Slimming Injections: Mounjaro Side Effects Explained (October 2026)

01 · The core answer

What can Mounjaro cause?

Mounjaro can cause mainly digestive side effects, plus a handful of other common ones. Like all medicines, Mounjaro can cause side effects, and most are digestive.¹

The most common — affecting more than 1 in 10 people — are nausea, diarrhoea, vomiting, constipation and abdominal discomfort. Common effects (up to 1 in 10) include indigestion, reduced appetite, dizziness, reflux, burping, hair shedding and injection-site reactions.¹

Tirzepatide is the first treatment of its kind to activate two natural gut-hormone receptors at once: GIP and GLP-1. Together these help regulate appetite, slow how quickly the stomach empties, and improve the way the body responds to insulin.¹ If you want the wider picture of how these treatments work, our guide to the Mounjaro slimming injection covers it.

02 · Your next step

Is this relevant to me, and what should I do next?

If you are considering Mounjaro, expect some digestive effects and plan for them rather than being surprised. Most people tolerate the gradual dose ladder well, and your prescriber can slow the pace if you do not. This is one reason treatment is only supplied after a consultation reviewed by a GPhC-registered prescriber.

If you already take Mounjaro and have a symptom that worries you, check the serious side effects information first. For everyday problems, our guides on dealing with Mounjaro side effects and how to minimise Mounjaro side effects go into practical detail.

If you are ready to start, the free online consultation is reviewed by a GPhC-registered prescriber, and our team supports you with side effects afterwards. Our weight loss clinic page explains the wider service.

03 · Digestive effects and trial rates

Digestive side effects and how common they are

Nausea is the most common side effect people notice, followed by diarrhoea, vomiting and constipation. 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

Diarrhoea was reported by 18.7% of people on 5mg, 20.8% on 10mg and 22.5% on 15mg, compared with 7.8% on placebo.¹ Trial averages do not predict your own experience, and some people have very little while others find certain doses harder.

The pattern over time matters more than any single number. Nausea, vomiting and diarrhoea were more common while the dose was being increased and became less frequent over time.¹ In practice, a rough week after a dose increase is common and often followed by an easier stretch.

Most people carry on without trouble, but a small minority stop. 1.9% of people on 5mg, 3.3% on 10mg and 4.3% on 15mg stopped treatment permanently because of gastrointestinal side effects, compared with 0.5% on placebo.¹

04 · Other common effects

Hair shedding, tiredness, dizziness and injection-site reactions

Beyond the stomach, the effects people most often raise are hair shedding, fatigue and reactions where they inject. Each has a straightforward explanation and a practical response.

Hair shedding. Hair loss was reported by 4.9% of people taking tirzepatide in the weight-management trials, compared with 1.0% on placebo.¹ Temporary hair shedding is a recognised side effect, linked to rapid weight loss itself rather than a direct drug effect. For most people it settles as weight stabilises. Protein intake and overall nutrition matter, which is another reason the reduced-calorie diet should stay balanced.¹

Fatigue. 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, 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.¹

Dizziness. Dizziness is listed among the common effects. If it is new or severe, tell your prescriber. If you feel faint, get medical help straight away.

Injection-site reactions. Injection-site reactions were reported by 8.0% of people taking tirzepatide in the weight-management trials, compared with 1.8% on placebo.¹ Mild redness, itching, a small bruise or a tender lump at the injection site is common and normally settles within a few days. Rotating sites each week and letting the pen warm briefly out of the fridge reduces stinging. Contact your clinical team if a reaction is spreading, hot, very painful, or lasts more than a week.¹

05 · Timeline and dose changes

When side effects start, peak and settle

Side effects usually start within days of a first dose or an increase, peak early and then fade. Most side effects follow a pattern: they appear in the first days after starting or increasing a dose, peak early, and settle as your body adapts.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.¹

This pattern is the reason for the dose ladder. Everyone starts on 2.5mg once a week, and the starting dose is about letting your body adjust, not about weight loss itself. After at least 4 weeks the dose increases to 5mg, and can then step up by 2.5mg at a minimum of 4-week intervals (7.5mg, 10mg, 12.5mg) up to a maximum of 15mg weekly, depending on how you respond and tolerate it.¹

Changing your dose if side effects are hard

You do not have to keep climbing the ladder if a dose is not working for you. 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 Many people settle at a maintenance dose of 5mg, 10mg or 15mg, and higher is not automatically better.

The same trial data show that a small share of people stop because of side effects. In SURMOUNT-1, 4.3% of people on 5mg, 7.1% on 10mg and 6.2% on 15mg stopped treatment because of side effects, compared with 2.6% on placebo.² Most people did not stop, and talking early to your prescriber is usually a better route than quitting.

Do not change doses yourself or take extra injections to catch up. If you miss a dose, take it as soon as you can within 4 days (96 hours) of the scheduled time, then carry on with your usual day. If more than 4 days have passed, skip the missed dose entirely and take the next one on your normal day.¹

06 · Managing nausea and constipation

What helps with nausea, constipation and other stomach effects

Small changes to how you eat help most people. Practical steps that help most people: 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.¹

Know the line between uncomfortable and urgent. If nausea or vomiting is persistent or you cannot keep fluids down, contact your clinical team — dehydration needs prompt attention.

Constipation

Constipation is very common on Mounjaro for weight management (it may affect more than 1 in 10 people), because food moves through the gut more slowly and many people eat and drink less.¹ What helps: fluid intake first, fibre from vegetables and whole grains, and daily movement. If it persists, a pharmacist can recommend a suitable laxative. Contact your clinical team for pain, bloating or more than a few days without a bowel movement.¹

What to eat in a day

There is no single approved Mounjaro menu. Aim to include protein at each meal, vegetables and fibre, plain lower-fat choices while you adjust and regular fluids, all alongside a reduced-calorie diet and increased physical activity. Your prescriber or a dietitian can tailor that to you. Our guide to minimising side effects has more food ideas.

07 · Serious side effects

Serious side effects and when to seek help

Serious side effects are uncommon, but they need urgent attention. Seek immediate medical help for: severe, persistent stomach pain that may spread to your back, with or without vomiting (possible pancreatitis); pain in the upper right abdomen, fever or yellowing of skin/eyes (possible gallbladder problems); or signs of a severe allergic reaction such as swelling of the face or throat and difficulty breathing.¹

Do not wait for the next dose day or an online reply for these. Get medical help straight away, call 999 or go to A&E, and tell staff you use Mounjaro.

Low blood sugar with other medicines

If you take insulin or a sulfonylurea alongside Mounjaro, watch for low blood sugar (shakiness, sweating, confusion). Your prescriber may need to adjust those medicines.¹ This is a dose adjustment issue, not a blanket ban, so talk to your prescriber rather than stopping anything yourself.

Oral contraception

Mounjaro can reduce absorption of oral contraceptives, because stomach emptying slows. 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 See the MHRA guidance on contraception with these medicines.⁷

For a wider list of less common effects, see our guide to the rare side effects of Mounjaro, and for how they are collected, our page on Mounjaro reported side effects.

08 · Long-term effects and who should avoid it

Long-term effects, who should not take it and what to avoid

The documented side effects are mostly digestive, and in the trials these became less frequent over time. For any lasting or new symptom, speak to your prescriber and report it through the Yellow Card scheme.¹⁹

Who should not take Mounjaro?

You should not take Mounjaro if you are allergic to tirzepatide or any of its ingredients, and it should not be used during pregnancy or while trying to conceive. Mounjaro must not be used during pregnancy, and it should be stopped at least one month before trying to conceive. If you become pregnant during treatment, stop the medicine and tell your prescriber promptly. Beyond that, suitability is a prescriber decision, which is why Mounjaro is a prescription-only medicine, supplied only after an online consultation reviewed by a GPhC-registered prescriber.¹ Your prescriber will take extra care, or may recommend against treatment, if you have a history of pancreatitis, severe gut or digestive disease, diabetic retinopathy, or gallbladder problems. Always share your full medical history and medicine list during consultation. In the UK it is licensed for weight management in adults and for type 2 diabetes.

What should I avoid?

Avoid pushing through severe symptoms, changing your dose yourself, or buying pens from sellers offering to supply without a consultation or prescription. The licensed information recommends use alongside a reduced-calorie diet and increased physical activity.

Is it a high-risk medication?

It is a prescription-only medicine with a well-documented side effect profile, most of it digestive and temporary. The serious effects are uncommon. Risk depends on your health, your other medicines and how well you are monitored. Regular prescriber reviews are part of safe use.

What is the black box warning?

"Black box warning" is a US labelling term. In the UK, the safety information sits in the product information and patient leaflet.¹³ Read the leaflet that comes with your pen.

09 · Mood, face and loose skin

Mood, "Ozempic face" and loose skin

Regulators have looked at mood, and the evidence does not support a causal link. The EMA's safety committee reviewed the evidence and concluded in 2024 that it does not support a causal link, and the MHRA keeps this under continuous review.¹⁷ Even so, any big change in weight, appetite and routine can affect how you feel. Tell your prescriber about any history of depression, and seek help promptly if your mood drops or you have distressing thoughts during treatment. Our page on Mounjaro psychological side effects goes deeper.

If you have thoughts of harming yourself or ending your life, get help straight away. Call 999 or go to A&E if you are in immediate danger. In England you can call NHS 111 and choose the mental health option, any time. You can also call Samaritans free on 116 123, day or night. Tell your prescriber about any change in your mood while taking Mounjaro.

"Ozempic face" is not a drug effect. It is what losing a significant amount of weight quickly can do to facial volume, on any method including diet or surgery. The same applies to loose skin. The counter-measures are the ones that protect lean mass: adequate protein, resistance exercise, and a loss pace guided by your prescriber rather than maximum speed. Skin commonly adapts gradually after weight stabilises.¹

10 · Reporting and support

Reporting side effects and getting support

Report any suspected side effect to your prescriber and to the MHRA Yellow Card scheme, which exists for patients as well as professionals.¹⁹ Reports help regulators spot patterns, but they do not prove a medicine caused a symptom.

With us, aftercare includes ongoing monthly reviews, dose-titration and follow-up assessments by the prescribing team, and side-effect support. Our support team is available Monday to Friday, 9am to 5pm, by email at support@everydaymeds.co.uk, through the account portal chat or by phone on 0333 207 0413. Repeat orders re-check suitability, and there is no auto-shipping.

Because side effects can also come from fake or poorly stored products, buy only from a registered pharmacy. We are GPhC-registered (9012878), and our Mounjaro comes through official LillyDirect supply. Keep pens in a fridge at 2–8°C and never freeze them. Once in use, a pen can be kept out of the fridge not above 30°C for up to 30 days, after which it should be discarded.¹

If you are weighing up different injections and their effects, our overview of slimming injections, evidence and UK access compares them, and our page on slimming injection side effects and price covers the cost angle.

NHS access is a separate route. Mounjaro is available on the NHS, but access is being phased in over several years.⁴