In your first week on Mounjaro (tirzepatide) you take a single 2.5mg starter injection — a dose designed to help your body adjust, not to drive weight loss. Most people notice a reduced appetite and feeling full sooner, and some get mild nausea, tiredness or bowel changes in the first few days. These usually settle as your body adapts.
Below is a practical, UK-focused guide to week one: what the starter dose does, the side effects to expect and when they peak, how your appetite is likely to change, and the simple habits that make the first injection easier. Mounjaro is a prescription-only medicine available only after a clinical consultation with a GPhC-registered prescriber.
What dose do you start Mounjaro on in the first week?
Everyone starts Mounjaro on the 2.5mg dose, taken as one injection under the skin once a week, regardless of your weight or BMI. This is a deliberately low starter dose whose job is to let your gut adjust to the medicine, which reduces the chance of severe nausea.
The 2.5mg dose is not the dose that produces most of the weight loss — it is a gentle introduction. After at least four weeks your prescriber may increase you to 5mg, then onwards through 7.5mg, 10mg, 12.5mg and up to the maximum licensed dose of 15mg, with each step reviewed. You can read the full schedule on the Mounjaro page.
How does Mounjaro work, and why start so low?
Mounjaro (tirzepatide) activates two gut-hormone receptors at once — GLP-1 and GIP — which reduce appetite, slow how quickly the stomach empties and help you feel full sooner and for longer. Because this changes how your digestive system behaves, a low starter dose gives your body time to adapt gradually.
Starting at 2.5mg and climbing slowly is how the licensed titration is designed to limit side effects. Jumping to a higher dose too soon is more likely to cause nausea and vomiting, which is exactly why the step-up schedule exists.
What side effects can you expect in the first week?
The most common first-week side effects of Mounjaro are gut-related, because the medicine slows digestion. Side effects affecting more than 1 in 10 people include:
- Nausea
- Diarrhoea
- Constipation
- Vomiting
- Decreased appetite
- Fatigue
In the first few days these are usually mild and tend to ease as your body adjusts. Serious but rare risks include severe allergic reaction, acute pancreatitis, severe low blood sugar (particularly if you also take diabetes medicines), severe gastrointestinal disease and, in people with diabetes, worsening diabetic retinopathy. Seek urgent medical help for severe or persistent abdominal pain, repeated vomiting or signs of an allergic reaction.
You can report any suspected side effect through the MHRA Yellow Card scheme at https://yellowcard.mhra.gov.uk/.
When do first-week side effects feel worst?
For many people, nausea and tiredness are most noticeable in the first 24 to 72 hours after the injection, then ease over the following days. This happens because the medicine is highest in your system shortly after your weekly dose, and your gut is still getting used to emptying more slowly.
If side effects are troublesome, it often helps to inject on a day when you can take things gently afterwards. Side effects that are severe, do not settle, or stop you keeping fluids down should be discussed with your prescriber before your next dose — contact us if you are unsure.
How will your appetite change in week one?
Most people notice their appetite is smaller within the first week — you feel full sooner, stay full longer, and food cravings often quieten. This is the medicine working as intended, not a side effect to resist.
Because you naturally eat less, it is important to still eat enough protein and stay hydrated. Very large, rich or fatty meals tend to feel uncomfortable now that the stomach empties more slowly, so smaller, balanced meals are usually better tolerated in the first week.
How do you inject Mounjaro and when?
Mounjaro is a once-weekly injection you give yourself under the skin of the abdomen, thigh or upper arm, on the same day each week. You can take it at any time of day, with or without food, and it does not need to be timed around meals.
- Choose a weekly "injection day" you will remember.
- Rotate the injection site each week to reduce irritation.
- If you miss a dose, follow the patient information leaflet and your prescriber's advice — do not double up.
Your pharmacy provides the device and instructions, and your prescriber will confirm how to use it before you start.
Practical tips for an easier first week
- Eat smaller portions and stop when you feel full, rather than finishing out of habit.
- Choose plainer, lower-fat foods on injection day if nausea bothers you.
- Sip water through the day to stay hydrated, especially with diarrhoea or vomiting.
- Keep fibre and fluids up if you tend towards constipation.
- Don't expect rapid weight change on 2.5mg — this dose is about adjusting, not results.
- Keep a note of any side effects to discuss at your next prescriber review.
How much weight will you lose in the first week?
The 2.5mg starter dose is not designed to produce meaningful weight loss, so you should not judge Mounjaro by week one. Weight change in the first week is often small and can be affected by fluid shifts and bowel changes.
The published efficacy figure reflects the full programme, not the starter dose: in the SURMOUNT-1 trial, participants on the 15mg dose lost on average 22.5% of body weight over 72 weeks, alongside lifestyle support. Individual results vary; not everyone reaches this. Compare your options across our treatments overview, which also covers the Wegovy injection and the Wegovy Tablet.
Who can start Mounjaro?
Mounjaro may be an option for adults aged 18 or over with a BMI of 30 or more, or a BMI of 27 or more alongside a weight-related condition such as type 2 diabetes, high blood pressure or sleep apnoea. A prescriber decides whether it is clinically appropriate for you.
Mounjaro is not suitable if you are pregnant, planning pregnancy or breastfeeding, have type 1 diabetes, a personal or family history of medullary thyroid cancer or MEN2, a history of pancreatitis, severe gastrointestinal disease, an active eating disorder, severe liver or kidney impairment, or a known allergy to tirzepatide. You can check your eligibility with a free online consultation.
This article is for information only and is not a substitute for personalised medical advice. Weight-loss medicines are prescription-only, are not suitable for everyone, and are prescribed only after a clinical consultation with a GPhC-registered prescriber. Individual results vary. Always speak to a UK-registered pharmacist or doctor before starting or changing any prescription medication.
