What are the most common Mounjaro side effects?

The most common are to do with your stomach: feeling sick (nausea), diarrhoea, constipation, being sick (vomiting) and stomach pain, all very common (more than 1 in 10 people). Feeling tired and less appetite are common too. They're usually worst when you start and after each dose increase, and settle over a few weeks.

When do the side effects start, and how long do they last?

They usually appear in the first days and are worst during the dose step-up, then ease over time. The leaflets say most stomach side effects are mild to moderate and don't last. Constipation can hang around a bit longer than nausea.

How do I stop the nausea?

Eat small meals slowly and stop when you're full, go easy on fatty, fried and very sugary foods, and sip fluids through the day. If you're being sick a lot, can't keep fluids down, or it isn't settling, speak to your prescriber or pharmacist.

When should I worry about a side effect?

Get urgent help for severe stomach or back pain that doesn't go away (a possible inflamed pancreas), signs of a severe allergic reaction like swelling of the throat or tongue or trouble breathing (call 999), severe pain in the upper right of your tummy or yellowing skin (a gallbladder problem), or being unable to keep fluids down. The MHRA strengthened the pancreatitis warning in 2026, so don't wait severe, lasting pain out.

Does Mounjaro cause hair loss, or cancer?

Hair loss is listed as a common side effect in the leaflet and often relates to rapid weight loss rather than the drug itself. The leaflets do not list cancer as a side effect of these medicines in people. If you're worried about a specific risk, talk it through with your prescriber.

Do the side effects mean it's working?

Not really. Side effects and weight loss both come from the same effect on your gut and appetite, but you can lose weight with few side effects, or have side effects without fast weight loss. Don't judge whether it's working by how rough you feel.

Side effects, and when to worry

The side effects of Mounjaro, Wegovy, Ozempic and Saxenda, straight from the UK leaflets: the common ones by how common they are, when they start and how long they last, and the serious signs to act on.

Updated 31 October 2026

Your medicine

Most side effects are to do with your stomach, worst when you start and after each dose increase, and they usually settle over a few weeks.

When to get medical help

Most of these are uncommon, but don’t wait them out. In an emergency call 999.

Inflamed pancreas (acute pancreatitis)Call 111: Severe pain in your stomach or back that comes on and doesn’t go away, often with being sick. Stop your injections and get urgent medical help. Call 111. The MHRA strengthened this warning in 2026 after rare severe cases, so don’t wait it out.
Severe allergic reaction (anaphylaxis)Call 999 now: Swelling of your face, lips, throat or tongue, difficulty breathing or swallowing, or a sudden widespread rash. Call 999 now. This is an emergency.
Gallstones or a gallbladder problemCall 111: Severe pain in the upper right of your tummy, yellowing of your skin or the whites of your eyes, a high temperature, or feeling sick. Get medical help. Call 111 or see a doctor.
Dehydration affecting your kidneysCall 111: A lot of vomiting or diarrhoea that you can’t keep on top of, leaving you unable to keep fluids down, very thirsty, dizzy or passing little urine. Get medical advice. Call 111. Being very dehydrated can affect your kidneys.
Low blood sugar (hypoglycaemia)Tell your prescriber: Sweating, shaking, a pounding heart, going pale, feeling hungry, dizzy or confused. Treat it with a fast-acting sugar, then a snack. It’s much more likely if you also take insulin or a sulfonylurea, so tell your prescriber, who may lower that other medicine. If someone passes out or has a fit, call 999.
Worsening diabetic eye disease (retinopathy)Tell your prescriber: If you have type 2 diabetes: changes in your vision, especially in the first months as your blood sugar improves. Tell your prescriber and keep up your diabetic eye checks.
Before an operation, sedation or endoscopyTell your care team: These medicines slow how fast your stomach empties, which matters for anaesthetic and raises the risk of breathing stomach contents into your lungs. Tell your anaesthetist or surgical team you’re on it before any planned operation, sedation or endoscopy.

The common side effects

From Mounjaro’s UK leaflet, grouped by how many people get them.

very common (more than 1 in 10 people)

Feeling sick (nausea)about 27% in trials
Diarrhoeaabout 22% in trials
Constipationabout 16% in trials
Being sick (vomiting)about 13% in trials
Stomach pain
Less appetite (usually part of how it works)
Feeling tired

common (between 1 in 100 and 1 in 10 people)

Indigestion, bloating, burping, wind or reflux
Dizziness
Hair loss
Gallstones
Redness or itching where you inject

Trial figures from tirzepatide trials (SURMOUNT). Your experience can be very different.

When they happen, and settling them

The leaflet says nausea, diarrhoea and vomiting happen mostly during the dose step-up and get less over time, and that most people’s stomach side effects are mild or moderate.

Eat small meals, slowly, and stop when you feel full.
Go easy on fatty, fried and very sugary foods, which make nausea worse.
Sip fluids through the day, especially if you have diarrhoea or are being sick.
For constipation, build up fibre and fluids and keep moving.

More on eating well and easing side effects →

Beyond the leaflet

What trial and post-market data add. These are patterns across many people, not a guide to your own risk.

In the trials: In the weight-loss trials the common stomach effects were the ones above (nausea about 1 in 4, diarrhoea about 1 in 5).
Post-market reports: Reports collected after launch most often back up the same stomach effects. Pancreatitis is rare, but it is the signal the MHRA acted on in 2026. Reporting through the Yellow Card scheme is what surfaced it.

Pregnancy, and staying safe

Don’t use Mounjaro in pregnancy or while trying to conceive. The MHRA says to stop it at least 1 month before trying for a baby. It can also make the contraceptive pill less reliable, so add a barrier method (such as condoms) for 4 weeks after you start and for 4 weeks after each dose increase.

Only use a pen from a pharmacy or a registered prescriber. The MHRA warns that fake weight-loss pens are sold on social media and by unregulated sellers, and you can’t tell what’s in them.

How long it stays in your system →Using your pen →

This is general information from the UK patient leaflets, the NHS and the MHRA, not medical advice and not a prediction for you. Everyone is different. Always read the leaflet in your pack, and talk to your prescriber or pharmacist about any side effect. In an emergency call 999. Sources: the Mounjaro leaflet, the NHS, and the MHRA. Report a side effect through the Yellow Card scheme.

Common questions

What are the most common Mounjaro side effects?: The most common are to do with your stomach: feeling sick (nausea), diarrhoea, constipation, being sick (vomiting) and stomach pain, all very common (more than 1 in 10 people). Feeling tired and less appetite are common too. They're usually worst when you start and after each dose increase, and settle over a few weeks.
When do the side effects start, and how long do they last?: They usually appear in the first days and are worst during the dose step-up, then ease over time. The leaflets say most stomach side effects are mild to moderate and don't last. Constipation can hang around a bit longer than nausea.
How do I stop the nausea?: Eat small meals slowly and stop when you're full, go easy on fatty, fried and very sugary foods, and sip fluids through the day. If you're being sick a lot, can't keep fluids down, or it isn't settling, speak to your prescriber or pharmacist.
When should I worry about a side effect?: Get urgent help for severe stomach or back pain that doesn't go away (a possible inflamed pancreas), signs of a severe allergic reaction like swelling of the throat or tongue or trouble breathing (call 999), severe pain in the upper right of your tummy or yellowing skin (a gallbladder problem), or being unable to keep fluids down. The MHRA strengthened the pancreatitis warning in 2026, so don't wait severe, lasting pain out.
Does Mounjaro cause hair loss, or cancer?: Hair loss is listed as a common side effect in the leaflet and often relates to rapid weight loss rather than the drug itself. The leaflets do not list cancer as a side effect of these medicines in people. If you're worried about a specific risk, talk it through with your prescriber.
Do the side effects mean it's working?: Not really. Side effects and weight loss both come from the same effect on your gut and appetite, but you can lose weight with few side effects, or have side effects without fast weight loss. Don't judge whether it's working by how rough you feel.

Side effects and frequencies from the UK patient leaflets for Mounjaro, Wegovy, Ozempic and Saxenda, the NHS and the MHRA. Trial rates from the pivotal trials and post-market reports.

General information from the leaflets, the NHS and the MHRA, not medical advice and not a prediction for you. Always read the leaflet in your pack and talk to your prescriber about any side effect. In an emergency call 999.