

The one thing worth checking before you put the pen down: confirm the dose counter has returned to zero (or shows the locked symbol, depending on pen version). That takes under a minute and tells you the plunger completed its full travel. If the counter did not reach zero, contact your prescriber before ordering the next pen.

What happens to the pen after that last dose

Once the fourth injection is complete, the pen's locking mechanism engages. You will not be able to depress the button again, and the dose window will show zero or a lock indicator. This is intentional and irreversible, the pen is single-patient-use and cannot be refilled or reset. Some people try pressing harder, assuming the pen has jammed, but forcing a locked pen risks needle-stick injury and will not release any further medication because there is none left to release.

The used needle should be detached and both the needle and pen disposed of in a suitable sharps container straight away. UK regulations require sharps to go into a rigid, puncture-resistant container rather than a household bin; your GP surgery or local council usually offers a sharps collection point. If you are curious about how long Mounjaro pens last across their full lifespan, including storage and in-use timelines, that guide can help you plan when to order your next supply so there is no gap in treatment.

Storage matters right up until that last dose too. Tirzepatide degrades outside the cold chain, so a pen that has been left unrefrigerated for longer than the window stated in the Patient Information Leaflet may have reduced potency even if it looks normal. The fridge-storage guidance page covers this in detail; always defer to the leaflet for the precise temperature limits.

Dose four and what to do next

The Mounjaro KwikPen delivers one pen per four-week period at your current dose. After that fourth weekly injection you will need a new pen for week five. This is not automatic, at a regulated online pharmacy like nume, every repeat supply involves a fresh clinical review by a GPhC-registered prescriber before dispatch. There are no rolling subscriptions, and no dose increase goes out without evidence that the previous dose has been tolerated. That policy exists because dose increases need proper assessment, not just a reminder in a calendar.

If you are mid-titration (say, moving from 5mg to 7.5mg) your next pen will be a different strength, and your prescriber will confirm that before it ships. If you want to understand how long a Mounjaro pen lasts, including what affects that window when your injection schedule shifts due to illness or travel, that guide is a useful reference. If you have ever wondered how long one Mounjaro pen is expected to last you, that page breaks it down clearly so you can time your reorders with confidence. Questions about your specific next steps belong with your prescriber, not a general webpage; the FAQs page covers the most common repeat-order queries.

Tirzepatide's place in weight management is well established. NICE's appraisal of tirzepatide (Technology Appraisal TA1026, published December 2024) recommends it for adults meeting specific BMI and comorbidity criteria, and the NHS tirzepatide medicines page is a reliable reference for how the medicine works and what to expect from each stage of treatment. Both are worth bookmarking alongside your Patient Information Leaflet.

Side effects to watch for around any injection, including the last one

There is nothing physiologically distinct about the fourth injection compared with the first three, the dose is identical and the side-effect profile is the same. The most commonly reported effects are gastrointestinal: nausea, loose stools or constipation, and a feeling of fullness that lingers longer than usual. These tend to be most noticeable in the days immediately after starting treatment or moving up a dose level, and they generally ease within a week or two. Keeping well hydrated and eating smaller meals can help.

A few symptoms need prompt medical attention regardless of which injection number you are on. Severe abdominal pain that travels through to your back, with or without vomiting, should be assessed urgently, this can be a sign of pancreatitis, which the MHRA highlighted as a known though infrequent risk in its Drug Safety Update of January 2026. Signs of a serious allergic reaction (facial swelling, difficulty breathing, rapid heartbeat) also require emergency help. For anything that concerns you between injections, our aftercare team is available seven days a week.

If you are thinking about starting Mounjaro and want to understand the full picture before committing, the Mounjaro overview page covers the treatment from eligibility through to what ongoing care looks like. And if cost is part of your decision, the Mounjaro cost page sets out what private treatment typically involves in the UK without any surprises.

When you are ready to take the next step, check your eligibility with a free consultation, a real prescriber reads your answers the same day.