Safety & Side Effects

Mounjaro Side Effects

Learn about Mounjaro side effects, including common symptoms, what to expect when starting, and how they change over time.

Whether you are starting Mounjaro for the first time or trying to make sense of symptoms a few weeks in, this guide explains the side effects of Mounjaro (tirzepatide) reported in clinical trials and on the UK product licence — what is common, what is rare, what to watch for, and when to seek help. LetsLoseWeight is an independent comparison site; we do not prescribe medication. The information below is for context, not personal medical advice.

How to read side-effect frequencies

Side effects on UK and European medicines licences are grouped into standardised frequency bands. Knowing what each band actually means makes the rest of the page easier to read.

Frequency Meaning
Very common Affects more than 1 in 10 people
Common Affects up to 1 in 10 people
Uncommon Affects up to 1 in 100 people
Rare Affects up to 1 in 1,000 people
Very rare Affects fewer than 1 in 10,000 people

These categories come from the Mounjaro Summary of Product Characteristics on the EMC, which is the authoritative UK reference for Mounjaro side effects. The figures are based on data from the SURPASS (type 2 diabetes) and SURMOUNT (weight management) clinical trial programmes.

Very common and common side effects

Most Mounjaro side effects are gastrointestinal — affecting the digestive system — and most settle within a few weeks as the body adjusts.

Very common (more than 1 in 10)

  • Nausea — most pronounced when starting treatment or after a dose increase, and the most frequently reported side effect across the SURMOUNT trials.
  • Diarrhoea
  • Vomiting — usually mild but can drive dehydration if persistent.
  • Constipation

Common (up to 1 in 10)

  • Abdominal pain or discomfort
  • Indigestion (dyspepsia)
  • Burping
  • Reduced appetite — listed as a side effect though it is partly the intended action of the medicine.
  • Bloating or excessive wind
  • Heartburn (gastro-oesophageal reflux)
  • Hair loss
  • Dizziness
  • Tiredness or fatigue
  • Injection-site reactions — redness, mild pain or itching where the injection was given. Rotating sites between the abdomen, thigh and upper arm reduces this.
  • Hypersensitivity reactions — including rash, itching and eczema-like skin changes.

If you have type 2 diabetes and take Mounjaro alongside insulin or a sulfonylurea, low blood sugar (hypoglycaemia) is also common and can be more likely than for someone using Mounjaro alone. Speak to your prescriber about adjusting other diabetes medicines if needed.

Uncommon and rare side effects

These happen to fewer people but are important to recognise because some need prompt attention.

Uncommon (up to 1 in 100)

  • Acute pancreatitis — inflammation of the pancreas. Symptoms include severe upper-abdominal pain that may radiate to the back, often with persistent vomiting. If this happens, stop taking Mounjaro and seek urgent medical attention.
  • Gallstones (cholelithiasis) and gallbladder inflammation (cholecystitis) — pain in the upper right abdomen, sometimes with nausea, may indicate a gallbladder problem.
  • Dehydration — secondary to severe vomiting or diarrhoea. Can lead to a temporary drop in kidney function.
  • Acute kidney injury — usually in the context of dehydration from gastrointestinal symptoms.
  • Increase in pancreatic enzymes (lipase and amylase) detected on blood tests — sometimes without symptoms.
  • Increased heart rate
  • Diabetic retinopathy worsening — in people with type 2 diabetes, rapid improvements in blood sugar can sometimes worsen pre-existing eye disease in the short term. Anyone with diabetic eye disease should be monitored more closely after starting.

Rare (up to 1 in 10,000)

  • Severe allergic reactions including anaphylaxis and angioedema — symptoms include rapid swelling of the face, lips, tongue or throat, breathing problems and a fast heartbeat. Call 999 or go to A&E immediately if these occur.

Boxed warning: thyroid C-cell tumours and MEN 2

In studies in rats, tirzepatide caused thyroid C-cell tumours, including medullary thyroid carcinoma (MTC). It is not known whether tirzepatide causes the same in humans. Because of this:

  • Mounjaro must not be used by anyone with a personal or family history of medullary thyroid carcinoma, or with multiple endocrine neoplasia syndrome type 2 (MEN 2).
  • Patients should report symptoms such as a lump or swelling in the neck, hoarseness, difficulty swallowing or shortness of breath to a clinician promptly.

This is one of the contraindications listed in the SmPC and is a reason a regulated UK prescriber will not issue a prescription. See our eligibility guide for the full list.

Side effects in pregnancy and breastfeeding

Mounjaro is not licensed in pregnancy. Anyone who becomes pregnant while taking Mounjaro should stop the medicine and contact their prescriber. The SmPC also advises stopping Mounjaro at least one month before a planned pregnancy because of the medicine's long half-life.

It is not licensed during breastfeeding either. There are no adequate data on whether tirzepatide passes into breast milk in humans, so the licence advises against use.

Side effects on hormonal contraception

The Mounjaro SmPC carries a specific warning about oral contraceptives. Tirzepatide can slow gastric emptying, which may delay or reduce the absorption of medicines taken by mouth. For people using a combined oral contraceptive pill, the SmPC recommends switching to a non-oral method (e.g. an intrauterine device, implant, injection) or adding a barrier method (e.g. condoms) for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase.

This is a practical contraception decision worth flagging with your prescriber when starting treatment.

What to do if you have side effects

Most common side effects settle within a few weeks of starting or stepping up the dose, and there are practical things that can help — see our reduce side effects guide for diet and lifestyle tips that target nausea, constipation and tiredness.

If side effects are severe or persistent, options to discuss with your prescriber include:

  • Pausing at the current dose for an additional 4 weeks before next step-up
  • Going back down a level to the previous tolerable dose
  • Stopping treatment if symptoms do not settle

Stopping Mounjaro is not dangerous in itself, though weight regain and (in people with type 2 diabetes) a rise in blood sugar are likely.

When to seek urgent medical help

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

  • Severe upper-abdominal pain that radiates to the back, often with persistent vomiting (signs of pancreatitis)
  • Rapid swelling of the face, lips, tongue or throat, or breathing difficulty (signs of a severe allergic reaction)
  • Severe allergic skin reactions (large rash, blistering)

Contact your GP or NHS 111 for:

  • Severe or persistent vomiting or diarrhoea leading to inability to keep fluids down
  • Severe abdominal pain that does not settle
  • Yellow skin or whites of the eyes (jaundice)
  • Lump or swelling in the neck, hoarseness, or trouble swallowing
  • Significant changes in vision (especially if you have type 2 diabetes)

Where there is any doubt, seek advice — pharmacists and NHS 111 are quick routes to a clinical opinion.

Reporting side effects

In the UK, suspected side effects to any medicine — including Mounjaro — can be reported through the MHRA Yellow Card scheme. Anyone can report: patients, parents, carers and healthcare professionals. Reports help the MHRA identify new safety issues and update prescribing guidance.

Mounjaro carries a black triangle (▼) in the BNF and patient information leaflet, which means it is under additional safety monitoring. Patients are encouraged to report any suspected side effect, however minor, while this monitoring is in place.

Frequently asked questions

Do Mounjaro side effects get better over time?
For most people, yes. Gastrointestinal side effects are usually most noticeable in the first 1–2 weeks after starting or stepping up a dose, and then settle. If they do not settle after 2–3 weeks at a stable dose, talk to your prescriber.

Can I drink alcohol on Mounjaro?
Mounjaro itself does not have a direct alcohol interaction listed in the SmPC, but alcohol can worsen nausea and gastrointestinal upset. People with type 2 diabetes should be careful because alcohol can lower blood sugar. Ask your prescriber for individual advice.

Are Mounjaro side effects worse than Wegovy's?
The two medicines have broadly similar side-effect profiles — both are dominated by gastrointestinal symptoms — but the rates differ. In the SURMOUNT-5 head-to-head trial, gastrointestinal side effects were broadly comparable between the two, with some differences in severity at the highest doses (Aronne et al., NEJM 2025). Tolerance varies between individuals.

Can Mounjaro affect mental health?
The MHRA, EMA and FDA have all reviewed reports of suicidal thoughts and self-harm in people taking GLP-1 receptor agonists. Reviews so far have not found a causal link, but a small risk cannot be definitively ruled out. Anyone experiencing low mood, suicidal thoughts or self-harm urges while on Mounjaro should contact their GP, NHS 111 (option 2 for mental health), or the Samaritans (116 123) and discuss with their prescriber.

Will my hair really fall out?
Hair loss (alopecia) is listed as a common side effect on the Mounjaro licence — affecting up to 1 in 10 people. It is usually temporary, often linked to rapid weight loss rather than the medicine itself, and tends to recover. Adequate protein intake and managing weight-loss pace can help.

Next steps

Sources

This guide is for general information only and is not a substitute for professional medical advice. If you experience symptoms after taking Mounjaro, talk to your prescriber, your GP, or call NHS 111.

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Mounjaro Side Effects UK: Common, Serious, What to Do | LetsLoseWeight