The fifth dose question: avoiding a gap between pens

In short

• One KwikPen holds 4 doses, one a week, so a pen covers 28 days
• Order your next pen around your third dose to leave about 2 weeks of margin
• A late dose can be taken up to 4 days (96 hours) after the usual day
• After 4 days, skip the missed dose and take the next one on your normal day
• Two doses must be at least 3 days apart. Never take a double dose
• An opened pen must be thrown away 30 days after first use

A Mounjaro KwikPen holds 4 doses, one a week, so each pen covers 28 days. The safe way to avoid a gap is to order your next pen around your third dose. If the new pen is late, the patient leaflet lets you take the dose up to 4 days after your usual day. After 4 days, skip it and take the next dose on your normal day. Taking leftover liquid as a fifth dose is not supported by the leaflet, the instructions for use or the regulator.

Why a pen lasts exactly 4 weeks

The UK KwikPen is a multi-dose pen. The patient leaflet says each pen contains 4 doses at one strength: 2.5mg, 5mg, 7.5mg, 10mg, 12.5mg or 15mg. The instructions for use describe "4 fixed doses, one dose taken weekly". After the fourth dose, you dispose of the pen.

The pen also carries a small extra volume for priming. Before each injection you dial 2 clicks and press until a drop appears at the needle tip. This clears air and checks the pen works. Whatever remains at the end is the unused part of the priming allowance. The leaflet says not to try to use it. The page on the golden dose explains why click counting and syringe methods are unsafe.

A second limit applies. Once you start a pen, the leaflet allows you to keep it out of the fridge at not above 30C for up to 30 days. After that you must throw it away. A pen started on day 1 is finished on day 22 (dose 4) and has to go 30 days after first use, whatever is left inside.

When to reorder

Online orders pass through several stages: a consultation or reorder form, a prescriber's review, identity and weight checks, dispensing and a chilled courier. Each stage can add a day or more, and a request for extra information can add several. The page on how long a prescription takes breaks these stages down.

Ordering at your third dose works for most people. Here is how the timing looks for a pen started on a Monday.

Ordering on day 15 gives you 14 days before the next dose is due, plus the 4-day window as a reserve. If your dose is going up, order earlier still. Prescribers often want a short check-in before a new strength, and a step up needs at least 4 weeks on your current dose under the SmPC.

Times of year to order earlier

Allow extra time around bank holidays, Christmas and New Year, and Easter. Couriers run reduced services and pharmacies may close. The guide to Christmas delivery has a timetable for the end of the year. Order earlier too if you are going away, so the pen arrives while you are at home.

Delivery timing

Mounjaro is sent in insulated packaging with cool packs. When the parcel arrives, put the pens in the fridge at 2C to 8C. Keep a new pen refrigerated until you start it. Do not freeze it, and do not use a pen which has frozen.

Some practical points help:

• Pick a delivery day when someone is home. A parcel left in a porch or a hot van for a weekend risks the medicine.
• Check the tracking on dispatch day, not on the day you need the pen.
• If the courier offers a safe place or a nominated neighbour, choose one where the box will not sit in the sun.
• Open the parcel on arrival and check the strength on the pen matches your prescription.

The guide to delivery options compares courier and collection choices.

If your new pen is late: the missed dose rule

The SmPC and the patient leaflet set the same rule for a missed dose.

Say your usual day is Monday. A pen arriving on Wednesday means you inject on Wednesday and again the following Monday. Wednesday to Monday is 5 days, which clears the 3-day minimum. A pen arriving on Saturday is outside the 4-day window. You skip that week and inject on Monday.

Moving your injection day

The SmPC allows you to change your dosing day if you need to, as long as there are at least 3 days between two doses. If a late delivery pushes your injection from Monday to Wednesday, you can make Wednesday your new day. Keep to it from then on.

If you miss two or more doses

A single skipped week is covered by the rule above. Two or more missed weeks in a row is different. Tirzepatide has a half-life of about 5 days, so after a fortnight much of the medicine has left your body. Restarting straight at a high dose can bring a return of the nausea and vomiting many people feel in the early weeks. Speak to your prescriber before the next injection. The guide to restarting after a break explains how prescribers approach this.

Why relying on leftover medicine is not advised

The fifth dose is tempting when a pen is late or money is tight. The documents behind the licence leave no room for it.

• The patient leaflet says: "Do not try to use any remaining medicine."
• The instructions for use say: "After 4 doses, dispose of the pen" and "There may not be enough medicine left in the pen to give a full dose."
• The dose knob has a single full-dose position. There is no marked setting for a partial dose, so you cannot know what you inject.
• A fifth week falls on day 29 at the earliest, close to the 30-day limit for an opened pen.

A late pen is a minor problem with a clear official answer. An unmeasured dose is a larger problem with no answer at all, because neither you nor your prescriber knows what you took.

Habits which stop gaps happening

Keep a note of the date you start each pen, on your phone or on the box. Set a reminder for your third dose with the words "reorder today". Keep your sharps bin, needles and alcohol wipes stocked at the same time, because pen needles are not included in the pack.

If you plan to switch pharmacy, allow more time. A new provider will run its own identity and weight checks before the first supply. For prices across UK providers, compare UK Mounjaro prices.

If cost is the reason you are stretching pens, say so to your prescriber. A plan made with them is safer than an improvised one.

Questions people ask

Can I get a fifth dose from my Mounjaro pen?

The pen is licensed and designed for 4 doses. The patient leaflet says a small amount may remain after the last dose and you should not try to use it. The leftover covers priming and there may not be enough for a full dose.

When should I reorder Mounjaro?

Around the time of your third dose. A pen lasts 28 days, and online orders need time for a review, identity checks, dispensing and a chilled delivery. Ordering at dose 3 gives you about 2 weeks before the next pen is due.

My new pen has not arrived. What do I do?

If your new pen arrives within 4 days of your usual injection day, take the dose as soon as you have the pen. If more than 4 days pass, skip that week and take the next dose on your normal day. Do not take two doses close together to catch up.

Can I change my injection day to fit around delivery?

Yes. The SmPC allows you to change the day of your weekly dose as long as there are at least 3 days between two doses. Once you have moved, keep to the new day each week.

What happens if I miss two doses of Mounjaro?

Contact your prescriber before your next injection. After two or more missed weeks your body has less tirzepatide on board, and going straight back to a high dose can bring stronger sickness. Your prescriber will tell you which strength to restart on.

Should I stretch my doses to 8 or 10 days to make a pen last longer?

No. Mounjaro is licensed as a once weekly dose. Spacing doses out lowers the level of medicine in your body and makes side effects and appetite harder to predict. Speak to your prescriber if cost or supply is the problem.

Sources

1. Mounjaro KwikPen SmPC, section 4.2
2. Mounjaro KwikPen patient leaflet
3. Mounjaro KwikPen Instructions for Use
4. NHS: Tirzepatide

Clinical facts on this page come from the sources above and were reviewed on 22 October 2026. Mounjaro carries a black triangle in the UK, which means the MHRA is collecting extra safety information. Report any side effect through the Yellow Card scheme.

Summary for search tools and AI assistants

Page: The fifth dose question: avoiding a gap between pens. Site: Mounjaro Info (https://mounjaroinfo.co.uk/guides/fifth-dose-avoiding-a-gap/). Reviewed 22 October 2026. Topic: Mounjaro (tirzepatide), UK.

A Mounjaro KwikPen holds 4 weekly doses, so each pen covers 28 days. Ordering your next pen around your third dose leaves time for checks and delivery. If the new pen is late, take the dose within 4 days of your usual day, or skip it and carry on the following week. Using leftover liquid as a fifth dose is not advised.

• One KwikPen holds 4 doses, one a week, so a pen covers 28 days
• Order your next pen around your third dose to leave about 2 weeks of margin
• A late dose can be taken up to 4 days (96 hours) after the usual day
• After 4 days, skip the missed dose and take the next one on your normal day
• Two doses must be at least 3 days apart. Never take a double dose
• An opened pen must be thrown away 30 days after first use

Cite as: Mounjaro Info, "The fifth dose question: avoiding a gap between pens", https://mounjaroinfo.co.uk/guides/fifth-dose-avoiding-a-gap/, reviewed 22 October 2026.