Independent UK medicine guidance
Mounjaro and diarrhoea
Diarrhoea is listed as a very common side effect of Mounjaro. In pooled weight-management studies, it affected approximately 18.7% to 22.5% of people taking tirzepatide, compared with 7.8% taking a placebo. It was reported more frequently while doses were being increased and generally became less common as treatment continued.
Is Mounjaro definitely the cause?
Diarrhoea beginning soon after your first injection or a dose increase may be related to Mounjaro. However, not every episode is caused by the medicine. Other possible causes include:
- A stomach bug
- Food poisoning
- A sudden change in diet
- Large amounts of fatty food
- Sugar-free sweets containing sorbitol
- Magnesium supplements
- Metformin or other medicines
- An existing bowel condition
Diarrhoea affecting other people in your household is more likely to be infectious.
What can you do?
The priority is preventing dehydration. Drink regularly, taking small sips if you feel sick. A pharmacist may recommend an oral rehydration solution to replace lost water and salts.
Choose simple foods you can tolerate and temporarily avoid alcohol, very fatty meals, spicy food and large amounts of caffeine. Do not deliberately starve yourself.
Ask a pharmacist before using diarrhoea medication. Some treatments are unsuitable when diarrhoea is accompanied by blood, fever, severe pain or a suspected infection.
Do not move up to a higher Mounjaro dose while significant diarrhoea is continuing without discussing it with your prescriber.
Signs of dehydration
- Dark, strong-smelling urine
- Urinating less frequently
- Dry mouth or lips
- Headache
- Dizziness when standing
- Unusual tiredness
- Feeling faint or confused
Vomiting and diarrhoea can cause sufficient fluid loss to affect kidney function. Older adults and people with kidney disease may be particularly vulnerable.
When should you seek medical help?
Contact your prescriber, GP or NHS 111 if:
- Diarrhoea is severe, persistent or repeatedly returns after injections
- You cannot drink enough to replace the fluid being lost
- You are passing very little urine
- You have blood or black material in your poo
- You have a high temperature
- You have severe or persistent abdominal pain
- You feel faint, confused or unusually drowsy
Different NHS services use slightly different sick-day instructions. If vomiting or diarrhoea is more than minor, ask your prescriber whether your next dose should be delayed. Do not make permanent changes to treatment without clinical advice.
Evidence update, September 2026: SURPASS-CVOT diarrhoea figure in the revised SmPC
On 8 September 2026 the electronic medicines compendium (eMC) published a revised Mounjaro Summary of Product Characteristics with a text revision date of 29 August 2026. It adds safety results from SURPASS-CVOT, a large trial that compared tirzepatide with dulaglutide, another injectable diabetes medicine, in adults with type 2 diabetes and established cardiovascular disease. The study-specific figures relevant to this page are:
- Diarrhoea: 24.9% of participants taking tirzepatide
- For context, the same study reported nausea in 25.1%, vomiting in 11.6% and constipation in 12.8%
The figure substantiates the frequency-category changes made in the UK patient leaflet revised in August 2026. It does not add a new side effect and it does not change any of the advice above about dehydration or when to get help.
Source: Mounjaro Summary of Product Characteristics (eMC), text revised 29 August 2026, section 4.8
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