Constipation on Mounjaro: Why It Happens and How to Manage It

Constipation on Mounjaro is one of the most frequently reported side effects of tirzepatide treatment. In the SURMOUNT clinical trials, up to 6 per cent of participants experienced constipation, making it the third most common gastrointestinal complaint after nausea and diarrhoea. While it is rarely dangerous, it can be uncomfortable enough to affect your quality of life and, if left unmanaged, may discourage you from continuing treatment.

The good news is that constipation on Mounjaro is usually manageable with straightforward dietary and lifestyle adjustments. At CutKilo, our doctors monitor every patient for side effects throughout treatment and can offer tailored advice if simple measures are not enough. This guide explains why tirzepatide causes constipation, what you can do about it, and when to seek medical advice.

Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist. One of GLP-1’s key effects is slowing gastric emptying, which means food moves more slowly through your entire digestive tract. This delay gives your intestines more time to absorb water from the stool, making it firmer and harder to pass. The effect is dose-dependent: it tends to be more noticeable after dose increases and often improves as your body adjusts over two to four weeks.

In the SURMOUNT-1 trial (NEJM, 2022), constipation was reported by 5.6 per cent of participants on the 15 mg dose compared with 1.2 per cent on placebo. The SURMOUNT-2 trial in people with type 2 diabetes showed similar rates. According to the MHRA-approved SmPC for tirzepatide, constipation is classified as a “common” adverse reaction (occurring in 1 to 10 per cent of patients).

Importantly, most cases were mild to moderate and resolved without stopping treatment. Severe constipation leading to discontinuation was rare in all SURMOUNT trial arms.

When you inject tirzepatide, the GLP-1 receptor activation sends signals to the smooth muscle of your stomach and intestines, telling them to slow down. This is the same mechanism that reduces appetite and helps you feel full for longer after eating. Slower motility through the colon allows more water to be reabsorbed, producing drier, harder stools that are more difficult to pass.

There is also emerging evidence, published in Gut (2023), that GLP-1 receptor agonists may influence the gut microbiome composition during treatment, which could contribute to changes in bowel habit. However, this research is still in early stages, and the primary driver of constipation remains the direct effect on motility.

Increasing your fibre intake is the single most effective dietary change. Aim for 25 to 30 grams per day from whole foods: oats, berries, flaxseeds, lentils, broccoli, and sweet potatoes are all excellent sources. If your appetite is reduced on Mounjaro (as it often is), focus on fibre-dense foods rather than large volumes. A tablespoon of ground flaxseed stirred into yoghurt or porridge adds roughly 3 grams of fibre without needing a big meal.

Hydration is equally important. When you increase fibre without increasing fluid, constipation can actually worsen. Aim for at least 1.5 to 2 litres of water per day. Herbal teas, diluted squash, and water-rich foods like cucumber and watermelon all count. If you are following the fibre and hydration guidance for GLP-1 therapy already on the CutKilo blog, you are on the right track.

Regular physical activity stimulates peristalsis, the wave-like contractions that move food through your digestive system. Even a 20-minute brisk walk each day can make a meaningful difference. You do not need intense exercise: gentle movement, stretching, and yoga-style core work all promote gut motility.

Try to establish a consistent toilet routine. The gastrocolic reflex (the urge to open your bowels after eating) is strongest in the morning, so sitting on the toilet for five to ten minutes after breakfast, without straining, can help train your body into a regular pattern. Elevating your feet on a small stool brings your knees above your hips, which straightens the anorectal angle and makes passing stools easier.

If dietary and lifestyle changes are not enough after two weeks, an over-the-counter osmotic laxative such as macrogol (Laxido or Movicol in the UK) can help. Macrogol works by drawing water into the bowel, softening the stool. It is generally well tolerated and is recommended by NICE as a first-line treatment for chronic constipation in adults.

Avoid stimulant laxatives (such as senna or bisacodyl) for regular use, as they can cause dependency and cramping. If you need a laxative for more than a few weeks, discuss this with your CutKilo prescriber, who may consider whether a dose adjustment or additional investigation is warranted.

In rare cases, GLP-1 receptor agonists have been associated with a more serious condition called ileus, where the bowel stops moving altogether. The MHRA SmPC for tirzepatide lists intestinal obstruction as an uncommon adverse event. You should seek urgent medical attention if you have not had a bowel movement for more than five days, if you develop severe abdominal pain or bloating, if you are vomiting alongside constipation, or if you notice blood in your stool.

These symptoms do not necessarily mean you have ileus, but they require assessment. Your CutKilo doctor can advise whether to pause treatment while the cause is investigated.

Many patients find that constipation appears or worsens shortly after a dose increase and then settles within two to four weeks as the body adapts. The SURMOUNT trials used a gradual titration schedule (starting at 2.5 mg and increasing every four weeks) specifically to minimise gastrointestinal side effects. CutKilo follows a similar approach, and your prescriber will only recommend moving to a higher dose when side effects from the current dose have stabilised. If you are curious about why dose increases happen, the dose increase guide on the CutKilo blog explains the rationale in detail.

Does constipation on Mounjaro go away on its own? For most patients, yes. Constipation tends to be worst in the first two to four weeks after starting treatment or increasing a dose. As your body adjusts to the new level of GLP-1 receptor activation, bowel habits usually return closer to normal.

Can I take fibre supplements like psyllium husk on Mounjaro? Yes. Psyllium husk (Fybogel in the UK) is a soluble fibre supplement that can help soften stools. Start with a low dose and build up gradually, and always take it with a full glass of water. Taking fibre supplements without adequate fluid can worsen constipation.

Should I reduce my Mounjaro dose if constipation is severe? Do not change your dose without speaking to your prescriber. In some cases, your CutKilo doctor may recommend staying at a lower dose for longer before titrating upward, which allows the gut to adapt more gradually.

Is constipation worse on higher doses of Mounjaro? The SURMOUNT data suggest that constipation rates are slightly higher at 10 mg and 15 mg compared with 5 mg, but the difference is modest. Individual variation matters more than absolute dose in most cases.

Can probiotics help with constipation on Mounjaro? There is limited clinical evidence that probiotics meaningfully improve GLP-1-related constipation. They are unlikely to cause harm, but dietary fibre, hydration, and physical activity have a much stronger evidence base.

Constipation on Mounjaro is common, usually mild, and almost always manageable. Increasing fibre intake, staying well hydrated, and moving your body daily are the three most effective interventions. If those are not enough, an osmotic laxative such as macrogol is a safe next step. Speak to your CutKilo doctor if constipation persists beyond four weeks or if you develop any warning signs such as severe pain, vomiting, or blood in your stool.