Check the pattern and severity
Loose stools after starting treatment or increasing a dose may come from tirzepatide, but timing proves nothing on its own. A stomach bug, food poisoning, another medicine or an existing bowel condition looks similar. Note how many episodes you have, how long they last and any pain, fever or blood.
How diarrhoea affects you matters as much as how often you go. Mild loose stools while you drink normally carry less risk than repeated fluid loss with dizziness. Older people and people with kidney problems can become unwell faster.
Replace lost fluid first
Drink regularly, taking small frequent sips if nausea makes larger drinks hard. A pharmacist can tell you whether an oral rehydration product suits you. These products replace water and salts in set amounts. Sports drinks and sugary drinks do not do the same job.
If you have a fluid restriction, heart failure or advanced kidney disease, ask for advice on your own plan and ignore general drinking targets. Watch how much urine you pass and look out for thirst, dizziness and growing weakness. Not feeling thirsty does not rule out dehydration.
Eating and anti-diarrhoea medicines
Eat what you can manage and avoid forcing down a large or fatty meal while symptoms last. Aim to get back to normal eating. Diarrhoea is no chance to speed up weight loss. Eating hardly anything for a long time adds weakness and slows recovery.
Ask a pharmacist before taking anti-diarrhoea medicine, especially with fever, bloody stools, marked abdominal pain or other health conditions. Some situations need a medical check before you slow the bowel down. Do not keep using these medicines to cope with a dose that keeps making you ill.
Your other prescriptions
Diarrhoea affects how well you absorb tablets and makes dehydration problems more likely with some treatments. If you have diabetes, follow your sick day guidance and ask your team about checking your glucose. Do not stop insulin or guess at changes to your treatment on your own.
If you take the contraceptive pill, follow the pill's own advice on severe diarrhoea as well as the extra contraception precautions for tirzepatide. A pharmacist or sexual health service can tell you whether you need emergency contraception if your protection may have failed.
When to get help and review the dose
Contact a clinician promptly if you cannot keep fluids down, pass hardly any urine, feel dizzy most of the time or have heavy or ongoing diarrhoea. Blood in your stool, severe pain, confusion or a marked decline needs urgent assessment. Severe persistent pain in the upper abdomen may need checking for pancreatitis. Take no further Mounjaro doses while pancreatitis is suspected.
Tell your prescriber before the next planned dose increase. Bring dates, symptoms and details of your other medicines. If the diarrhoea clears but returns with each dose, that pattern needs a treatment review, not repeated self-treatment with no explanation.
Sources
Your own medicine leaflet and prescriber come first. A source from another country does not mean the same rule applies in the UK.
- Mounjaro UK Summary of Product Characteristics, updated September 2026 medicines.org.uk
- Mounjaro UK patient information leaflet medicines.org.uk
- NHS: diarrhoea and vomiting nhs.uk
- NHS: dehydration nhs.uk
- MHRA: GLP-1 medicines, what you need to know gov.uk
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