Constipation on GLP-1 Medication

Constipation is one of the most common side effects of Mounjaro (tirzepatide), Wegovy (semaglutide) and other GLP-1 medicines. It happens because these medicines slow digestion, and because people usually eat and drink much less on treatment, so less fibre and fluid reach the bowel. In trials, it affected roughly one in eight to one in six people on tirzepatide and around a quarter on semaglutide 2.4 mg. For most people it can be managed by gradually increasing fibre, drinking more, staying active and, if needed, using a laxative recommended by a pharmacist. A few warning signs need prompt medical attention.

What counts as constipation?

The NHS describes constipation as not having a poo at least three times a week, or passing stools that are unusually large or small, dry, hard or lumpy, or difficult to pass. You may also feel that you have not fully emptied your bowels, have stomach ache, or feel bloated or sick.

Bowel habits vary a lot between people, and they naturally change when you eat much less. Going less often on GLP-1 treatment is not automatically a problem, if stools remain soft and easy to pass and you feel comfortable. What matters is a change that causes straining, discomfort or hard stools.

How common is constipation on GLP-1 medicines?

Mounjaro (tirzepatide)

In the SURMOUNT-1 trial of tirzepatide for weight management, constipation was reported by:

The Mounjaro product information lists constipation as very common in the weight-management setting.

Wegovy (semaglutide 2.4 mg)

The Wegovy product information, based on pooled trial data, reports constipation in about 24% of people on semaglutide 2.4 mg. As with other gastrointestinal effects, it was predominantly mild to moderate.

Timing

Unlike nausea, which tends to be concentrated around starting treatment and dose increases, constipation can appear at any point and may persist for longer for some people, because it is closely linked to how much you eat and drink.

Why GLP-1 medicines cause constipation

Several factors combine:

1. Slower movement through the gut. GLP-1 medicines slow stomach emptying and can slow movement further along the digestive tract. The longer stools stay in the bowel, the more water is absorbed, making them harder and drier.
2. Eating less. Smaller meals mean less bulk to stimulate the bowel.
3. Less fibre. When appetite is low, people often gravitate towards lighter, more refined foods and eat fewer vegetables, wholegrains and pulses.
4. Less fluid. Feeling full can make people drink less, and fibre needs fluid to work.
5. More protein, less fibre. A focus on protein, which is important on GLP-1 treatment, can crowd out fibre if meals are not balanced.
6. Less movement. Fatigue on treatment can reduce activity, and physical activity helps keep the bowel moving.

Self-help: what you can do

Increase fibre gradually

The Scientific Advisory Committee on Nutrition recommends that adults aim for 30 g of fibre a day. Most UK adults fall short of this, and it is harder still when appetite is reduced. The key is to increase fibre gradually over a couple of weeks, because a sudden large increase can cause wind, bloating and cramps, which are already common on GLP-1 treatment.

Both types of fibre matter:

• Soluble fibre (oats, pulses, fruit such as apples and pears, root vegetables, linseed) absorbs water and softens stools.
• Insoluble fibre (wholegrains, wholemeal bread, bran, nuts, seeds, the skins of fruit and vegetables) adds bulk and helps things move along.

The 2025 joint advisory on nutritional priorities for GLP-1 therapy noted that increasing soluble and insoluble fibre, including prunes or other dried fruits, can be helpful for constipation.

Fibre content of common foods

Figures are approximate and vary by product and portion. Check labels where available.

Simple swaps to add fibre to small meals

Drink more fluids

Fibre works by holding water in the stool. Increasing fibre without increasing fluids can make constipation worse. The NHS recommends six to eight cups or glasses of fluid a day for most adults, and more in hot weather or with exercise. Sip steadily through the day, ideally between meals. See our guide to hydration on GLP-1 medication.

Keep active

Physical activity stimulates the bowel. Even a daily walk can help. The UK guidelines recommend at least 150 minutes of moderate activity a week. Our guide to exercise on GLP-1 medication explains how to build up.

Toilet habits

The NHS suggests:

• going to the toilet when you feel the urge, rather than waiting
• giving yourself enough time and privacy
• trying a regular time each day, such as after breakfast, when the bowel is naturally more active
• resting your feet on a low stool while on the toilet, so your knees are higher than your hips, which can make it easier to pass stools

Foods that may help

Some foods are traditionally used for constipation and have some supporting evidence:

• Prunes and prune juice: contain fibre and sorbitol, which has a mild laxative effect. Start with a few prunes or a small glass of juice.
• Kiwi fruit: some people find two kiwis a day helpful.
• Ground linseed (flaxseed): can be stirred into yoghurt or porridge; take with plenty of fluid.
• Pears and apples: contain fibre and natural sugars that can soften stools.

A sample higher-fibre day for a small appetite

The example below shows how fibre can add up across small meals. It is illustrative only, and the figures are approximate. If you currently eat much less fibre than this, build up over a couple of weeks.

Remember to drink more fluid as you add fibre. A glass of water or another drink with each meal and snack, plus extra between meals, is a good habit.

Common mistakes that can make constipation worse

• Adding lots of fibre all at once. This can cause wind, bloating and cramping. Increase gradually.
• Adding fibre without extra fluid. Fibre needs water to soften stools. Without it, bulk-forming fibre can make stools harder.
• Relying on protein shakes and bars. These are often low in fibre and some contain sugar alcohols that cause wind. If you use them, make sure the rest of your diet includes fibre-rich foods.
• Ignoring the urge to go. Holding on allows more water to be absorbed, making stools harder.
• Becoming less active. Fatigue on treatment can reduce movement. Even short walks help.
• Waiting too long to ask for help. A pharmacist can advise early, before constipation becomes severe and uncomfortable.
• Overusing stimulant laxatives. These are generally intended for short-term use. If you are reaching for them often, speak to your prescriber.

Laxatives

If self-help measures are not enough after a few days, a pharmacist can recommend a laxative. The NHS describes several types:

Points to keep in mind:

• Ask a pharmacist which laxative suits you, especially if you have other health conditions or take other medicines.
• Leave a gap between bulk-forming laxatives and other oral medicines, as they can affect absorption.
• Do not use laxatives long term without medical advice.
• If you find you need laxatives regularly, tell your prescriber. They may want to review your dose or treatment plan.

Constipation and other side effects

Constipation can cause nausea

A backed-up bowel can cause nausea, bloating, abdominal discomfort and loss of appetite. If you feel sick and have not opened your bowels for several days, treating the constipation may help. See our guide to managing nausea on GLP-1 medication.

Alternating constipation and diarrhoea

Some people experience both on GLP-1 treatment, sometimes alternating. Occasionally, severe constipation can lead to "overflow", where liquid stool leaks around a hard blockage and looks like diarrhoea. If you have a history of constipation followed by sudden loose stools you cannot control, see a GP. Our guide to diarrhoea on GLP-1 medication covers loose stools.

Haemorrhoids and anal fissures

Straining and hard stools can lead to piles (haemorrhoids) or small tears (fissures), which can cause pain and bright red blood on the toilet paper. Softening stools usually helps. Speak to a pharmacist or GP, and always get any rectal bleeding checked if it is new, persistent or you are unsure of the cause.

Who is at higher risk?

You may be more likely to develop constipation on GLP-1 treatment if you:

• already tend towards constipation
• are older
• are relatively inactive
• take other medicines that can cause constipation, such as some painkillers (especially opioids like codeine), iron supplements, some antacids, some antidepressants and some blood pressure medicines
• eat very little, or very little fibre
• drink relatively little fluid

If you take any of these medicines, do not stop them without advice, but mention constipation to your GP or pharmacist. The NHS notes that iron supplements above 20 mg a day can cause constipation, which is worth bearing in mind if you are considering one. See our guide to vitamins and supplements on GLP-1 treatment.

Can constipation be a sign of something more serious?

Usually not. However, it is important to be aware that:

• Bowel obstruction is uncommon but serious. Symptoms include severe or cramping abdominal pain, a swollen tummy, vomiting, and being unable to pass wind or stools. This needs urgent medical attention.
• Faecal impaction, where a large mass of hard stool gets stuck, can occur with severe or prolonged constipation, particularly in older adults.
• A persistent change in bowel habit, blood in your stools, or unexplained symptoms should be checked by a GP, even if you think it is due to your medicine. Do not assume weight loss or bowel changes are only caused by GLP-1 treatment.

The NHS bowel cancer screening programme offers home testing kits to adults in eligible age groups in England, with similar programmes in Scotland, Wales and Northern Ireland. If you receive a kit, complete it.

How providers can help

Your prescriber should explain how to manage common side effects and when to get in touch. If constipation is persistent or severe, they may suggest staying on a dose for longer before increasing, or reviewing your treatment. Support levels vary between services; our provider comparison and our guide on how to choose an online pharmacy for weight loss explain what to ask. For more on the medicines, see our weight loss injections hub, our Mounjaro side effects guide and the Mounjaro constipation page. For eating ideas, see our guide to what to eat on GLP-1 medication.

When to speak to a professional

Speak to a pharmacist, your GP or your prescriber if:

• self-help measures have not helped after a week or so
• you are regularly using laxatives
• constipation is causing significant discomfort, straining or affecting your appetite
• you have had constipation for more than three weeks
• you notice blood in your stools, or on the toilet paper, especially if it is new or persistent
• you have a persistent change in your bowel habit, or tiredness and breathlessness that could suggest anaemia
• you take medicines that can cause constipation, such as opioid painkillers

Seek urgent medical help (call NHS 111, or 999 in an emergency) if you have:

• severe abdominal pain that does not go away
• a swollen, bloated or hard tummy, especially with vomiting
• vomiting together with an inability to pass wind or stools
• dark, black or tarry stools, or a large amount of blood in your stools
• a high temperature with abdominal pain
• severe pain in the upper abdomen spreading to the back (possible pancreatitis)

Decisions about your medication, including dose changes, belong to your prescriber.