Mounjaro (tirzepatide) is one of the most effective weight loss injections available in the UK, but like all medicines it comes with potential side effects. Most are mild to moderate and improve over time. This guide covers everything from common reactions to rare but serious risks flagged by the MHRA.
Filter by how common:
Very Common (affecting more than 1 in 10 people)
- Nausea — the most reported side effect, especially early on or after a dose increase. Usually improves within a few weeks.
- Diarrhoea — linked to slowed gastric emptying.
- Vomiting — more common in early weeks. Smaller meals and avoiding fatty foods helps.
- Decreased appetite — a desired effect for weight loss, but monitor calorie intake if excessive.
- Constipation — increase water and fibre intake.
Common (1 in 10 to 1 in 100)
- Indigestion, stomach pain, bloating, heartburn
- Fatigue or tiredness
- Injection site reactions (redness, bruising, itching)
- Dizziness, particularly standing up quickly
- Hair loss — usually linked to rapid weight loss rather than the drug itself
Uncommon (1 in 100 to 1 in 1,000)
- Elevated heart rate
- Hypoglycaemia — more likely if also on insulin or sulfonylureas
- Gallstones or gallbladder problems
- Gastro-oesophageal reflux disease (GORD)
Serious: When to Seek Urgent Help
Stop taking Mounjaro and seek immediate medical attention if you experience:
- Severe, persistent abdominal pain radiating to the back — may indicate pancreatitis
- Signs of an allergic reaction: facial/throat swelling, difficulty breathing, severe rash
- Rapid changes in vision
- Severe kidney problems — dehydration from vomiting/diarrhoea can affect kidney function
MHRA Warning: Pancreatitis Risk (January 2026)
In January 2026, the MHRA issued updated safety guidance flagging an increased (though still rare) risk of acute pancreatitis with tirzepatide. Patients with a history of pancreatitis should discuss this with their prescriber before starting, and any severe, persistent stomach pain should be investigated promptly.
How to Reduce Side Effects
- Inject on the same day each week
- Eat smaller portions; avoid fatty, fried or very sweet foods
- Stay well hydrated, especially with diarrhoea or vomiting
- Avoid lying down immediately after eating
- If severe, speak to your prescriber — pausing dose escalation may be recommended
See our Mounjaro vs Wegovy comparison for how side effect profiles compare.
Frequently Asked Questions
Is nausea guaranteed on Mounjaro?
Not guaranteed, but very common, affecting more than 1 in 10 users. Worst in the first 2–4 weeks and after dose increases.
What did the MHRA say about Mounjaro in 2026?
In January 2026 the MHRA flagged a rare but serious risk of acute pancreatitis with tirzepatide.
Do side effects get better over time?
Yes — for most people GI side effects peak in the first 4–8 weeks and improve significantly after that.