Diarrhoea is one of the most common side effects of Mounjaro (tirzepatide), affecting around 12–17% of people in clinical trials. If you're dealing with loose stools or more frequent trips to the bathroom, you're far from alone.

Diarrhoea happens on Mounjaro because the medication changes how quickly food moves through your intestines. It's usually temporary and improves within 2–4 weeks on a stable dose. Staying hydrated, eating bland foods, and avoiding triggers can help you manage symptoms whilst your body adjusts.

Key takeaways

Diarrhoea affects around 12–17% of people taking Mounjaro and is caused by the medication speeding up transit through your lower intestines

Symptoms typically improve within two to four weeks on a stable dose, though they may return briefly after dose increases

Stay hydrated with 2–3 litres of fluid daily, eat smaller bland meals, and avoid fatty, spicy, or high-fibre foods during flare-ups

Seek medical advice if diarrhoea lasts more than a few days, you notice blood in your stool, or you show signs of dehydration that don't improve

Why does Mounjaro cause diarrhoea?

Mounjaro contains tirzepatide, which works on two appetite-regulating hormones: GLP-1 and GIP. By mimicking these hormones, the medication helps you feel fuller for longer and reduces appetite, which is how it supports weight loss. These hormones also affect your digestive system.

Simplified diagram showing how Mounjaro affects digestion, with the stomach slowing down and the lower intestines speeding up, causing diarrhoea.

Mounjaro primarily slows gastric emptying (how quickly food leaves your stomach and enters your small intestine), but it can have the opposite effect further along your digestive tract. In some people, food moves through the lower intestines faster than usual. When this happens, your body doesn't have enough time to absorb water from the stool, which results in loose or watery stools.

Several factors can make diarrhoea more likely:

Starting the medication or increasing your dose triggers a fresh adjustment period
Eating high-fat or greasy foods puts extra strain on your digestive system
Consuming large meals overwhelms your altered digestive pace
Drinking alcohol can irritate your gut and worsen symptoms
Taking other medications that affect digestion may compound the effect

Diarrhoea is one of several digestive side effects you might experience on Mounjaro. Some people alternate between diarrhoea and constipation as their digestive system adjusts.

How long does Mounjaro diarrhoea last?

For most people, diarrhoea improves within the first two to four weeks on a stable dose. Your digestive system needs time to adapt to how Mounjaro affects the speed of digestion, and this adjustment period varies from person to person.

You might notice diarrhoea returning temporarily each time you move up to a higher dose. This is normal, as each increase essentially restarts the adjustment process. The pattern typically goes:

Timeline

Period	What to expect
Week 1	Symptoms may appear or worsen after starting or increasing dose
Weeks 2–3	Gradual improvement as your body adjusts
Week 4 onwards	Most people find symptoms have reduced
After dose increases	Pattern may briefly repeat, often less severely

Timeline diagram showing the typical pattern of diarrhoea on Mounjaro: symptoms in week 1, gradual improvement in weeks 2-3, and significant reduction by week 4.

If diarrhoea persists for more than a few weeks at the same dose, or if it's severe enough to interfere with your daily life, speak to your GP or prescriber. They might recommend adjusting your treatment plan.

How to manage diarrhoea on Mounjaro

You can't completely prevent digestive changes from the medication, but several strategies can help reduce the frequency and severity of diarrhoea and provide relief whilst your body adjusts.

Stay hydrated

This is the most important thing you can do. Diarrhoea causes you to lose fluids and electrolytes (essential minerals like sodium and potassium that help your body function properly) more quickly than usual. The NHS recommends 6–8 glasses of fluid daily as a baseline, but you'll need more when experiencing diarrhoea.

Aim for at least 2–3 litres of fluid daily, and consider adding an electrolyte drink or oral rehydration sachets like Dioralyte (available from pharmacies without prescription) if diarrhoea is frequent. These replace lost salts and sugars alongside fluids.

Good options include:

Water (sip throughout the day)
Diluted squash
Clear broths
Oral rehydration sachets such as Dioralyte
Coconut water (naturally contains electrolytes)

Diarrhoea can cause you to lose up to a litre or more of fluid per day during severe episodes, so replacing this is essential. Signs of dehydration to watch for:

Dark yellow urine
Dry mouth or lips
Dizziness or lightheadedness
Fatigue beyond normal Mounjaro tiredness
Reduced urination

Infographic showing five signs of dehydration: dark urine, dry mouth, thirst, headache, and fatigue, each with an icon and label.

If you notice these signs, increase your fluid intake immediately.

Follow the BRAT diet temporarily

When diarrhoea is at its worst, bland, easy-to-digest foods can help. The BRAT diet (bananas, rice, applesauce, and toast) has long been recommended for settling upset digestive systems, as outlined in NHS guidance on managing diarrhoea. These foods are low in fibre and gentle on your gut.

Other good options include:

Plain crackers
Boiled potatoes (without butter or oil)
Chicken broth
Plain chicken or fish
White pasta (without rich sauces)

Recipe card showing the BRAT diet foods for settling an upset stomach: bananas, rice, applesauce, and toast, alongside other bland foods like crackers and plain chicken.

Once symptoms improve, gradually reintroduce other foods.

Avoid trigger foods

Certain foods are more likely to worsen diarrhoea

Fatty or fried foods (slow to digest and can irritate the gut)
Spicy dishes (can stimulate bowel movements)
Dairy products (especially if you're lactose sensitive)
Caffeine and alcohol (both can stimulate the bowel)
Artificial sweeteners like sorbitol, which have a laxative effect
High-fibre foods during acute episodes (reintroduce gradually once settled)

Comparison infographic showing foods to eat versus foods to avoid when experiencing diarrhoea on Mounjaro, with green ticks for helpful foods and red crosses for trigger foods.

Eat smaller, more frequent meals

Large meals can overwhelm your digestive system and trigger diarrhoea. Eating five or six smaller meals throughout the day puts less strain on your gut and gives your body a better chance of absorbing nutrients properly.

Stop eating when you feel satisfied rather than full. Your slowed stomach emptying combined with faster intestinal transit creates a tricky balance that smaller portions help manage.

Consider over-the-counter remedies

Medications like loperamide (sold as Imodium) can help control diarrhoea by slowing how quickly food moves through your bowels. These are generally safe to use with Mounjaro according to the Mounjaro summary of product characteristics, but check with your pharmacist first, especially if you're taking other medications.

Probiotic supplements might also help by supporting healthy gut bacteria. Some evidence suggests that products containing Lactobacillus or Saccharomyces boulardii may help manage diarrhoea.

Don't stop taking Mounjaro without speaking to your prescriber first. Diarrhoea typically improves as your body adjusts, and your prescriber can advise if any changes to your treatment are needed.

Diarrhoea alongside other digestive symptoms

Diarrhoea on Mounjaro often comes with other digestive side effects. You might also experience:

Feeling nauseous, especially early in treatment
Abdominal cramping or cramping
Mounjaro-related bloating and gas
Sulphur burps

These symptoms share a common cause: Mounjaro's effect on your digestive system. The strategies that help with diarrhoea (smaller meals, staying hydrated, avoiding trigger foods) generally help with these other symptoms too.

Some people find themselves alternating between diarrhoea and constipation. This can be frustrating, but it usually stabilises as your body adjusts to the medication over several weeks.

Does diarrhoea affect weight loss?

Some people worry that diarrhoea might reduce the effectiveness of Mounjaro for weight loss. The concern is understandable: if food is moving through your system too quickly, are you absorbing the medication properly?

Because Mounjaro is injected under the skin rather than taken orally, diarrhoea doesn't affect how the medication enters your bloodstream. Your body absorbs the full dose regardless of what's happening in your digestive tract.

Severe or prolonged diarrhoea can lead to dehydration and electrolyte imbalances, which can affect your overall wellbeing and energy levels. Managing diarrhoea helps you stay healthy and maintain the lifestyle changes that support weight loss.

Finding the right provider

If gut side effects are knocking your confidence in your current provider, it's worth seeing what else is out there. Provider quality varies a lot when it comes to ongoing clinical support, dose escalation guidance, and how easy it is to pause or adjust treatment when symptoms flare.

Last reviewed: December 2024

Sources

SURMOUNT-1 trial results – New England Journal of Medicine
Mounjaro summary of product characteristics – Electronic Medicines Compendium
NHS guidance on diarrhoea – NHS
NHS water, drinks and hydration – NHS
NHS dehydration guidance – NHS