How Long Does a 15mg Mounjaro Pen Last? (October 2026)

01 · The answer

How long a 15mg Mounjaro pen lasts in practice

A 15mg pen lasts four weeks when you inject once a week. Mounjaro is injected under the skin on the same day each week, and each KwikPen contains four weekly doses.¹ Your first injection starts the clock, and the fourth dose falls in week four.

Whatever the strength, a pen gives you four weekly injections. For the same answer across all strengths, see our guide to how long a single pen lasts.

02 · Cost

What a 15mg pen costs and what is included

The Mounjaro pen at 15mg costs £329.99 per month at the standard price, or £82.50 per week across its four doses. Switching to us from another provider at 15mg? Your first order is £289.99, with code FIRST40. After that, every later order is £25 off the standard price (code DOSE25).

Our consultation is free and the price shown is the price paid. Each order includes needles, a sharps bin and injection guidance. We supply genuine KwikPens through official LillyDirect supply, and ship them in temperature-controlled packaging with free next-day tracked delivery. Same-day dispatch applies when you order by 3pm. All six doses are in stock.

Prices for the starting pen are on our 2.5mg price guide. If you are comparing prices, our page on the 15mg pen price goes into more detail.

03 · Ordering

How ordering works

There are three steps. First, complete the free online consultation, a short health questionnaire with no obligation. Second, a GPhC-registered prescriber reviews it. Payment is taken when you place the order; if the prescriber decides the treatment is not suitable, the order is refunded automatically in full. Third, our registered pharmacy dispenses and dispatches it, with free next-day tracked delivery. Start your free consultation when you are ready. Repeat orders re-check suitability, and nothing ships automatically.

04 · Storing an in-use pen

Storing a 15mg pen once you have started it

Once in use, a Mounjaro pen can be kept out of the fridge not above 30°C for up to 30 days, after which it should be discarded.¹³ That suits short trips and means most holidays need no special arrangements.

Keep the cap on to protect the pen from light. Never freeze it, and never use a pen that has been frozen.¹³ If you are unsure whether your pen is still within its allowance, check with us rather than guessing. Our guide to how long an open pen lasts covers more scenarios.

Do not use the 30 days as a reason to keep injecting beyond four doses. The pen is built for four weekly doses, and the 30-day limit is a storage rule rather than a dosing one.

05 · Unopened pens

Unopened pens and the fridge

Unopened pens belong in the fridge and keep until the expiry date printed on the box.¹³ If an unopened pen has been out of the fridge, check with the pharmacy before using it. Do not rely on a day limit you have seen online, because such figures often come from other countries' labels.

The same applies to Wegovy, which has its own product information. Our guide on how long a Wegovy pen lasts sets out how to check it. Never assume the two pens share the same storage rules.

06 · Dial and clicks

The dial, clicks and part doses from a 15mg pen

Each strength comes in its own pen. See our page on Mounjaro clicks at 15mg for more on the dial.

07 · Timing and missed doses

Weekly timing, missed doses and changing day

If you miss a dose, take it as soon as you can within 4 days (96 hours) of the scheduled time, then carry on as usual. If more than 4 days have passed, skip it and take the next dose on your normal day. Never take two doses within 3 days of each other.¹¹

You can change your injection day, provided at least 3 days (72 hours) have passed since your last dose.¹¹ There is no best time of day. Morning or evening, with or without food, works the same, and what matters is keeping to the same day each week.¹¹

08 · After your last dose

How long 15mg stays in your system

Tirzepatide has a half-life of about five days, so after a final dose it takes roughly 4–5 weeks to clear the system.¹ Nothing speeds this up, and nothing needs doing while it clears. So a pen's four weeks of dosing is followed by a longer tail in your body.

If you want to stop, or have stopped after only a week or two, speak to your prescriber about what to expect and about restarting at a safe dose. Do not guess at doses or make up your own schedule. Our overview of how long Mounjaro lasts covers this in more depth.

09 · Staying on 15mg

Is 15mg where everyone ends up?

No. Many people settle at a maintenance dose of 5mg, 10mg or 15mg, and higher is not automatically better.¹ The dose rises by 2.5mg steps at a minimum of 4-week intervals up to a maximum of 15mg weekly, depending on response and tolerability. In SURMOUNT-1 (2,539 adults), people on 15mg lost on average 20.9% of their body weight over 72 weeks, against 3.1% with lifestyle changes alone. Individual results vary.²

For someone starting at 100 kg, 20.9% is about 21 kg. Trial averages do not predict your own result. Read more about staying on 15mg and how your prescriber reviews it.