Mounjaro Side Effects Week 1: What's Normal and What Isn't (October 2026)

01 · The core answer

What to expect from Mounjaro in the first week

Expect mild to moderate stomach-related effects, most likely in the first days after your first injection. The commonest are nausea, diarrhoea, vomiting, constipation and abdominal discomfort. Reduced appetite, indigestion, reflux, burping, dizziness and injection-site reactions are also recognised.¹

Side effects are most likely when starting treatment or moving up a dose, and for most people they ease within days to a few weeks as the body adjusts.¹³ That is exactly why treatment begins on the lowest strength. The 2.5mg starting weeks are about adjustment, and experiences differ widely. Some people notice appetite change in the first week, while others feel little until a higher dose. Both are normal.¹

02 · Your next step

Is what I'm feeling normal, and what do I do next?

If your symptoms are mild, short-lived and settling, they fit the normal pattern. Most side effects appear in the first days after starting or increasing a dose, peak early and settle as your body adapts.¹ Keep a simple note of what you feel and when, because it helps your clinical team if you need to ask for advice.

If symptoms are hard to tolerate, speak to your clinical team rather than pushing through. Prescribers can hold the current dose for longer before stepping up.¹ Our guides on how to minimise side effects and dealing with them day to day go further.

Every order is reviewed by a GPhC-registered prescriber, and our weight loss clinic includes side-effect support.

03 · How the first week unfolds

How the first week unfolds

The first week follows a loose pattern rather than a fixed timetable. Most side effects appear in the first days after the injection, peak early and then settle as your body adapts.¹ Mild nausea, altered appetite and tiredness in the first days after each injection are the commonest early effects, and they usually settle.¹

Because Mounjaro is a once-weekly medicine, you take it on the same day each week. The days straight after the injection are when you are most likely to notice something. The days just before the next dose are often the quietest. Noticing that rhythm helps you plan lighter meals and a lighter schedule around your injection day.

You may also feel nothing at all. That does not mean the medicine is not working or that you have done something wrong. Effects and benefits vary widely at the starting dose. If you want to know how the very first day can feel, our guide to the first day after your injection covers it, and our first-week overview sets out the wider picture.

04 · Nausea and digestive effects

Nausea, diarrhoea and other digestive effects

Digestive effects are the headline of week one because of how Mounjaro works. Tirzepatide activates the GIP and GLP-1 receptors, which helps regulate appetite and slows how quickly the stomach empties.¹ Most people feel fuller sooner and less hungry between meals. A slower-emptying stomach is also why large portions can sit heavily.

The trial figures give useful context, although they were measured across the dose range rather than at the starting dose alone. In the trials supporting Mounjaro's weight-management licence, nausea was reported by 24.6% of people on 5mg, 29.0% on 10mg and 28.0% on 15mg, compared with 8.5% on placebo.¹ Nausea, vomiting and diarrhoea were more common while the dose was being increased and became less frequent over time.¹

Practical steps that help most people are to eat smaller meals slowly, stop at the first feeling of fullness, favour plain lower-fat foods while adjusting, stay well hydrated and avoid lying down straight after eating.¹³ If nausea or vomiting is persistent or you cannot keep fluids down, contact your clinical team — dehydration needs prompt attention.¹³

05 · Constipation and bowels

Constipation in the first week

Constipation is very common on Mounjaro (it may affect more than 1 in 10 people), because food moves through the gut more slowly and many people eat and drink less.¹³ What helps is fluid first, with regular water through the day. Then add fibre from vegetables and whole grains, and keep up daily movement.

If it persists, a pharmacist can recommend a suitable laxative. Contact your clinical team about pain, bloating or more than a few days without a bowel movement.¹³ Diarrhoea is just as recognised, and the same hydration advice applies. If either pattern drags on, our guide to dealing with Mounjaro side effects lays out what to try in order.

06 · Tiredness and appetite

Tiredness, low energy and appetite changes

Fatigue is a recognised common side effect, most noticeable in the first weeks and after dose increases. It usually reflects a mix of the body adjusting and simply eating less.¹³ So keep meals balanced, protein especially, stay hydrated and keep gentle activity going. For most people energy returns to normal within a few weeks. Tell your clinical team if tiredness is severe or not settling.

Reduced appetite is also recognised. You may feel full far sooner than usual, and snacking loses its pull. That is the medicine doing what it is designed to do, but eating too little can add to tiredness and dizziness. Dizziness is a recognised common effect, so stand up slowly and keep drinking fluids.

Headache is not listed as a side effect in the UK Mounjaro patient leaflet. If you develop a new symptom that worries you, discuss it with your prescriber and report it through the Yellow Card scheme.¹⁹

07 · Injection-site reactions

Injection-site reactions

Mild redness, itching, a small bruise or a tender lump at the injection site is common and normally settles within a few days.¹³ Injection-site reactions were reported by 8.0% of people taking tirzepatide in the weight-management trials, compared with 1.8% on placebo.¹

Rotating sites each week and letting the pen warm briefly out of the fridge reduces stinging.¹³ Contact your clinical team if a reaction is spreading, hot, very painful, or lasts more than a week. For technique and common worries, see our injection side effects guide.

08 · When to get help

When to get urgent help

Serious side effects are uncommon but need urgent attention. Seek immediate medical help for severe, persistent stomach pain that may spread to your back, with or without vomiting, which could be pancreatitis.¹ Also seek immediate medical help for pain in the upper right abdomen, fever or yellowing of the skin or eyes, which could mean gallbladder problems. Signs of a severe allergic reaction, such as swelling of the face or throat and difficulty breathing, need immediate medical help.¹

If you cannot keep fluids down, get medical help straight away. Dehydration needs prompt attention, and your clinical team can advise.¹³ You can report any suspected side effect through the Yellow Card scheme.¹⁹ For the less usual effects, see our page on rare side effects of Mounjaro.

09 · Medicines and food to be careful with

What to avoid and which medicines to mention

Mounjaro slows stomach emptying, which can reduce how well oral contraceptive pills are absorbed. The SmPC advises switching to a non-oral method, or adding a barrier method (such as condoms), for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase.¹ If you take insulin or a sulfonylurea alongside Mounjaro, watch for low blood sugar, such as shakiness, sweating and confusion. Your prescriber may need to adjust those medicines.¹

Beyond those, tell your prescriber about every medicine and supplement you take. On food, the practical guidance is plain lower-fat foods while adjusting, smaller meals and not lying down straight after eating. Very fatty, fried or very sugary food and large portions can sit heavily because the stomach empties more slowly, so ease off them in the early days.¹³

10 · Snacks and tips

Practical tips and snack ideas for the first week

Keep it simple: small, plain, protein-led food eaten slowly. Good options include yoghurt, boiled eggs, plain chicken, soup, crackers with a little cottage cheese, or soft fruit. Sip water through the day rather than gulping it with meals.

Plan your injection day sensibly. Choose a day when a quieter evening will not be a problem, and have easy meals ready. Stop eating at the first feeling of fullness, even if the plate is not finished.¹³ Our guide to minimising Mounjaro side effects expands each of these, and the wider GLP-1 side effects overview is useful background.

11 · Weight loss in the first week

How much weight to expect in the first week

Expect little, and do not judge the medicine by the first weigh-in. Weight loss on Mounjaro is gradual by design. The first weeks on 2.5mg are an adjustment phase and losses are usually modest. Most measurable progress builds from 5mg onward, month by month.²

Early loss includes water weight, so the trend over weeks matters more than any single weigh-in.² The headline SURMOUNT-1 trial averages were measured at 72 weeks, not in the first days, and individual results vary. Think in months, not days. Treatment is reviewed by your prescriber, including after 6 months on the highest dose you can tolerate.

12 · Moving on to the second week and beyond

What comes after the first week

The second week usually looks like a continuation of the first, at the same 2.5mg dose. Symptoms that appeared early should be easing, and you will start to learn your own rhythm around injection day. The dose does not change until the starting period is complete, and side effects are most likely again when you move up a dose.¹

If a new dose is hard to tolerate, prescribers can hold the current dose longer before stepping up. Speak to your clinical team rather than pushing through severe symptoms.¹ To see how things typically look a little further on, read about side effects in the third week. If you have missed your injection day, our missed dose guide explains what to do.