Mounjaro side effects — UK patient reference

Common (≥1 in 10)

• Nausea
• Diarrhoea
• Decreased appetite
• Constipation
• Vomiting
• Abdominal discomfort / pain
• Injection-site reactions (redness, itching)
• Fatigue
• Headache

Less common but reported

• Gallstones / cholecystitis (gallbladder inflammation)
• Gastro-oesophageal reflux, eructation (burping), heartburn
• Hair loss
• Hypoglycaemia (particularly when combined with insulin or sulfonylureas in T2DM)
• Increased heart rate
• Tachycardia at higher doses

Serious (uncommon but important)

• Pancreatitis. A recognised risk of all GLP-1 receptor agonists. Red flags: severe persistent abdominal pain radiating to the back, with or without nausea/vomiting. Stop the medication and seek urgent medical attention. See GLP-1 pancreatitis.
• Severe gallbladder disease. Acute right-upper-quadrant pain, fever, jaundice — seek urgent care. See GLP-1 gallbladder.
• Severe hypoglycaemia when combined with insulin or sulfonylureas in T2DM. Your prescriber should adjust other diabetes medication when starting Mounjaro.
• Hypersensitivity / anaphylaxis. Rare but reported. Difficulty breathing, swelling of face/throat — emergency response.

Pregnancy and breastfeeding

Mounjaro is contraindicated in pregnancy. Stop at least two months before planned conception. Mounjaro can affect the absorption of oral contraceptives in the first weeks of use, and weight loss itself can restore fertility — barrier contraception is the conventional advice during titration. See GLP-1 pregnancy and fertility for the detail.

What to do for common GI effects

• Hydrate. Small frequent sips beat occasional large drinks.
• Eat smaller, simpler, lower-fat meals. Bland carbohydrate (toast, rice, crackers) is often tolerated when nothing else is.
• Avoid alcohol during the first week of any new dose step.
• Slow the titration if your prescriber agrees.
• For constipation: fluid + fibre first; laxatives next; see GLP-1 constipation for the escalation ladder.

When to contact your prescriber urgently

• Severe abdominal pain, especially radiating to the back.
• Severe right-upper-quadrant pain, fever, or jaundice.
• Severe persistent vomiting or diarrhoea (dehydration risk).
• Signs of severe hypoglycaemia (sweating, shaking, confusion) — applies to T2DM patients on insulin / sulfonylureas.
• New severe headache or visual changes.
• Mental-health changes including thoughts of self-harm.

How to report a Mounjaro side effect

Use Yellow Card — the official UK adverse-event reporting scheme. You can submit a report yourself; you don’t need to go through your prescriber. Also tell your prescriber so they can adjust the dose or product as needed.