This article is not about whether you should take it or not. That decision has already been made: your doctor made it and it is written on a prescription. This is about what happens after the prescription, which is where half of people get lost — and where a pharmacy with five people waiting cannot always stop for ten minutes.
First of all, to make it clear what this page is about.
Tirzepatide is a prescription-only medicine. This is
health information intended for those who already have the treatment prescribed — it is not
advertising, does not promote any brand and is not intended to help decide whether to start, stop or change a
dose: that is for your doctor. Advertising prescription medicines to the general public is prohibited
(RD 1416/1994 in Spain; please check your local regulatory agency for equivalent regulations), and that is why there is no promotional material from any laboratory here nor any way to obtain it. This site
has no relationship with any manufacturer and charges
nothing for what is explained here.
The primary source that overrides this page is your package leaflet, which
you can consult via CIMA (AEMPS, the Spanish medicines regulator — please check your own national regulator for regulatory information).
First: what it is and how to inject it
Mounjaro is the brand name for tirzepatide. It is injected under the skin once a week, on the day of your choice, at any time of day, with or without food. Usual injection sites are the abdomen, thigh or upper arm, and you should rotate injection sites.
It is not "insulin", although it is injected similarly. It acts on two hormones your own gut produces when you eat (GIP and GLP-1) and does three things at once: prompts the pancreas to release insulin when sugar is present, slows down stomach emptying, and brings on the feeling of fullness sooner. That slower emptying quickly explains a good part of the rest of this article: constipation, digestive discomfort and even the effect on the contraceptive pill.
Not all "weight loss injectables" are the same
They are all lumped together in conversations, but they are not the same inside:
| Tirzepatide (Mounjaro) | Semaglutide (Wegovy, Ozempic) | |
|---|---|---|
| Acts on | Two receptors: GIP and GLP-1 | One: GLP-1 |
| Frequency | Once a week | Once a week |
| Titration | 2.5 → 15 mg, in 2.5 mg steps | 0.25 → 2.4 mg (Wegovy) |
That second hormone, GIP, is what distinguishes tirzepatide from everything before it. This is not just a laboratory detail: it is considered largely responsible for why, for an equivalent effect, digestive tolerability tends to be better than with a drug that acts only on GLP-1. If someone has told you about their experience with a different injection, do not assume yours will be the same.
The dosing schedule: why you start so low
Nobody starts on the dose they will end up taking. You start on 2.5 mg, which is a starting dose — designed to let the body get used to it, not to have a therapeutic effect — and you move up in steps with a minimum of four weeks on each:
| Weeks | Dose | Purpose of that step |
|---|---|---|
| 1 – 4 | 2.5 mg | Allowing the digestive system to get used to it. Not a maintenance dose. |
| 5 – 8 | 5 mg | First dose with a maintenance effect. |
| 9 onwards | 7.5 · 10 · 12.5 · 15 mg | Increased in 2.5 mg steps, and only if needed, with at least 4 weeks at each step. |
This schedule is not bureaucracy. If digestive discomfort occurs, it depends much more on the speed at which you increase the dose than on the final dose you reach — which is why the steps exist and why they have a minimum duration. If your doctor decides to extend one step to six or eight weeks, it is not a delay: it is exactly what it is intended for.
15 mg is not "the goal". The right dose is the lowest one with which you achieve your goal and tolerate well. Some people stay on 5 mg and others reach 15 mg; neither is doing it wrong.
How much it really costs
Let's start with the cost of each pack. These are the retail selling prices (RRP) of the six presentations of Mounjaro KwikPen — a 2.4 ml pre-filled pen — checked against a professional medicines database on 2 September 2026:
| Presentation | RRP per pack | Weeks covered |
|---|---|---|
| 2.5 mg (starting dose) | 207.91 € | 4 |
| 5 mg | 271.35 € | 4 |
| 7.5 mg | 358.71 € | 4 |
| 10 mg | 358.71 € | 4 |
| 12.5 mg | 446.07 € | 4 |
| 15 mg | 446.07 € | 4 |
The fact that 7.5 mg and 10 mg share a price, and 12.5 mg and 15 mg do too, is not a typo: that is how it is in the table. Prices change without warning, so the one that counts is the price given at your pharmacy on the day you collect it.
With those figures in front of you, there is a calculation that almost everyone gets wrong, and always in the same direction: it falls short.
A KwikPen contains 2.4 ml, which is four weekly doses of 0.6 ml. Four doses equal four weeks. And the year has 52 weeks, not 48. In other words: 13 packs are needed per year, not 12. Calculating expenditure as "the cost of the box × 12" leaves an entire pack out — around 8% less than what you are actually going to pay.
There is a second thing that also does not appear in any published price: what is left over in the pen when you change dose gets thrown away. A 5 mg KwikPen cannot deliver 7.5 mg. If a dose titration step lasts six weeks instead of four, two packs of that strength are needed and two doses are left unused, which are paid for all the same.
We have created a cost calculator that puts together the complete schedule — including dose escalation — and tells you how many packs and how much money, month by month. The prices are intentionally editable fields: the correct one is what you are told at your pharmacy, not the one circulating on the internet.
What about the NHS?
It is not funded. Mounjaro is paid for in full, with a private prescription — the price in the table above is what comes out of your pocket, with no reimbursement tier discounting anything afterwards. The same applies to Wegovy.
It is worth being clear about this before starting, because it is an expense that does not decrease: for as long as treatment lasts, that figure repeats every four weeks.
How to store it (and the most common mistake)
Unopened pens go in the fridge, between 2 and 8 ºC, and remain stable there until the pack expiry date. Not in the fridge door, though: that is the area with the greatest temperature fluctuation. And never in the freezer — if a pen has ever been frozen, it must be discarded, even if it looks perfect on the outside.
It can be kept outside the fridge, but the number of days is not the same for all presentations, and that is where the most common mistake happens:
| Presentation | In the fridge | Out of the fridge (below 30 ºC) |
|---|---|---|
| KwikPen (multidose pen, 4 doses) | 2 – 8 ºC until expiry | Up to 30 days after first use; then discarded |
| Single-dose vial | 2 – 8 ºC until expiry | Up to 21 days; then discarded |
Check which one you have before relying on a number. Someone who has read "30 days" on a forum and has a vial is exceeding it by nine days. If in doubt, check the patient information leaflet inside your box.
Travelling with the pen
You can travel, including by plane. Three rules and none of them are optional:
If you change time zones, keep it simple: what matters is that at least 3 days pass between one injection and the next. Take your dose on your usual day of the week wherever you happen to be.
I missed a dose. What should I do?
This is the question most frequently asked at the pharmacy counter, and it has a short answer:
Less than 4 days (96 hours) since the scheduled day: take the dose as soon as you remember and continue with your usual schedule.
More than 4 days: skip that dose and take the next one on the usual scheduled day.
Never take two doses together to make up for a missed one. Taking a double dose does not make up for anything and significantly increases the likelihood of feeling unwell.
What if you want to change the day of the week because Thursdays are completely unsuitable? You can, provided that at least 3 days (72 hours) pass between one injection and the next. From then on, that is your new fixed day.
Effects: what is normal and what is not
You might not notice anything. Many people tolerate it well from start to finish, and tirzepatide usually settles better than injectables that only act on GLP-1. But if you do notice anything, it is most likely to be digestive, to appear at the start or when stepping up a dose, and to go away on its own in a few days. The trick is knowing how to tell that apart from the rest.
| What may appear | What helps |
|---|---|
| Nausea | Eating smaller, more frequent meals, stopping before you feel full, and avoiding fried and very fatty foods. |
| Constipation | Drinking water throughout the day, fibre, keeping active. If it sets in, speak to us or your GP — there are options. |
| Diarrhoea or vomiting | Hydration. If they last more than a couple of days, seek advice: this is how dehydration sets in. |
| Injection site discomfort | Changing the site with each injection. |
| Reduced appetite | This is part of the effect. Be careful not to end up eating too little and eating poorly: you must make sure you get enough protein. |
This is not "getting used to it": this is calling for help.
- Severe, non-resolving abdominal pain, especially if it radiates towards the back, with or without vomiting. This is a warning sign of pancreatitis and must be addressed immediately; do not wait.
- Vomiting or diarrhoea that will not stop: dehydration is what ultimately damages the kidneys, and it happens sooner than you might think.
- Hypos / low blood sugar (cold sweat, shaking, dizziness, confusion), especially if you are also taking insulin or a sulphonylurea such as glimepiride or gliclazide. With that combination, your other medication may need to be adjusted, and that is a decision for your doctor.
- Visual disturbances if you have diabetic retinopathy, or severe pain in the upper right side under the ribs (gallbladder).
The interaction hardly anyone talks about: the pill
Because tirzepatide delays gastric emptying, it can reduce the absorption of oral contraceptives. This is not a minor detail and it is not always pointed out.
Agencies in several countries — including the UK and Australia — recommend that anyone taking the pill switch to a non-oral contraceptive, or add a barrier method during the 4 weeks following the start of treatment and for the 4 weeks following each dose increase. The manufacturer maintains that the measured reduction is not clinically relevant, and in Spain the OCU (Spanish Consumers' Association) even asked the AEMPS (the Spanish medicines regulator) to clarify the matter. Please check your own national regulator (such as the MHRA in the UK) for official regulatory guidance.
In short: there is a discrepancy between reputable sources. When that happens, the honest approach is not to pick the one you like best, but to state it — and when dealing with something as irreversible as an unplanned pregnancy, asking your doctor or pharmacist and checking the patient information leaflet in your pack is very cheap insurance.
Where to ask when this isn't enough
Three places, in this exact order, and none of them is an internet forum:
- The patient information leaflet in your pack. This is the document that takes precedence over any website, including this one. You can also find it on CIMA, the search engine of the AEMPS (the Spanish medicines agency), by searching for the name of your medicine (remember to consult your own national regulator's database, such as MHRA or EMA, for local regulatory information).
- Your pharmacy. For injection technique, storage, missed doses and to review the rest of your medication. It is a walk-in consultation at no cost, and it is literally what we are here for.
- Your doctor. For everything relating to doses, changes, persistent side effects and whether your case meets NHS funding criteria.
If the pharmacy gives you supporting materials with your box — such as how to use the device or tracking sheets — keep them: they are the easiest things to lose and the things you will miss the most in the second week.
What this article cannot tell you
It cannot tell you whether tirzepatide is right for you, what dose you need, for how long, or what happens when you stop taking it. Nor can it tell you if your case meets your local healthcare board's funding criteria. All of that depends on your medical history, and the person who has that in front of them is your doctor.
What it can do is save you from the three most common pitfalls seen at the pharmacy counter: a pen frozen in checked luggage, taking two doses together to make up for a missed one, and budgeting for twelve boxes when you needed thirteen.
Frequently asked questions
How many days can the pen be kept out of the fridge?
It depends on the presentation, and mixing them up is a frequent mistake. The multidose KwikPen can be kept for up to 30 days below 30 ºC after first use; the single-dose vial, for 21 days. Unopened pens must be stored in the fridge between 2 and 8 ºC until their expiry date. Never freeze it: if it has been frozen, throw it away even if it looks normal.
I forgot to take my injection, what should I do?
If less than 4 days (96 hours) have passed since your scheduled day, take the dose as soon as you remember and continue with your normal schedule. If more than 4 days have passed, skip that dose and take the next one on your usual day. Never take two doses together to make up for a missed one.
Can I change the day of the week I take my injection?
Yes, as long as at least 3 days, meaning 72 hours, have passed between one injection and the next. Once you have made the change, that becomes your new fixed day.
Can I take the pen on an aeroplane?
Yes, and always in your hand luggage: an aircraft hold can drop below freezing temperatures, and a frozen pen has to be thrown away. Take the pack with its label, the patient information leaflet and a letter or prescription, as needles must travel with the medication. Use a cool bag with a cool pack, never ice in direct contact.
Does tirzepatide affect the contraceptive pill?
It can reduce its absorption because it delays gastric emptying. The recommendation in several countries is to switch to a non-oral contraceptive or add a barrier method for the 4 weeks following the start of treatment and for the 4 weeks following each dose increase. In Spain, it is advisable to consult your doctor or pharmacist and follow the instructions in your product's patient information leaflet (for regulatory advice, always check your own national regulator).
Sigue por aquí
- GLP-1 cost calculator — the complete schedule with dose escalation and real packs.
- GLP-1 pharmacy counter guide — the version for the dispensing pharmacist.
- GLP-1s and obesity: the 2026 landscape.
- Interactions checker — to cross-reference your entire medication list.
Sources
- Summary of product characteristics and patient information leaflet for Mounjaro (tirzepatide) — CIMA, the Spanish Agency of Medicines and Medical Devices (AEMPS) (note that readers should check their own national regulator for regulatory information). This is the source for the dose escalation, missed dose guidance, storage conditions for each presentation, and the 4 doses of 0.6 ml per KwikPen.
- Interaction with oral contraceptives — note from the General Council of Official Pharmacists' Associations (Consejo General) on the reduced effectiveness of oral contraceptives with tirzepatide, and the OCU's request to the Spanish agency AEMPS for clarification.
- Patient co-payment in pharmaceutical provision — Royal Decree-Law 16/2012.
The retail prices shown here were checked against a professional medicines database on 2 September 2026. Prices can change without notice and this page may not reflect those changes, so the price charged by your pharmacy on the day you collect your pack is the one that applies; they are editable fields in the calculator for this very reason. If you spot an error in any of the data in this article, please write to us: it will be corrected and a note of the correction will be published.