Mounjaro (tirzepatide) side effects — a UK guide

What is Mounjaro (tirzepatide)?

Mounjaro is the UK brand name for tirzepatide, a once-weekly injection made by Eli Lilly. It activates two gut-hormone receptors (GLP-1 and GIP), which reduce appetite, slow stomach emptying and improve blood-sugar control. In the UK it is licensed both for weight management (alongside diet and physical activity) and for type 2 diabetes. Because it slows how quickly the stomach empties, many of its side effects are digestive. Tirzepatide should only ever be started, titrated and monitored by a prescriber — this guide deliberately does not give doses. For a plain-English overview, see the NHS page on tirzepatide.

Common side effects

According to the NHS, the more common side effects of tirzepatide affect roughly more than 1 in 100 people. They are mostly digestive and tend to be at their most noticeable soon after a dose or a dose increase, easing as your body gets used to the medicine. They include:

• Feeling sick (nausea) or being sick (vomiting)
• Constipation or diarrhoea
• Stomach (tummy) pain and indigestion
• Feeling tired
• Itchy or red skin where you inject — redness may be harder to see on brown or black skin
• Hair loss

These are not usually serious, but tell your pharmacist or GP if they are severe, do not improve, or stop you eating and drinking normally — ongoing vomiting or diarrhoea can leave you dehydrated. Never change your own dose to manage side effects; speak to your prescriber first.

Serious side effects — when to act quickly

Serious side effects are uncommon, but it is important to recognise them. The NHS advises calling your doctor or contacting 111 straight away if you notice any of the following:

• Acute pancreatitis — an inflamed pancreas, which can cause severe pain in your stomach or back.
• Gallstones or gallbladder inflammation (cholecystitis) — which can cause severe tummy pain.
• Low blood sugar (hypoglycaemia) — more likely if you also take other diabetes medicines such as insulin or a sulfonylurea; symptoms include feeling shaky, sweaty, hungry or confused.

The MHRA acute pancreatitis warning (January 2026)

On 29 January 2026, the MHRA issued a Drug Safety Update strengthening the warnings on acute pancreatitis for all GLP-1 and dual GLP-1/GIP receptor agonists, including tirzepatide. It reported that between 2007 and October 2025 the MHRA had received 1,296 Yellow Card reports of pancreatitis linked to this group of medicines; while the overall frequency remains uncommon, some reports were severe — 24 were necrotising and 19 were fatal. The product information for these medicines was updated to highlight the risk of severe acute pancreatitis, including rare necrotising and fatal cases.

For context on this class-wide safety signal, see our guide to GLP-1 medicines and pancreatitis.

Contraception, pregnancy and breastfeeding

Because tirzepatide slows stomach emptying, it can reduce how well the contraceptive pill is absorbed, particularly around starting the medicine and after each dose increase. The NHS advises women who could become pregnant to use an additional barrier method of contraception (such as condoms, a diaphragm or cap), or switch to a non-oral method such as the coil or implant, for four weeks after starting tirzepatide and for four weeks after each dose increase. Talk to your prescriber or pharmacist about the best option for you.

• Pregnancy: tirzepatide is not used in pregnancy, as there is not enough information to say it is safe for the baby. If you are planning a pregnancy, the NHS advises stopping it at least four weeks before you start trying.
• Breastfeeding: tirzepatide is not used while breastfeeding, as it is not known whether it passes into breast milk.

Reducing and managing side effects

• Most gut symptoms settle over a few days to a few weeks as your body adjusts — they are often worst right after a dose or dose increase.
• Smaller, plainer meals and drinking enough fluids can help with nausea; your pharmacist can advise on managing constipation or diarrhoea.
• Keep any prescriber follow-up appointments so your dose is increased at the pace they recommend, not faster.
• Only ever buy tirzepatide from a UK-registered pharmacy against a valid prescription — unregulated sellers and falsified pens are a real safety risk.
• Never start, stop, split or change your dose without advice from your prescriber or pharmacist.

How to report a side effect (Yellow Card)

If you think you have had a side effect from tirzepatide, you can report it to the MHRA through the Yellow Card scheme. Reporting is open to patients and carers as well as health professionals, and you do not need to be certain the medicine caused the problem — a suspicion is enough. These reports help the MHRA detect new or rare risks, as they did with the pancreatitis signal above. Reporting a side effect does not replace getting medical help: if you are unwell, still speak to your pharmacist, GP or NHS 111, and call 999 in an emergency.

Where to get help

• Your pharmacist — for advice on common side effects, dehydration, and interactions with your other medicines.
• Your GP or diabetes/weight-management team — for side effects that are severe, persistent, or affect your daily life, and any dose questions.
• NHS 111 — if you are worried and unsure how urgently you need to be seen.
• 999 or A&E — for a suspected serious allergic reaction, or severe, persistent tummy or back pain that could be pancreatitis.

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