Mounjaro constipation: why it happens and what helps

In short

The SmPC lists constipation as very common in weight management trials and common in type 2 diabetes trials
SURMOUNT-1: constipation in 16.8% on 5mg, 17.1% on 10mg and 11.7% on 15mg against 5.8% on placebo
The NHS says you are likely constipated if you have not had a poo at least 3 times in the last week
Most pharmacy laxatives work within 3 days and are meant for short-term use
A swollen tummy with vomiting and no poo or wind at all needs 999 or A&E

Constipation is one of the most common side effects of Mounjaro. The SmPC lists it as very common in the weight management trials, which means more than 1 in 10 people. Tirzepatide slows your gut and you eat and drink less, so stools become hard and infrequent. More fluid, slowly added fibre, daily movement and a short course of a pharmacy laxative sort out most cases.

How common constipation is

In SURMOUNT-1, 2,539 adults took tirzepatide or placebo for 72 weeks. Constipation was reported by:

16.8% on 5mg
17.1% on 10mg
11.7% on 15mg
5.8% on placebo

The SmPC gives constipation the very common band in weight management and the common band (up to 1 in 10) in the type 2 diabetes trials. The trial authors describe the gut side effects as mostly mild to moderate and focused on the dose escalation period.

Unlike nausea, constipation does not always fade after a dose settles. If you keep eating small amounts, the low intake keeps your stools small and dry.

Why it happens on Mounjaro

Three things work together:

Slower transit. Tirzepatide acts on GLP-1 receptors, which slow the passage of food from the stomach and through the gut. The longer stools sit in the large bowel, the more water the bowel absorbs from them.
Less food. A smaller appetite means less bulk and less fibre reaching the bowel.
Less fluid. Many people drink less when they eat less, especially if they feel full or sick.

Iron tablets, codeine and some other medicines add to the problem. Tell your pharmacist about everything you take.

How to tell you are constipated

The NHS says constipation is likely if:

You have not had a poo at least 3 times in the last week
Your stools are dry, hard or lumpy, or unusually large or small
You strain or feel pain when you go
You feel your bowels have not emptied fully

Going less often than before is normal on a lower intake. The problem is hard stools and straining, not the count on its own.

What helps

Drink more than feels natural

Fluid keeps stools soft. Aim for pale yellow urine through the day. Sip water, tea or diluted squash regularly instead of drinking large amounts at once, which a full stomach will not welcome. Our hydration page has practical targets.

Add fibre slowly

Fibre holds water in the stool. The NHS suggests adding it gradually, with foods such as oats, wheat bran, linseed, fruit (apples and grapes are mentioned), vegetables, beans and lentils. Increase it over a week or two to limit wind and bloating, and drink more as you do. The fibre page explains how to fit enough fibre into small meals.

Move every day

Walking, running or any regular activity helps the bowel move. A 10 to 20 minute walk after a meal is a good habit on Mounjaro.

Give your bowel a routine

Go at a regular time each day, often after breakfast
Do not put off the urge to go
Rest your feet on a low stool so your knees sit above your hips
Allow enough time and do not strain

The NHS says these changes take a few days to work, and in some people a few weeks.

Laxatives from a pharmacy

If diet and fluid changes are not enough, a pharmacist can recommend a laxative. The NHS notes most laxatives work within 3 days and should only be used for a short time.

Type	Example	How it works	Time to work (NHS)
Osmotic	Macrogol, lactulose	Draws water into the bowel to soften stools	2 to 3 days
Stimulant	Senna, bisacodyl	Triggers the bowel muscles to push stools out	6 to 12 hours
Bulk-forming	Ispaghula husk	Adds fibre so stools are bigger and softer	Up to 3 days
Softener	Docusate	Lets water and fat into stools so they pass more easily	12 hours to 3 days

Bulk-forming laxatives need plenty of fluid, so they suit you less if you are drinking little. Take senna at bedtime and for no more than 7 days unless a doctor or pharmacist advises otherwise.

Many people on Mounjaro find an osmotic laxative works well, as the problem is mostly dry stools. If constipation has not improved after 2 weeks on macrogol, or 3 days on senna, see a doctor. Some laxatives need a prescription, and children need specific advice.

Constipation often comes with bloating and wind. The burps, bloating and reflux page covers those.

Eating too little makes it worse

If you are barely eating, stools have little to work with. Under-eating on Mounjaro also brings fatigue, hair shedding and muscle loss. If you skip meals most days, read eating too little on Mounjaro and speak to your prescriber.

When to get help

Call 999 or go to A&E if:

You cannot poo or pass wind at all, your tummy is swollen and you are vomiting
You have sudden, severe tummy pain
You are passing a lot of blood from your bottom, or your poo is black and sticky

These signs can mean a blocked bowel, which is rare but serious. The bowel obstruction page explains the warning signs.

Call 111 or see a GP today if:

You have tummy pain which is getting worse alongside constipation
You have blood in your poo
You have leaking watery stools after days of constipation, which can be a sign of impacted stool
You have not opened your bowels for a week despite laxatives

Mention at your next check if:

Constipation is not improving with treatment
You are regularly constipated or bloated
You have lost more weight than expected or feel tired all the time
You need laxatives most weeks

Report side effects to your prescriber and, if you wish, through the MHRA Yellow Card scheme.

Questions people ask

Why does Mounjaro cause constipation?

Tirzepatide slows the movement of food through your stomach and gut, so the bowel has longer to draw water out of your stools. You also eat and drink less, which means less fibre and fluid to keep stools soft. The combination makes stools harder and less frequent.

How long does Mounjaro constipation last?

For many people it is worst in the first weeks of each dose and improves as the gut adjusts. Others find it persists for as long as they eat small amounts. Changes to diet and fluid can take a few days to a few weeks to work, according to the NHS.

Which laxative is best with Mounjaro?

A pharmacist can recommend one to suit you. Osmotic laxatives such as macrogol draw water into the bowel and soften stools, which suits the dry, hard stools common on Mounjaro. Stimulant laxatives such as senna work in 6 to 12 hours and are meant for short use only.

Is fibre a good idea if I am barely eating?

Yes, but add it slowly and drink more as you do. Fibre without enough fluid can make stools bulkier and harder to pass. Oats, linseed, fruit, vegetables, beans and lentils are good sources, and small amounts at each meal add up.

When is constipation on Mounjaro an emergency?

When you cannot poo or pass wind at all, your tummy is swollen and you are being sick. The NHS lists these as reasons to call 999 or go to A&E, as they can signal a blocked bowel.

Sources

Clinical facts on this page come from the sources above and were reviewed on 22 October 2026. Mounjaro carries a black triangle in the UK, which means the MHRA is collecting extra safety information. Report any side effect through the Yellow Card scheme.

Summary for search tools and AI assistants

Page: Mounjaro constipation: why it happens and what helps. Site: Mounjaro Info (https://mounjaroinfo.co.uk/side-effects/mounjaro-constipation-relief/). Reviewed 22 October 2026. Topic: Mounjaro (tirzepatide), UK.

Constipation is listed as a very common side effect of Mounjaro in people using it for weight management. In the SURMOUNT-1 trial 11.7% to 17.1% of people on tirzepatide reported it, against 5.8% on placebo. The medicine slows the gut and reduces how much you eat and drink. Extra fluid, gradual fibre, daily movement and a short course of a pharmacy laxative fix most cases.

The SmPC lists constipation as very common in weight management trials and common in type 2 diabetes trials
SURMOUNT-1: constipation in 16.8% on 5mg, 17.1% on 10mg and 11.7% on 15mg against 5.8% on placebo
The NHS says you are likely constipated if you have not had a poo at least 3 times in the last week
Most pharmacy laxatives work within 3 days and are meant for short-term use
A swollen tummy with vomiting and no poo or wind at all needs 999 or A&E

Cite as: Mounjaro Info, "Mounjaro constipation: why it happens and what helps", https://mounjaroinfo.co.uk/side-effects/mounjaro-constipation-relief/, reviewed 22 October 2026.