What are the most common side effects?

Stomach ones. Feeling sick (nausea), diarrhoea, constipation and being sick (vomiting) are very common, meaning more than 1 in 10 people. In the weight-loss trials nausea affected roughly 1 in 4 people on Mounjaro. Reduced appetite is also very common, and is usually part of how the medicine works.

What can I do to ease the stomach side effects?

Eat smaller meals, stop when you feel full, and go easy on fatty, fried and very sugary foods, which make nausea worse. Keep sipping fluids, especially if you have diarrhoea or are being sick, to avoid getting dehydrated. If it is severe or you cannot keep fluids down, contact your prescriber or pharmacist.

What are the serious signs to act on?

Severe, lasting stomach pain, which may spread to your back and come with vomiting, can be inflammation of the pancreas: stop your injections and get urgent help, call 111. The MHRA strengthened this warning in 2026. Signs of a serious allergic reaction, a gallbladder problem, severe dehydration, or low blood sugar (mainly if you also take insulin or a sulfonylurea) also need prompt medical attention.

Mounjaro and Wegovy side effects: what to expect

Most side effects of these medicines are stomach ones, worst during the dose step-up and easing over time. A few are serious but rare, and knowing the signs matters. Here is what the UK leaflets say, and when to act.

Last updated 3 November 2026

The short version

Most common are stomach effects: nausea, diarrhoea, constipation, being sick. Very common (over 1 in 10).
They are usually worst during a dose increase and ease as your body adjusts.
Severe, lasting stomach pain (possible pancreatitis): stop your injections and call 111.
Report any side effect through the Yellow Card scheme. It is what surfaced the pancreatitis signal.

The common side effects

The very common side effects, affecting more than 1 in 10 people, are stomach related: feeling sick (nausea), diarrhoea, constipation, being sick (vomiting), stomach pain, reduced appetite (usually part of how it works) and feeling tired. In the weight-loss trials, nausea affected about 1 in 4 people on Mounjaro and diarrhoea about 1 in 5. Common effects, affecting between 1 in 100 and 1 in 10 people, include indigestion, bloating, burping, wind or reflux, dizziness, hair loss, gallstones, and redness or itching where you inject.

When they happen, and how long they last

The leaflets are clear that nausea, diarrhoea and vomiting happen mostly during the dose step-up and get less over time, and that for most people the stomach effects are mild or moderate. Expect them to be at their worst in the days after a dose increase, then settle. For how the benefits build over the same period, see what to expect in your first month.

Easing the stomach side effects

Eat smaller meals and stop when you feel full.
Go easy on fatty, fried and very sugary foods, which make nausea worse.
Keep sipping fluids, especially with diarrhoea or vomiting, to avoid dehydration.
If it is severe or you cannot keep fluids down, contact your prescriber or pharmacist.

Serious signs: act on these

These are rare, but do not wait them out:

Inflamed pancreas: severe, lasting stomach pain that may spread to your back, often with being sick. Stop your injections and get urgent help, call 111. The MHRA strengthened this warning in 2026.
Serious allergic reaction: swelling of the face, lips, mouth or throat, or trouble breathing. Call 999.
Gallbladder problem: pain in the upper right stomach, yellowing of the skin or eyes, fever.
Severe dehydration: from a lot of vomiting or diarrhoea, which can affect the kidneys.
Low blood sugar: mainly if you also take insulin or a sulfonylurea. Shaking, sweating, confusion.

Report side effects

Report any side effect to the MHRA through the Yellow Card scheme. Reporting is exactly what surfaced the rare pancreatitis cases that led to the strengthened 2026 warning, so it protects other people as well as building the picture.

See it laid out, and keep a diary

Our side effects tool groups the effects by how common they are, shows the serious signs to watch for, and lets you keep a simple diary with a Yellow Card link. If a side effect is hard to manage, a switch of medicine is one option your prescriber may discuss.

Common questions

What are the most common side effects?: Stomach ones. Feeling sick (nausea), diarrhoea, constipation and being sick (vomiting) are very common, meaning more than 1 in 10 people. In the weight-loss trials nausea affected roughly 1 in 4 people on Mounjaro. Reduced appetite is also very common, and is usually part of how the medicine works.
When do side effects start, and how long do they last?: The leaflets say nausea, diarrhoea and vomiting happen mostly during the dose step-up and get less over time, and that for most people the stomach effects are mild or moderate. They tend to be worst in the days after a dose increase and settle as your body adjusts.
What can I do to ease the stomach side effects?: Eat smaller meals, stop when you feel full, and go easy on fatty, fried and very sugary foods, which make nausea worse. Keep sipping fluids, especially if you have diarrhoea or are being sick, to avoid getting dehydrated. If it is severe or you cannot keep fluids down, contact your prescriber or pharmacist.
What are the serious signs to act on?: Severe, lasting stomach pain, which may spread to your back and come with vomiting, can be inflammation of the pancreas: stop your injections and get urgent help, call 111. The MHRA strengthened this warning in 2026. Signs of a serious allergic reaction, a gallbladder problem, severe dehydration, or low blood sugar (mainly if you also take insulin or a sulfonylurea) also need prompt medical attention.

Sources

From the UK leaflets, NHS pages and MHRA updates as of 3 November 2026. This is educational reference, not medical advice. Always read the leaflet in your pack, and speak to your prescriber or pharmacist about your own symptoms.