Mounjaro starter guide: your first 12 weeks step by step

In short

Weeks 1 to 4: 2.5mg once a week. This is a starting dose to let your gut adjust
Weeks 5 to 8: 5mg, the first maintenance dose. From week 9 you either stay on 5mg or move to 7.5mg
Each KwikPen holds 4 doses, so you use one pen per dose step
Missed dose: take it within 4 days (96 hours), otherwise skip it. Keep at least 3 days between doses
If you take the pill for contraception, add a barrier method for 4 weeks after starting and 4 weeks after each increase

Your first 12 weeks on Mounjaro follow a fixed pattern set by the SmPC. You take 2.5mg once a week for 4 weeks, then 5mg for at least 4 weeks, then either stay on 5mg or move to 7.5mg. Nausea, constipation and reduced appetite are most common in these weeks, and they often flare for a few days after each dose increase. Weight loss is often small in the first month and steadier from the second. The plan below covers what to do before your first dose, week by week.

Before your first injection

Get these sorted before day one:

Store the pen in the fridge at 2C to 8C. Do not freeze it. Keep it refrigerated until you start it.
Get a sharps bin. Needles must not go in household rubbish.
Choose an injection day you can keep each week. Pick one with a quiet day after, in case of nausea.
If you take an oral contraceptive pill, plan a barrier method such as condoms for 4 weeks from your first dose.
Tell your GP, especially if you take diabetes medicines, warfarin or other tablets which need careful dosing.
Stock up on easy foods: yoghurt, eggs, soup, crackers, fruit and lean protein.

Read the leaflet in the box. The KwikPen guide shows each step of the injection.

The 12 weeks at a glance

Weeks	Dose	What usually happens
1 to 4	2.5mg	Appetite falls for many people, mild nausea, small or no weight change
5 to 8	5mg	Side effects flare for a few days after the step, then settle. Weight loss becomes steadier
9 to 12	5mg or 7.5mg	Stay on 5mg if it works, or step up. Habits settle

Each KwikPen holds 4 weekly doses at one strength. In practice you use one pen per step. Expect a short check-in with your prescriber before each new pen.

Weeks 1 to 4: the 2.5mg starting dose

The SmPC sets 2.5mg as the starting dose, taken once a week for 4 weeks. It does not list 2.5mg as a maintenance dose. Its job is to let your stomach and gut get used to the medicine.

What you may notice:

Feeling full sooner, and less interest in snacks.
Mild nausea, for some people worse in the day or two after the injection.
Constipation, since food moves through more slowly.
Tiredness, which the SmPC lists as very common.

Some people lose a few kilograms in the first month, partly water. Others lose little. Neither result predicts the months ahead. Avoid judging the medicine on week 3.

Weeks 5 to 8: the move to 5mg

From week 5 your dose rises to 5mg, the first of the three maintenance doses listed in the SmPC (5mg, 10mg and 15mg). Expect side effects to return for a few days after the first 5mg injection, then ease. For most people, the appetite effect at 5mg is clearer than at 2.5mg.

If you take an oral contraceptive, add a barrier method again for 4 weeks after this increase. The NHS gives the same advice after each dose increase. The page on Mounjaro and the contraceptive pill explains why.

Weeks 9 to 12: stay or step up

After at least 4 weeks on 5mg, the SmPC allows a step to 7.5mg if needed. "If needed" matters. If your appetite is under control and your weight is falling, you can stay on 5mg. Many people do well on 5mg for months. If hunger has crept back or progress has stalled for several weeks, your prescriber may suggest 7.5mg. The dose steps guide explains how each step is decided.

By week 12 most people have a routine: a set injection day, smaller meals, more water, and a clearer sense of which foods sit badly.

Injection day habits

You can inject at any time of day, with or without food. Use the fatty tissue of your tummy, thigh or the back of your upper arm. Rotate sites each week and do not inject into the same spot twice running.

Habits which help:

Eat a light meal on injection day and the day after. Large or fatty meals make nausea worse.
Drink fluids steadily. The NHS suggests 6 to 8 cups or glasses a day for most adults, more if you are vomiting or have diarrhoea.
Set a weekly phone reminder.
Note the date and dose each week, so you know when the pen is due to run out.

The page on what to eat on injection day has meal ideas.

What to eat in the first 12 weeks

Mounjaro shrinks your appetite, so each meal carries more weight. Aim for:

Protein at each meal, such as eggs, fish, chicken, beans, lentils, tofu or Greek yoghurt. This helps protect muscle.
Fibre from vegetables, fruit, oats and wholegrains, which helps with constipation.
Smaller plates and slower eating. Stop when you feel satisfied, before you feel full.
Less fried, greasy or sugary food, which many people find harder to tolerate.

Do not skip meals to speed things up. Eating too little leads to tiredness, dizziness, hair shedding and muscle loss.

Missed doses and changing your day

If you miss a dose, take it within 4 days (96 hours) of the usual time. 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.

You can change your injection day, as long as at least 3 days have passed since your last dose.

Side effects: what is normal and what is not

The SmPC lists nausea, diarrhoea, vomiting, constipation, tummy pain, reduced appetite and tiredness as very common, affecting more than 1 in 10 people. Most are mild to moderate and fade over days to weeks. The page on nausea and vomiting covers what helps.

Vomiting and diarrhoea can dehydrate you, and the SmPC warns dehydration can affect your kidneys. Sip fluids often, and seek advice if you cannot keep fluids down.

When to get help

Call 999 or go to A&E if you have:

swelling of the lips, mouth, throat or tongue, trouble breathing, or a sudden rash with feeling faint (signs of a severe allergic reaction)
severe tummy pain which does not go away, especially if it spreads to your back or comes with vomiting (a possible sign of pancreatitis)

Call 111 or speak to your prescriber the same day if you have:

vomiting or diarrhoea for more than a day, or signs of dehydration such as dark urine or passing little urine
pain in the upper right of your tummy, a high temperature or yellow skin or eyes, which can point to gallbladder problems

Mention at your next check:

mild nausea, constipation or tiredness which is settling
injection site redness or itching
hair shedding

The guide on when to contact your prescriber lists more symptoms by urgency. If you feel dizzy or your vision is affected, do not drive until it passes. You can report any suspected side effect to the MHRA Yellow Card scheme.

What to expect by week 12

Every person's curve looks different. By week 12 you will have had 4 weeks on the starting dose and 4 to 8 weeks on a maintenance dose. Weight loss so far gives a rough guide, not a final verdict. The SmPC places the formal review at 6 months after reaching your highest tolerated dose. For the longer view, see what to expect on Mounjaro.

Questions people ask

How long does it take for Mounjaro to start working?

Many people notice less appetite within the first few days. Weight loss in the first 4 weeks on 2.5mg is often modest, and steadier loss usually starts once you reach 5mg. The SmPC asks for a review if you have lost under 5% after 6 months on your highest tolerated dose.

Should I go up to 7.5mg after 8 weeks?

Only if you need to. The SmPC says dose increases can be made if needed, after at least 4 weeks on the current dose. If 5mg controls your appetite and your weight is falling, staying on 5mg for longer is a normal choice.

When are Mounjaro side effects worst?

Side effects are most common in the first weeks of treatment and in the days after each dose increase. Most are mild to moderate and ease as your body adjusts. Persistent vomiting, severe tummy pain or signs of dehydration need prompt advice.

What is the best day to inject Mounjaro?

Any day you can keep to every week. Many people pick a day with a quiet evening or morning after, in case of nausea. You can inject at any time of day, with or without food, and you can change the day if at least 3 days have passed since your last dose.

Do I need to tell my GP I am starting Mounjaro?

It is safer if you do, especially if you take diabetes medicines, warfarin, or tablets which depend on steady absorption. GPhC guidance asks online prescribers for your GP details and consent to share information.

Sources

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: Mounjaro starter guide: your first 12 weeks step by step. Site: Mounjaro Info (https://mounjaroinfo.co.uk/guides/mounjaro-starter-guide-first-12-weeks/). Reviewed 22 October 2026. Topic: Mounjaro (tirzepatide), UK.

The first 12 weeks on Mounjaro follow the dose schedule in the SmPC. You take 2.5mg once a week for 4 weeks, then 5mg for at least 4 weeks, then stay on 5mg or move to 7.5mg. Side effects such as nausea and constipation are most common in this period, especially after each dose increase. Weight loss is often small in the first month and steadier from the second.

Weeks 1 to 4: 2.5mg once a week. This is a starting dose to let your gut adjust
Weeks 5 to 8: 5mg, the first maintenance dose. From week 9 you either stay on 5mg or move to 7.5mg
Each KwikPen holds 4 doses, so you use one pen per dose step
Missed dose: take it within 4 days (96 hours), otherwise skip it. Keep at least 3 days between doses
If you take the pill for contraception, add a barrier method for 4 weeks after starting and 4 weeks after each increase

Cite as: Mounjaro Info, "Mounjaro starter guide: your first 12 weeks step by step", https://mounjaroinfo.co.uk/guides/mounjaro-starter-guide-first-12-weeks/, reviewed 22 October 2026.