If you're starting treatment and wondering what to expect in your first week on Mounjaro, the short answer is one small injection of the lowest dose, and possibly some digestive side effects or a change in appetite as your body adjusts. This guide covers what is normal, what helps, and when to get advice straight away.
Mounjaro (tirzepatide) is a prescription-only medicine, and a prescriber decides whether it's suitable for you. This article is information only, not medical advice. The leaflet in your pack, and anything your prescriber or pharmacist has told you, come first.
What to expect in your first week on Mounjaro
The first week has one job: to begin at the lowest dose so your body can adjust. The licensed starting dose is 2.5 mg once a week for four weeks. Your prescriber will then usually move you to 5 mg; our dosage page sets out the later steps.
The UK patient leaflet explains that Mounjaro mainly works by regulating appetite, so you feel fuller, less hungry and have fewer food cravings.
Don't judge it by the scales yet
Headline weight-loss figures come from the 72-week SURMOUNT-1 trial, reported in the SmPC (the technical information for prescribers). It included adults with obesity, or overweight plus a weight-related condition, without type 2 diabetes. Alongside a reduced-calorie diet and more activity, average loss was 16.0% of body weight at 5 mg, 21.4% at 10 mg and 22.5% at 15 mg, against 2.4% on placebo. One week on a starter dose tells you very little. For now, how you are tolerating it matters more.
Injection day: getting the first dose done
Choose a day that fits your routine. You can inject at any time of day, with or without meals, ideally on the same day each week. The Instructions for Use include a calendar for noting your injection day and dose dates. Each KwikPen holds four weekly doses; once in use, it can be kept out of the fridge at up to 30°C for up to 30 days, and then it must be thrown away. Our guide to storage and fridge rules has more.
New to injecting? Our first-injection guide walks through the pen step by step. Note where the first dose went so you can use a fresh spot next week; see injection sites and rotation.
Common first-week side effects and what helps
The leaflet says nausea, diarrhoea and vomiting are usually not severe and ease over time in most people. These are some of the effects listed as common or very common in weight management:
| What you might notice | How common | What can help |
|---|---|---|
| Feeling sick (nausea) | Very common | Smaller, more frequent meals; avoid hot, fried or greasy food; eat slowly; sip a cold drink regularly |
| Diarrhoea or being sick | Very common | Drink plenty of fluids; get advice if it doesn't settle |
| Constipation | Very common | Ask your pharmacist what is suitable |
| Tummy pain, indigestion, bloating or burping | Very common (tummy pain); common (the others) | Tell your pharmacist if it bothers you; get urgent help for severe pain that won't go away |
| Feeling tired | Common | Tell your pharmacist or prescriber if it doesn't ease |
| Dizziness | Common | Don't drive or use machinery until it passes; tell your pharmacist or prescriber. It can also be a sign of dehydration |
| Itching or redness where you injected | Common | Use a different spot each week |
The leaflet also lists other common effects, including wind, reflux or heartburn, hair loss, low blood pressure, allergic reactions such as rash or itching, and gallstones. The food tips come from the NHS's general advice on feeling sick. Our side effects page has trial frequencies by dose.
Fluids, contraception and other medicines
Keep drinking
The leaflet warns that when you start treatment, vomiting, nausea or diarrhoea can cause dehydration, which may reduce kidney function, so drink plenty of fluids. The NHS lists signs of dehydration including dark yellow, strong-smelling pee, peeing less often than usual, and feeling dizzy or light-headed.
If you take the contraceptive pill
If you take an oral contraceptive, the MHRA says you should add a barrier method, such as a condom, for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase, or switch to a non-oral method such as the coil or implant. The NHS and the leaflet give similar advice. Mounjaro should not be used during pregnancy. If you're planning a pregnancy, the MHRA says to stop Mounjaro at least 1 month before you start trying; talk to your prescriber first.
If you have diabetes, or surgery coming up
If you use insulin or a sulphonylurea (a diabetes tablet), low blood sugar can occur and your doctor may lower those doses, so test your blood glucose as instructed. If you're due an operation under anaesthetic, tell your doctor and the anaesthetist that you use Mounjaro.
When to get help straight away
- Severe, persistent pain in the stomach area, which may also be felt in your back, with or without nausea and vomiting: stop using Mounjaro and get urgent medical help. It could be a sign of acute pancreatitis, which is serious. Gallstones or an inflamed gallbladder can also cause severe tummy pain.
- Signs of a severe allergic reaction, such as breathing problems or rapid swelling of the lips, tongue or throat: get immediate medical help.
- Vomiting or diarrhoea that won't stop, or signs of dehydration: contact your prescriber or call NHS 111.
For anything that bothers you or doesn't go away, speak to your pharmacist or prescriber. You can report any suspected side effect through the MHRA Yellow Card scheme.
Looking ahead to weeks two to four
You stay on 2.5 mg for the rest of the first month, on the same day each week. If you miss a dose, inject as soon as you remember if it's 4 days or less since it was due; after that, skip it and keep your usual day. Never double up. Our missed dose guide has examples.
A short weekly note of how you feel makes the review before your first step up easier. Side effects can come back for a while after a dose increase; our guide to Mounjaro dose steps explains what changes at 5 mg. The MHRA says the only way to be sure of a genuine medicine is a prescription and a legitimate pharmacy, and warns against unregulated sellers such as beauty salons or social media. Don't use "compounded" versions either.
Frequently asked questions
Will I lose weight in my first week on Mounjaro?
Week one isn't a fair test. The 2.5 mg dose is a starter dose, not a treatment dose, and the SURMOUNT-1 results came from 72 weeks of treatment, including a 20-week build-up to doses of 5, 10 or 15 mg. People differ, so ask your prescriber what is reasonable for you.
How soon does Mounjaro start working?
The leaflet gives no timetable for noticing less hunger, and people differ. The SmPC says blood levels peak 8 to 72 hours after each injection, but that isn't a guide to when you'll feel a change. Ask your pharmacist or prescriber what to look out for.
Is it normal to feel sick after my first Mounjaro injection?
Yes. Nausea is very common. The leaflet says it is usually not severe, is most common when you first start, and decreases over time in most people. Get urgent help for severe tummy pain that won't go away.
Should I inject again if I don't notice anything?
No. Take one injection a week, on your usual day, at your prescribed dose. Don't take extra or change your dose unless your prescriber tells you to. If you've injected too much, talk to your doctor straight away or call NHS 111.
Do I need to change my contraception when I start Mounjaro?
Yes, if you take the pill. The MHRA says to add a barrier method, such as a condom, for 4 weeks after starting and for 4 weeks after each dose increase, or switch to a non-oral method such as the coil or implant. Ask your prescriber or pharmacist which suits you.