Diarrhoea on GLP-1 Medication

Diarrhoea is one of the most common side effects of GLP-1 weight-loss medicines such as Mounjaro (tirzepatide) and Wegovy (semaglutide). It is listed as "very common" in both UK Summaries of Product Characteristics (SmPCs), meaning it affects more than 1 in 10 people. For most, it is mild to moderate, appears in the days after an injection or a dose increase, and eases as the body adjusts. The main risks are dehydration and, for some people, reduced absorption of oral contraceptives. This guide explains why it happens, how to manage it sensibly, and which symptoms need prompt medical advice.

How common is diarrhoea on GLP-1 medication?

The most reliable figures come from the medicines' official product information, which summarises the large clinical trials submitted to regulators.

A few points help put these numbers in context:

• The placebo rates are not zero. People on dummy injections also reported diarrhoea, partly because trial participants were also changing their diet and partly because diarrhoea is simply common in everyday life. The difference between the two columns is a better guide to the effect of the medicine.
• The trials are not head-to-head. The Mounjaro and Wegovy figures come from different studies, with different durations and populations, so you cannot reliably conclude that one medicine causes more diarrhoea than the other from this table. For a fuller look at how the two compare, see our Mounjaro vs Wegovy comparison.
• Most cases are mild. The Mounjaro SmPC reports that gastrointestinal side effects in the weight-management trials were mostly mild (61%) or moderate (34%). The Wegovy SmPC describes most gastrointestinal events as mild to moderate and of short duration.
• Few people stop because of it. In the pooled Mounjaro weight-management trials, between about 2% and 4% of people (depending on dose) stopped treatment permanently because of a gastrointestinal side effect. Across Wegovy's phase 3 programme, 4.3% stopped because of gastrointestinal events.

The Wegovy 7.2 mg dose appears in the current UK SmPC (revised September 2026) as an option for some adults with obesity after at least four weeks on 2.4 mg. As with any recently added dose, check the latest SmPC and speak to your prescriber about whether it applies to you.

Oral GLP-1 medicines, including semaglutide tablets, also list diarrhoea among their common side effects. Our guide to oral GLP-1 side effects covers tablets in more detail.

Why do GLP-1 medicines cause diarrhoea?

The exact mechanism is not fully understood, and it may differ between people. Several effects of GLP-1 medicines on the digestive system are likely to contribute.

Changes in gut movement

GLP-1 medicines slow the emptying of the stomach, which is one reason you feel full sooner and for longer. They also influence how the intestines contract and move contents along. In some people, this seems to disrupt the normal rhythm further down the gut, leading to looser or more frequent stools. Interestingly, the same medicines cause constipation in other people, which shows how individual the response is. If you have the opposite problem, see our guide to constipation on GLP-1 medication.

Changes in what and how you eat

Starting a GLP-1 medicine usually means eating much less, often at different times and sometimes choosing different foods. A sudden change in diet, such as eating more fibre, protein shakes, sugar-free products or meal replacements, can itself change bowel habits. Sugar alcohols such as sorbitol, xylitol and maltitol, found in many "sugar-free" sweets, drinks and protein bars, are a well-known cause of loose stools.

Fat digestion

Many people notice that a fatty or rich meal triggers diarrhoea, bloating or discomfort when they are on a GLP-1. Food sits longer in the stomach, and large amounts of fat may be harder to tolerate. This is a common pattern reported by patients and clinicians, although high-quality research specifically on this question is limited.

Dose escalation

The SmPCs for both Mounjaro and Wegovy note that nausea, vomiting and diarrhoea are more frequent during the dose escalation period and decrease over time. This is why both medicines are started at a low dose and increased gradually, usually every four weeks. Each step up can bring a temporary return of symptoms.

When does diarrhoea usually happen?

There is a typical pattern, although your own experience may differ.

If diarrhoea starts suddenly after months of stable treatment, do not assume it is the medicine. Infections, other medicines (such as antibiotics or metformin), food intolerance and bowel conditions can all cause diarrhoea. Speak to a pharmacist or GP if it is new, unexplained or not settling.

Managing diarrhoea: practical self-care

The aim is to stay hydrated, reduce triggers and give your gut time to adjust. These are general self-care measures, not a replacement for advice from your prescriber or pharmacist.

1. Put fluids first

The NHS advises drinking plenty of fluids when you have diarrhoea, and taking small sips if you feel sick. Water is the basis, but if you are losing a lot of fluid, oral rehydration solutions (available from pharmacies) replace salts and sugars more effectively than water alone. They are particularly useful if you are older, have kidney problems, or take medicines that affect fluid balance, such as diuretics.

Because GLP-1 medicines reduce appetite and sometimes thirst, many people underestimate how little they are drinking. Our guide to hydration on GLP-1 medication gives practical ways to keep fluid intake up.

2. Eat smaller, plainer meals

• Eat small portions and stop as soon as you feel full.
• Choose plain, easily digested foods while symptoms are active, such as rice, toast, potatoes, bananas, eggs, plain chicken or fish.
• Go easy on very fatty, fried or creamy foods, which many people find trigger symptoms.
• Eat slowly and avoid lying down straight after eating.

3. Identify your triggers

Common culprits reported by people on GLP-1 medicines include:

• Greasy takeaways, fried food and rich sauces
• Large meals, especially late in the evening
• Alcohol
• Very spicy food
• Sugar-free products containing sugar alcohols
• Large amounts of caffeine
• Sudden big increases in fibre or protein supplements

Keeping a simple food and symptom diary for two or three weeks can reveal patterns. Our article on foods that can worsen GLP-1 side effects goes into more detail.

4. Introduce fibre gradually

Soluble fibre, found in oats, bananas and peeled fruit, can help firm up stools for some people. However, adding a lot of fibre quickly can cause wind and bloating. Build up gradually and drink plenty alongside it.

5. Protect your skin

Frequent loose stools can make the skin around the bottom sore. Gentle cleansing with water, patting dry and a barrier cream can help. A pharmacist can advise on suitable products.

6. Practise good hygiene

If there is any chance the diarrhoea is due to an infection, the NHS advises washing hands thoroughly with soap and water, not sharing towels, and staying off work or school until 48 hours after symptoms stop.

What about anti-diarrhoeal medicines?

Over-the-counter medicines such as loperamide can reduce the frequency of loose stools for short periods. However:

• Ask a pharmacist first. They can check whether it is suitable alongside your other medicines and conditions.
• Do not use them if you have warning signs. A high temperature, blood or mucus in your stool, or severe abdominal pain may point to an infection or another condition that needs assessment.
• They treat the symptom, not the cause. If you need anti-diarrhoeal medicine regularly, tell your prescriber. It may mean your dose escalation is too fast for you.

Do not take extra doses of any medicine to "make up" for diarrhoea, and do not stop or change your GLP-1 dose without talking to your prescriber.

How prescribers manage persistent diarrhoea

If self-care is not enough, your prescriber has several options. The decision always depends on your individual circumstances.

• Slowing the titration. The Wegovy SmPC states that in case of significant gastrointestinal symptoms, prescribers can consider delaying dose escalation or lowering to the previous dose until symptoms improve. Mounjaro dose increases are made after a minimum of four weeks, so staying longer on a dose is an option.
• Staying on a lower maintenance dose. Not everyone needs the highest dose. Some people achieve meaningful results on a lower dose they tolerate better.
• Reviewing other medicines. Metformin, some antibiotics, magnesium supplements and certain other medicines can also cause diarrhoea. For people with type 2 diabetes taking metformin alongside a GLP-1, this combination is worth discussing.
• Checking for other causes. Persistent symptoms may need tests to rule out infection, coeliac disease, inflammatory bowel disease, bile acid problems or other conditions.
• Considering an alternative. In some cases, a different medicine or approach may suit you better.

Our guide to Mounjaro dosing explains the standard titration steps, and the Wegovy dosing schedule covers semaglutide.

Dehydration and kidney function

Both the Mounjaro and Wegovy SmPCs warn that nausea, vomiting and diarrhoea can cause dehydration, which in rare cases can lead to a deterioration in kidney function. This matters most for people who:

• Already have reduced kidney function
• Take medicines such as diuretics ("water tablets"), ACE inhibitors, angiotensin receptor blockers or NSAIDs such as ibuprofen
• Are older
• Have both vomiting and diarrhoea at the same time
• Are in hot weather or exercising heavily

Signs of dehydration

According to the NHS, signs of dehydration include:

• Feeling thirsty
• Dark yellow, strong-smelling urine
• Peeing less often than usual
• Feeling dizzy or lightheaded
• Feeling tired
• A dry mouth, lips and eyes

Mild dehydration can usually be managed with fluids and oral rehydration solutions. Severe dehydration, such as not peeing at all, confusion, a fast heartbeat, or feeling faint when standing up, needs urgent medical help.

Some prescribers recommend that people at higher risk temporarily pause certain medicines during an episode of significant diarrhoea or vomiting (sometimes called "sick day rules"). This must be agreed with your own GP or prescriber, because it depends on exactly which medicines you take.

Diarrhoea and other medicines you take

Oral contraceptives

This is one of the most important practical issues for women and anyone who can become pregnant.

• Diarrhoea and absorption. Severe or prolonged diarrhoea can stop the contraceptive pill being fully absorbed. The patient leaflet for your pill explains what to do, and a pharmacist can advise.
• Mounjaro's specific advice. Independently of diarrhoea, the Mounjaro SmPC states that reduced effectiveness of oral contraceptives cannot be excluded in women with obesity or overweight. It advises switching to a non-oral method, or adding a barrier method such as condoms, for four weeks after starting tirzepatide and for four weeks after each dose increase. The MHRA highlighted this in its June 2025 guidance.

We cover this topic in detail in our guide to Mounjaro, GLP-1s and contraception.

Diabetes medicines

People with type 2 diabetes who take insulin or sulfonylureas alongside a GLP-1 medicine may be more vulnerable to low blood sugar if they are eating very little because of gastrointestinal symptoms. If you have diabetes and are unwell, follow the sick day advice from your diabetes team and contact them if you are unsure.

Medicines with a narrow therapeutic range

GLP-1 medicines slow stomach emptying, which can affect how quickly some oral medicines are absorbed. Diarrhoea can add to this effect. If you take medicines where precise blood levels matter, such as warfarin, some epilepsy medicines or lithium, ask your prescriber or pharmacist whether any extra monitoring is needed.

Could it be something else?

GLP-1 medicines are a common cause of diarrhoea in people taking them, but they are not the only cause. Consider other explanations, especially if:

• The diarrhoea started suddenly with vomiting, fever or after eating out, which may suggest food poisoning or a stomach bug
• Other people around you are also unwell
• You have recently taken antibiotics
• There is blood or mucus in your stool
• You are waking at night to open your bowels
• You have unexplained weight loss beyond what you would expect from treatment, or a change in bowel habit lasting several weeks

A persistent change in bowel habit, particularly with blood in the stool, always needs medical assessment regardless of whether you are taking a GLP-1.

Pancreatitis and gallbladder problems

Rarely, diarrhoea can be part of a more serious picture. Acute pancreatitis and gallbladder problems are recognised, less common side effects of GLP-1 medicines. These typically cause severe abdominal pain rather than diarrhoea alone. Read more in our guides to GLP-1 medication and pancreatitis and GLP-1 medication and gallbladder problems.

Diarrhoea and choosing or reviewing treatment

How well you tolerate a medicine is a legitimate part of deciding whether it is right for you. If gastrointestinal side effects are affecting your work, sleep, social life or quality of life, tell your prescriber. Good prescribers, whether NHS or private, should:

• Explain common side effects before you start
• Give you a clear titration plan and the option to slow down
• Be reachable if you have problems
• Review your progress and side effects regularly

If you are comparing how different regulated online services handle follow-up and side-effect support, our provider comparison sets out what each service offers, without ranking providers.

For a broader overview of everything to expect on tirzepatide, see the Mounjaro side effects guide and the dedicated Mounjaro diarrhoea page. For semaglutide, see the Wegovy side effects guide. You can also return to our overview of weight-loss injections.

Reporting side effects

If you experience diarrhoea or any other suspected side effect, you can report it to the MHRA through the Yellow Card scheme, online or via the Yellow Card app. You do not need to be certain the medicine caused the problem. Reports from patients and healthcare professionals help the MHRA spot patterns and keep product information up to date. This applies whether your medicine came from the NHS or a private provider.

When to speak to a professional

Most GLP-1 diarrhoea can be managed at home, but some symptoms need medical advice.

Contact your prescriber, pharmacist or GP if:

• Diarrhoea lasts more than about a week, or returns severely after every dose
• You need anti-diarrhoeal medicine regularly
• Diarrhoea is stopping you eating or drinking enough
• You take medicines for diabetes, blood pressure or kidney problems and are unsure what to do while unwell
• You rely on the contraceptive pill and have had severe or prolonged diarrhoea

Call NHS 111 or get an urgent GP appointment if:

• You have signs of dehydration that are not improving with fluids, such as peeing very little, dark urine or dizziness
• There is blood in your stool or bleeding from your bottom
• You have a high temperature or feel very unwell
• Diarrhoea has lasted more than seven days, or vomiting more than two days

Call 999 or go to A&E if:

• You have severe, persistent abdominal pain, especially if it spreads to your back, with or without vomiting. This can be a sign of acute pancreatitis.
• You have pain in the upper right of your tummy with a high temperature or yellowing of the skin or the whites of your eyes (jaundice), which may suggest a gallbladder problem
• You are vomiting blood or what looks like coffee grounds, or passing black, tarry stools
• You are confused, very drowsy, or not peeing at all
• You have signs of a severe allergic reaction, such as swelling of the face or throat or difficulty breathing

If at any point you are struggling with your mental health or having thoughts of harming yourself, call NHS 111 (option 2 for mental health in many areas), contact Samaritans on 116 123, or call 999 in an emergency.