EN
Home/United Kingdom/Blog/What are the side effects of Mounjaro, and how do you report them?
UKUnited Kingdom Side effects and safety

What are the side effects of Mounjaro, and how do you report them?

Common and serious side effects of tirzepatide (Mounjaro) from the UK label, by dose, and how to report one through the MHRA Yellow Card scheme.

GLP Guidebook teamPublished 12 Oct 20268 min read11 sourcesWritten from published sources

Written for the UK. Reading from India? India version

The short answer

The most common are feeling sick, diarrhoea, vomiting, constipation and tummy pain, mostly while the dose is going up. In the UK label's weight trials, 25% to 29% felt sick, against 9% on a dummy injection. Serious risks include pancreatitis. Report side effects to the MHRA on a Yellow Card.

Key takeaways

  • In the UK Mounjaro label’s weight trials, 24.6% to 29.0% felt sick on tirzepatide 5 to 15 mg, against 8.5% on a dummy injection.1
  • The UK Mounjaro label says nausea, vomiting and diarrhoea were higher while the dose was rising and fell over time.1
  • Stomach side effects led 1.9% to 4.3% to stop Mounjaro for good in the weight trials, against 0.5% on a dummy injection.1
  • Acute pancreatitis is uncommon on the UK Mounjaro label, and in January 2026 the MHRA added warnings about rare necrotising and fatal cases.1,2
  • The MHRA’s Yellow Card scheme takes reports from anyone, online or in its app, including suspected fake products.3,4
On this page
  1. What are the most common Mounjaro side effects?
  2. Are side effects worse at higher doses?
  3. When do side effects start, and do they settle?
  4. What about hair loss, low blood pressure and allergic reactions?
  5. What serious risks does the UK label list?
  6. How do you report a side effect on a Yellow Card?
  7. Questions people ask
  8. How we researched this

What are the most common Mounjaro side effects?

Feeling sick, diarrhoea, vomiting, constipation and tummy pain. In the UK label's weight trials, about 1 in 4 people on Mounjaro felt sick, against fewer than 1 in 10 on a dummy injection.1

Before you go further
  • GLP‑1 medicines are prescription-only in the UK. On the NHS or privately, a healthcare professional has to assess you before a prescription is issued.4
  • The label facts here come from the UK product information (the SmPC, the official label each maker publishes on the electronic Medicines Compendium). Where we use a US or EU label instead, we say so.
  • Every risk of these medicines is in Guide 5: Side effects and risks.
  • This post describes labels, guidance and trials. It is not a plan for your treatment, and no dose or brand guarantees any result.

Mounjaro is tirzepatide, a weekly injection. It acts on two gut-hormone receptors, GLP‑1 and GIP, that affect appetite and blood sugar.4 In the UK it is licensed for type 2 diabetes and for weight management.1

The UK label groups side effects by how often they happen. "Very common" means more than 1 in 10 people. "Common" means 1 in 10 to 1 in 100.1

  • Very common: feeling sick (nausea), diarrhoea, vomiting, constipation and tummy pain, some of them only in the weight trials. Hypos are very common with insulin or a sulfonylurea.1
  • Common: indigestion, bloating, burping, wind, reflux, dizziness, low blood pressure, hair loss, allergic-type reactions and injection-site reactions.1
  • Uncommon: a changed sense of taste, injection-site pain, a slower-emptying stomach and acute pancreatitis.1

Are side effects worse at higher doses?

In the diabetes trials, stomach effects rose with the dose. In the weight trials, they were similar at 5, 10 and 15 mg, at about 56%, against 30% on a dummy.1

Any stomach or gut side effect, by weekly dose
% of adults in each group, two pools of placebo-controlled trials, UK Mounjaro label
UK Mounjaro label (SmPC), section 4.8, revised 29 August 2026. The two pools differ in people and length, so compare each bar with its own dummy figure.1

Feeling sick. In the weight trials, 24.6%, 29.0% and 28.0% felt sick on 5, 10 and 15 mg, against 8.5% on the dummy.1 So about 16 to 21 people in 100 felt sick because of the medicine itself, by our arithmetic. Diarrhoea affected 18.7% to 22.5%, against 7.8%.1

Stopping because of it. In the weight trials, 1.9% to 4.3% stopped for good because of a stomach side effect, against 0.5% on the dummy.1 Most stomach effects were mild (61%) or moderate (34%).1

Your prescriber sets the dose. The UK label starts at 2.5 mg a week and raises it in 2.5 mg steps, at least 4 weeks apart, up to 15 mg.1 Our post Wegovy vs Mounjaro compares the two.

When do side effects start, and do they settle?

Mostly while the dose is going up. The UK label says nausea, vomiting and diarrhoea were higher during dose increases and fell over time.1

The MHRA says most side effects are mild to moderate or short-lived. It warns that some, such as vomiting and diarrhoea, can lead to severe dehydration and a hospital stay.4 The UK label says dehydration from stomach side effects can harm the kidneys, including sudden kidney failure.1

The NHS says to sip fluids slowly if you have been sick, and that a pharmacist can suggest rehydration drinks.5 Practical tips for nausea are in Guide 5.

What about hair loss, low blood pressure and allergic reactions?

Each is common in the weight trials on the UK label. Hair loss affected 4.9% on Mounjaro against 1.0% on a dummy injection.1

  • Hair loss was mostly mild, and most people recovered while still on treatment. No one stopped tirzepatide because of it.1
  • Allergic-type reactions, such as hives, eczema or rash, were reported in 5.0% on Mounjaro against 3.8% on a dummy in the weight trials. Severe reactions (anaphylaxis) are rare.1
  • Low blood pressure events were common in the weight trials, and 94% were mild to moderate.1
  • Mood. After a review, the MHRA concluded the data do not support a link between GLP‑1 medicines and depression or suicidal thoughts. It keeps watching.4
Free toolQuestions to take to your next review

Builds a printable list for your GP, nurse or pharmacist, including side effects so far. No sign-up.

Open the question list

What serious risks does the UK label list?

Pancreatitis, gallbladder problems, kidney injury from dehydration, hypos with some diabetes medicines, and breathing in stomach contents under anaesthetic.1

  • Pancreatitis is uncommon (1 in 100 to 1 in 1,000). In January 2026 the MHRA added warnings of rare necrotising and fatal cases.1,2 See pancreatitis and gallbladder.
  • Gallstones are common in the weight trials: 1.1% against 1.0% on a dummy. An inflamed gallbladder affected 0.6% on Mounjaro against 0.2% on a dummy.1
  • Hypos. With a sulfonylurea, 10% to 14% had a significant or severe hypo; with basal insulin, 14% to 19%.1
  • Diabetic eye disease can worsen for a time when blood sugar improves quickly, the label warns.1
  • Surgery. A slow stomach raises the risk of breathing in stomach contents under general anaesthetic or deep sedation.1,6 See Mounjaro before surgery.
  • Pregnancy and the pill. Not for use in pregnancy, and it may make the pill less reliable.1 See pregnancy and the pill.
Get help now: call 999 for a life-threatening emergency.7 Call for severe, persistent stomach pain that may spread to your back, with or without being sick: the MHRA’s warning sign for pancreatitis.2 Call if someone with a low is unconscious and glucagon isn’t available or hasn’t worked.8 For urgent advice that is not an emergency, use NHS 111 in England, Scotland and Wales, online or by phone.9 Northern Ireland has no 111 service: call your GP practice or the GP out-of-hours service.10

How do you report a side effect on a Yellow Card?

Report it to the MHRA's Yellow Card scheme, online or in the Yellow Card app. Anyone can report, including as a guest without an account.3

The MHRA asks people to report suspected side effects of GLP‑1 medicines, and any product they suspect is not genuine.4 It says stomach effects make up most of the Yellow Card reports it receives for these medicines.4

  • Extra monitoring. Mounjaro carries a black triangle on its UK label, meaning it is under additional monitoring for new safety information.1
  • What helps a report: the MHRA asks for as much as possible, including medical history, other medicines, when it started, treatment dates, batch number and brand.6
  • Pancreatitis in hospital: reporting it can lead to an invitation to the Yellow Card Biobank, a genetics study.11

The Yellow Card site cannot give medical advice; for worries about your health it points you to your doctor, pharmacist or NHS 111.3 Private prescriptions may not show in your NHS records, the MHRA warns, so tell any doctor you see.2

RisksThe risks, in brief
  • Stomach effects are very common: 24.6% to 29.0% felt sick in the weight trials, against 8.5% on a dummy.1
  • Pancreatitis is uncommon (1 in 100 to 1 in 1,000).1
  • Gallstones are common (1 in 10 to 1 in 100) in the weight trials.1
  • Dehydration can lead to kidney injury.1
  • Hypos are more likely with insulin or a sulfonylurea.1
Full detail in Guide 5: Side effects and risks →

Questions people ask

What is the most common side effect of Mounjaro?

Feeling sick (nausea). In the UK label’s weight trials, 24.6% to 29.0% felt sick on Mounjaro against 8.5% on a dummy injection.1

How long do Mounjaro side effects last?

The UK label says nausea, vomiting and diarrhoea were higher while the dose was rising and decreased over time.1

Does Mounjaro cause hair loss?

It can. In the weight trials, 4.9% reported hair loss on Mounjaro against 1.0% on a dummy. It was mostly mild, and most recovered while still on treatment.1

How do I report a Mounjaro side effect?

Use the MHRA Yellow Card scheme, on the website or in the Yellow Card app. Anyone can report, including as a guest.3

When is a Mounjaro side effect an emergency?

The MHRA says to get urgent help for severe, persistent stomach pain that may spread to the back, with or without vomiting.2 Call 999 for a life-threatening emergency.7

Shorter answers: How do I report a side effect (Yellow Card)? and When should I call 999 or 111 on a GLP-1?

How we researched this

On 12 Oct 2026 we read section 4 of the UK Mounjaro label (SmPC, revised 29 August 2026). We also read the MHRA’s patient guidance and safety updates, and the Yellow Card website. All side-effect figures come from the UK label.

The UK label gives no separate vomiting or constipation figure by dose for the weight trials, so we give the frequency band instead. Trials of tirzepatide were paid for by its maker, Eli Lilly.

Before you move on

  • Stomach effects are the most common, mostly while the dose is going up.
  • About 1 in 4 felt sick in the weight trials, against under 1 in 10 on a dummy.
  • Pancreatitis is uncommon, but severe stomach pain needs urgent help.
  • Report suspected side effects or fake pens on a Yellow Card.
I'm already on itLife on a GLP-1Doctor questions tool

More from the UK blog

UKUnited KingdomPancreatitis and gallbladder: the MHRA warning UKUnited KingdomWegovy vs Mounjaro in the UK UKUnited KingdomFake Mounjaro and illegal jabs
About GLP Guidebook

The monthly UK update

An independent guide to GLP‑1 medicines. We don't sell medicines, take drug-company money or earn clinic commissions. Once a month: NHS access changes, approvals, safety notices and recalls. How we write

Was this post easy to understand?

Sources

  1. Eli Lilly. Mounjaro KwikPen (tirzepatide): UK summary of product characteristics (SmPC, the UK label), date of revision 29 August 2026. Sections 4.2, 4.4, 4.5, 4.6, 4.8, 5.2, 6.4. emc
  2. MHRA. Drug Safety Update: GLP‑1 receptor agonists and dual GLP‑1/GIP receptor agonists: strengthened warnings on acute pancreatitis, including necrotising and fatal cases. Volume 19, issue 6, 29 January 2026. gov.uk
  3. MHRA. Yellow Card: making medicines and medical devices safer (reporting site; guest reports and the Yellow Card app). Opened 12 October 2026. yellowcard.mhra.gov.uk
  4. MHRA (Medicines and Healthcare products Regulatory Agency). GLP‑1 medicines for weight loss and diabetes: what you need to know. First published 5 June 2025, updated 5 February 2026. gov.uk
  5. NHS. Dehydration. Opened 12 October 2026. nhs.uk
  6. MHRA. Drug Safety Update: GLP‑1 and dual GIP/GLP‑1 receptor agonists: potential risk of pulmonary aspiration during general anaesthesia or deep sedation. Volume 18, issue 6, 28 January 2025. gov.uk
  7. NHS. When to call 999. Page last reviewed 3 February 2023. nhs.uk
  8. NHS. Low blood sugar (hypoglycaemia). Opened 12 October 2026. nhs.uk
  9. NHS. When to use NHS 111 online or call 111. Opened 12 October 2026. nhs.uk
  10. nidirect (Northern Ireland government). GP out of hours service. Opened 12 October 2026. nidirect.gov.uk
  11. MHRA. If you take a GLP‑1 medicine and have been hospitalised by acute pancreatitis, the Yellow Card Biobank wants to hear from you. Press release, 26 June 2025. gov.uk

Last checked 12 Oct 2026 · Next review due 12 Oct 2027 · Report an error

How this post was made. Written by the GLP Guidebook team from the sources above. It's general information, not medical advice. Talk to a qualified doctor, nurse or pharmacist before starting, stopping or changing any medicine. How we write