If you have recently started Mounjaro and your bowel habits have slowed to a crawl — or stopped entirely — you are dealing with one of the most common side effects of GLP-1 weight loss medications. Constipation on Mounjaro is uncomfortable, sometimes painful, and can feel alarming if you are someone who previously had regular bowel movements.
The reassuring news is that this side effect is well understood, highly manageable, and for most patients, temporary. This guide explains exactly why Mounjaro causes constipation, provides a comprehensive list of evidence-based remedies, covers what is available in UK pharmacies, and outlines when you should seek medical advice.
Does Mounjaro Cause Constipation? The Evidence
Yes, it does — and it is one of the most commonly reported gastrointestinal side effects. In the SURMOUNT clinical trials, which tested tirzepatide (the active ingredient in Mounjaro) for weight management, constipation was reported by approximately 12–18% of participants depending on the dose. At higher doses (10 mg, 12.5 mg, and 15 mg), the rates were at the upper end of this range.
To put that in context: if you were in a room with ten other people taking Mounjaro, at least one or two of them would be experiencing constipation alongside you. And those are just the trial figures — real-world reporting suggests the true number may be higher, as patients in clinical trials are often given more dietary guidance than those in standard care.
So if you are wondering whether the medication is to blame for your bowel changes, the answer is almost certainly yes.
Why GLP-1 Medications Cause Constipation
Understanding the mechanism behind Mounjaro constipation helps you target the right remedies. There are several interconnected reasons why the medication affects your bowels, and most patients experience a combination of all of them.
1. Slowed Gastric Emptying Affects the Entire GI Tract
The primary mechanism by which Mounjaro suppresses appetite is delayed gastric emptying — food stays in your stomach for significantly longer than it normally would, which makes you feel fuller for longer and eat less. But this slowing effect does not stop at the stomach. It extends through the entire gastrointestinal tract, including the small intestine and large intestine (colon).
When transit through the colon is slower, the body has more time to absorb water from the stool. The longer waste material sits in the colon, the drier and harder it becomes. Hard, dry stool is more difficult to pass, requires more straining, and moves through the bowel less efficiently — the textbook definition of constipation.
This mechanism is the same reason why other GLP-1 medications like Wegovy (semaglutide) also cause constipation. Any drug that slows gut motility will have this downstream effect on bowel function.
2. Significantly Reduced Food Intake
Mounjaro’s appetite suppression is powerful. Many patients find their daily food intake drops dramatically — sometimes by half or more compared to pre-medication levels. Less food going in means less waste to come out, which naturally reduces bowel movement frequency.
But more importantly, reduced food intake usually means reduced fibre intake. Dietary fibre — found in fruits, vegetables, wholegrains, legumes, nuts, and seeds — is the single most important nutrient for maintaining regular bowel movements. Fibre adds bulk to stool, draws water into the colon, and stimulates the muscular contractions (peristalsis) that move waste through your digestive system.
When you eat less food overall, and particularly when appetite suppression leads you to skip meals or choose low-volume convenience foods, fibre intake can plummet. The NHS recommends 30g of fibre per day for adults — a target that most British adults already fail to meet even when eating normally. On Mounjaro, hitting this target becomes genuinely challenging without deliberate effort.
3. Dehydration
Dehydration is a major contributor to constipation on Mounjaro, and it often goes unrecognised. There are several reasons why patients on the medication tend to be under-hydrated:
- Less food means less water from food. Around 20–30% of your daily fluid intake comes from the food you eat. Fruits, vegetables, soups, and sauces all contain significant water. When you eat less, this hidden hydration source diminishes.
- Nausea reduces drinking. Many patients find that nausea — another common Mounjaro side effect — makes drinking water feel unpleasant, particularly in large quantities.
- Changed routines. When your eating pattern changes, your drinking pattern often changes too. If you previously drank water with meals and are now skipping meals, those hydration opportunities disappear.
Adequate water is essential for soft, well-formed stools. When the body is dehydrated, the colon absorbs even more water from waste material than usual, producing stools that are hard, dry, and difficult to pass.
4. Reduced Physical Activity
Some patients experience fatigue on Mounjaro, particularly in the first few weeks and after dose increases. This tiredness can lead to reduced physical activity, which in turn affects bowel function. Exercise — even gentle exercise like walking — stimulates peristalsis and helps move waste through the digestive system. When activity levels drop, the bowels slow down further.
5. Changes in Gut Microbiome
Emerging research suggests that GLP-1 medications may alter the composition of the gut microbiome — the community of bacteria that lives in your digestive system. These bacteria play an important role in digestion and bowel regularity. While the research is still in early stages, changes to the microbiome could contribute to bowel disturbance, including constipation.
Evidence-Based Remedies for Mounjaro Constipation
The strategies below are listed in order of priority — start with the foundations (hydration and fibre) before moving to supplements and over-the-counter treatments. Most patients find that addressing the first two or three items resolves the problem without needing medication.
1. Increase Your Water Intake to 2.5 Litres or More
This is the single most important step. Aim for a minimum of 2.5 litres of fluid per day — more if you are physically active or in warm weather. Water, herbal teas, sugar-free squash, and clear broths all count.
Practical tips for increasing fluid intake on Mounjaro:
- Keep a water bottle with you at all times and sip throughout the day rather than drinking large amounts at once
- Set hourly phone reminders if you tend to forget
- Drink a full glass of water first thing in the morning before eating or drinking anything else — this can stimulate bowel activity
- If plain water feels unappealing due to nausea, try warm water with lemon, peppermint tea, or chilled sparkling water
- Track your intake using a free app like WaterMinder or simply mark levels on your water bottle with a pen
2. Prioritise High-Fibre Foods
Even when appetite is suppressed, you can make fibre-rich choices that pack a lot of bulk into small portions. Focus on including at least one high-fibre food at every meal. Here is a practical list of high-fibre foods that work well for patients on Mounjaro:
Fruits (choose these when appetite is low — they are easy to eat and hydrating):
- Raspberries — 8g fibre per 125g punnet
- Pears (with skin) — 5.5g fibre per pear
- Apples (with skin) — 4.4g fibre per apple
- Oranges — 3g fibre per orange
- Kiwi fruit — 2.3g fibre per kiwi (also contains actinidin, an enzyme that promotes motility)
Vegetables:
- Broccoli — 5g fibre per 150g serving
- Sweet potatoes — 4g fibre per medium potato
- Carrots — 3.5g fibre per 150g serving
- Peas — 5g fibre per 80g serving
- Spinach — add to smoothies or scrambled eggs for easy fibre
Wholegrains and legumes:
- Baked beans — 10g fibre per standard tin (widely available and affordable across UK supermarkets)
- Lentils — 8g fibre per 100g cooked
- Porridge oats — 4g fibre per 40g serving
- Wholemeal bread — 3g fibre per slice
- Chickpeas — 7g fibre per 100g cooked
3. Add Psyllium Husk
Psyllium husk is a soluble fibre supplement that absorbs water in the gut to form a gel-like substance, making stools softer, bulkier, and easier to pass. It is one of the most evidence-based natural remedies for constipation and is particularly well-suited to Mounjaro patients because it works gently without causing cramping.
Start with one teaspoon (approximately 5g) mixed into a large glass of water once daily, and increase to twice daily if needed. It is essential to drink plenty of water when taking psyllium — without adequate fluid, it can actually worsen constipation by creating a dry, bulky mass in the colon.
Psyllium husk is available in UK health food shops and online. Brands like Whole Psyllium Husks by Nature’s Best and Fybogel (which contains ispaghula husk, the same active ingredient) are widely stocked.
4. Try Ground Flaxseed
Ground flaxseed (also called linseed) is an excellent fibre supplement that provides both soluble and insoluble fibre, plus omega-3 fatty acids. Two tablespoons of ground flaxseed provide approximately 4g of fibre and can be easily stirred into porridge, yoghurt, smoothies, or sprinkled on salads.
Like psyllium, ground flaxseed works by absorbing water and adding bulk to stool. It also has a mild lubricating effect that can make stools easier to pass. Make sure to use ground flaxseed rather than whole seeds, as whole seeds can pass through the digestive system undigested. You can find ground flaxseed at Tesco, Sainsbury’s, Aldi, and most UK supermarkets.
5. Walk for 20–30 Minutes Daily
Regular gentle exercise — walking is ideal — stimulates the muscular contractions of the colon that move waste through the digestive system. Even a short daily walk can make a meaningful difference to bowel regularity. Many patients find that a morning walk is particularly effective, as the combination of movement and the body’s natural morning cortisol rise helps trigger the urge to go.
If fatigue on Mounjaro is making exercise difficult, start small. Even a 10-minute walk around the block is better than none. As your energy improves (which it typically does within a few weeks of each dose), you can gradually increase the duration.
6. Consider Magnesium Supplements
Magnesium is a mineral that plays a role in muscle contraction, including the smooth muscle of the digestive tract. Magnesium citrate, in particular, has a mild osmotic laxative effect — it draws water into the colon, softening stool and stimulating bowel movements.
A dose of 200–400mg of magnesium citrate taken in the evening can help with both constipation and sleep quality (magnesium also supports relaxation). It is widely available from UK pharmacies and supplement retailers including Holland & Barrett, Boots, and Amazon UK.
Note: if you have kidney problems, check with your doctor before supplementing with magnesium, as the kidneys are responsible for excreting excess magnesium from the body.
7. Include Probiotic Foods
Probiotic-rich foods support a healthy gut microbiome, which in turn supports regular bowel function. Consider including the following in your diet:
- Natural live yoghurt — look for labels that say “live cultures” or “contains L. acidophilus and B. bifidum”
- Kefir — a fermented milk drink with a high concentration of beneficial bacteria, available in most UK supermarkets (Biotiful is a popular brand)
- Sauerkraut — fermented cabbage, choose unpasteurised varieties from the chilled section
- Kimchi — Korean fermented vegetables, increasingly available in UK shops
- Miso — fermented soy paste, used to make miso soup
If you prefer to take a probiotic supplement, look for one containing Bifidobacterium lactis or Lactobacillus rhamnosus, both of which have evidence supporting their role in improving bowel regularity.
8. Try Kiwi Fruit
This deserves its own mention because kiwi fruit have surprisingly strong evidence for relieving constipation. Two green kiwi fruit per day have been shown in clinical trials to significantly improve bowel frequency, stool consistency, and straining. Kiwi fruit contain a unique combination of fibre, water, and an enzyme called actinidin that supports digestive motility.
For many Mounjaro patients, two kiwi fruit per day is an easy, natural addition that requires no supplements or medication. They are small enough to eat even when appetite is suppressed, and they are available year-round in UK supermarkets.
Over-the-Counter Options Available in UK Pharmacies
If dietary changes and natural supplements are not sufficient, several over-the-counter products can provide additional relief. These are all available without a prescription from UK pharmacies including Boots, Superdrug, Lloyds Pharmacy, and Well Pharmacy.
Fybogel (Ispaghula Husk)
Fybogel is the UK’s best-known bulk-forming laxative. It contains ispaghula husk (the same active ingredient as psyllium) in convenient sachets that dissolve in water. Take one sachet twice daily with plenty of water. It is gentle, well-tolerated, and safe for long-term use. Available in orange and lemon flavours.
Lactulose
Lactulose is an osmotic laxative available as a liquid. It works by drawing water into the colon, softening stool and stimulating bowel movements. The typical dose is 15ml twice daily. It usually takes 24–48 hours to work. Lactulose is gentle and safe, though some patients experience bloating and wind when they first start taking it.
Dulcoease (Docusate Sodium)
Dulcoease is a stool softener that works by allowing water and fat to penetrate the stool, making it softer and easier to pass. It is particularly useful if your stools are very hard and dry. The usual dose is one to two capsules daily. It typically works within one to three days.
Glycerol Suppositories
For more immediate relief when you feel the urge to go but cannot pass a stool, glycerol suppositories can help. They work by lubricating and stimulating the rectum, usually producing a bowel movement within 15–60 minutes. They are available over the counter at any UK pharmacy and are useful as an occasional rescue remedy rather than a daily treatment.
Senna (Senokot)
Senna is a stimulant laxative that works by irritating the lining of the colon to stimulate contractions. It is effective but should be used sparingly and not on a daily basis, as regular use can lead to the bowel becoming dependent on it. Reserve senna for situations where other approaches have not worked and you need relief. The typical dose is one to two tablets at bedtime, with a bowel movement expected the following morning.
A Note on Laxative Use
Bulk-forming laxatives (Fybogel, psyllium) and stool softeners (Dulcoease) are safe for regular use alongside Mounjaro. Osmotic laxatives (lactulose) are also generally safe for ongoing use. Stimulant laxatives (senna, bisacodyl) should be used only occasionally and not as a long-term solution. If you find yourself needing stimulant laxatives regularly, speak to your clinician about adjusting your treatment plan.
Tips for Preventing Constipation From the Start
If you are about to start Mounjaro or have just begun your first dose, these proactive steps can help prevent constipation from becoming an issue in the first place:
- Start increasing your water intake now. Build the habit of drinking 2.5+ litres daily before the medication begins to suppress your appetite and reduce your instinct to drink.
- Stock your kitchen with high-fibre foods. When appetite is low, you will reach for whatever is most convenient. Make sure convenient options include fruit, wholemeal bread, baked beans, and porridge oats.
- Begin a daily psyllium or ground flaxseed habit. Adding one teaspoon of psyllium husk or two tablespoons of ground flaxseed to your morning routine creates a baseline of fibre that protects bowel function even when overall food intake drops.
- Maintain daily movement. Even a short walk counts. Build exercise into your routine before you start the medication so it becomes automatic rather than something you need to motivate yourself to do when tiredness kicks in.
- Establish a toilet routine. The bowel responds well to routine. Try to sit on the toilet at the same time each day — usually after breakfast or your morning coffee — even if you do not feel an urge. Over time, this trains the bowel to expect activity at that time.
- Use a footstool. Placing a small stool (around 20cm high) under your feet while on the toilet tilts the pelvis into a squatting position, which straightens the anorectal angle and makes it easier to pass stool. This simple change can make a surprising difference. The Squatty Potty brand is widely available in the UK, though any small stool or box will work.
When Constipation Is a Medical Concern
For most patients, Mounjaro constipation is uncomfortable but not dangerous. However, there are situations where you should seek medical advice promptly:
- No bowel movement for five or more days — while reduced frequency is normal on Mounjaro, going five or more days without any movement warrants assessment
- Severe abdominal pain or cramping — particularly if it is localised to one area rather than general discomfort
- Blood in your stool or on the toilet paper — this could indicate haemorrhoids (common with straining) or other conditions that need evaluation
- Nausea, vomiting, and inability to pass gas alongside constipation — this combination could indicate a bowel obstruction, which is a medical emergency
- Persistent bloating that does not improve — significant distension of the abdomen should be assessed
- Constipation that does not respond to any of the remedies listed above — your clinician may need to investigate further or consider adjusting your medication
In rare cases, severe constipation on GLP-1 medications can lead to faecal impaction — a condition where a large, hard mass of stool becomes stuck in the rectum and cannot be passed naturally. This is uncommon but serious and requires medical treatment. Seeking help early, before constipation becomes severe, is always the best approach.
How Evernu Supports Your Digestive Health on Mounjaro
At Evernu, we take a holistic approach to weight loss treatment that goes beyond simply prescribing medication. Our clinical team understands that side effects like constipation can significantly impact your quality of life and, if unmanaged, may lead patients to discontinue treatment prematurely — missing out on the benefits they started the medication to achieve.
When you start Mounjaro with Evernu, you receive:
- Nutritional guidance that specifically addresses fibre intake, hydration, and dietary adjustments to support digestive health alongside weight loss
- Regular clinician check-ins where you can discuss side effects including constipation and receive personalised advice
- Dose adjustment support — if constipation is severe and not responding to management strategies, your clinician can discuss whether slowing your dose escalation may be appropriate
- Ongoing access to your care team for questions and concerns between scheduled appointments
Our aim is to help you get the best possible results from Mounjaro while minimising discomfort along the way. If you are interested in starting a supported weight loss programme in the UK, visit our weight loss treatment page to learn more.
Key Takeaways
- Constipation is one of the most common side effects of Mounjaro, reported by 12–18% of participants in clinical trials
- It is caused by slowed gastric and intestinal motility, reduced food and fibre intake, and dehydration
- Drinking at least 2.5 litres of fluid daily is the single most important step for relief
- Prioritise high-fibre foods: raspberries, pears, baked beans, lentils, porridge oats, and wholemeal bread
- Psyllium husk and ground flaxseed are effective, gentle fibre supplements
- Magnesium citrate (200–400mg in the evening) can support both bowel function and sleep
- UK pharmacy options include Fybogel, lactulose, and Dulcoease for additional relief
- Two kiwi fruit per day have clinical evidence supporting improved bowel regularity
- Seek medical advice if you have no bowel movement for five or more days, experience severe pain, or notice blood in your stool
- Prevention is easier than treatment — build hydration, fibre, and walking habits before or as soon as you start Mounjaro
