The UK's most complete GLP-1 guide
Every Question Answered. Every Side Effect Explained.
- How GLP-1 medications work
- Mounjaro vs Wegovy compared
- Injection instructions
- Side effects & solutions
- What to eat on treatment
- FAQs answered
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Mounjaro and Constipation — A UK Health Guide
Constipation is a common side effect of Mounjaro (tirzepatide), affecting how quickly food moves through your digestive system. While it's usually manageable with simple lifestyle adjustments, understanding why it happens and how to ease it can make your treatment more comfortable. This guide explains the causes, practical management strategies, and when to seek help from your GP.
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Enter your height and weight below. Supports both imperial (stone & lbs) and metric (kg). Results include NHS category, healthy weight range, and ethnicity adjustments.
What Is BMI and Why Does It Matter?
Body Mass Index (BMI) is the most widely used screening tool for assessing whether your weight falls within a healthy range. Used by the NHS, GPs, and weight management services across the UK, BMI provides a quick snapshot of your weight relative to your height. The formula is simple: your weight in kilograms divided by your height in metres squared (kg/m²).
While BMI isn't a perfect measure — it doesn't account for muscle mass, bone density, or fat distribution — it remains the standard first step in identifying potential weight-related health risks including type 2 diabetes, cardiovascular disease, high blood pressure, and certain cancers.
The NHS also recommends adjusted thresholds for people of South Asian, Chinese, or Black African heritage — a BMI of 23 or above is classified as overweight, and 27.5 as obese. Our calculator includes this adjustment as a one-click toggle.
Why Mounjaro Causes Constipation
Mounjaro works as a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways to help regulate appetite and support weight loss. One side effect of this mechanism is that it slows gastric emptying — the rate at which food moves from your stomach into your small intestine. This slowdown can lead to constipation, particularly in the first few weeks of treatment or when your dose increases. Constipation is very common on Mounjaro, affecting more than 1 in 10 people in clinical trials. The good news is that this side effect often improves over time as your body adjusts to the medication. Additionally, Mounjaro may reduce your overall appetite, which can mean eating less food overall — and less food in your digestive system can also contribute to constipation if you're not mindful of your fibre intake.
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How It Works
Three simple steps to understand your weight and take action.
Enter your height and weight in our free calculator. Get your BMI, NHS category, healthy weight range, and ethnicity-adjusted thresholds instantly.
If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
Use our progress tracker to log your weight weekly, visualise your journey with charts, and stay motivated — all stored privately in your browser.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Managing Constipation While on Mounjaro
Most constipation related to Mounjaro can be managed with practical lifestyle changes. Drink plenty of water — aim for at least 6–8 glasses daily, as dehydration worsens constipation. Eat high-fibre foods gradually: soluble fibre from oats, fruits, and vegetables, plus insoluble fibre from whole grains and legumes. Increase physical activity — even a 20-minute walk daily stimulates bowel movement. Avoid foods that are high in fat or processed, as these move more slowly through your system. If these steps don't help within a few days, you can use over-the-counter laxatives (stool softeners like docusate or osmotic laxatives like lactulose), but always check the patient information leaflet first. Keep a food and symptom diary using our free progress tracker to identify patterns and share with your GP if needed.
- Dual GIP/GLP-1 receptor agonist
- Up to 22.5% body weight loss (SURMOUNT-1)
- Weekly injection, dose escalation over 20+ weeks
- Available via private prescription in the UK
- Doses: 2.5mg → 5mg → 7.5mg → 10mg → 12.5mg → 15mg
- GLP-1 receptor agonist
- Up to 15–17% body weight loss (STEP trials)
- Weekly injection, 5-step dose escalation
- NHS & private prescription available
- Doses: 0.25mg → 0.5mg → 1mg → 1.7mg → 2.4mg
Mounjaro vs Wegovy: Head-to-Head
A side-by-side comparison of the two most popular GLP-1 weight loss medications available in the UK.
| Feature | Mounjaro | Wegovy |
|---|---|---|
| Drug class | Dual GIP/GLP-1 agonist | GLP-1 agonist |
| Active ingredient | Tirzepatide | Semaglutide |
| Avg. weight loss | 15–22.5% | 12–17% |
| Key trial | SURMOUNT-1 (2022) | STEP 1 (2021) |
| Max dose | 15 mg weekly | 2.4 mg weekly |
| Escalation period | 20–32 weeks | 16–20 weeks |
| Administration | Weekly injection (pen) | Weekly injection (pen) |
| NHS availability | Private prescription only | NHS & private |
| Approx. private cost | £150–£250/month | £150–£300/month |
| Common side effects | Nausea, diarrhoea, constipation | Nausea, vomiting, diarrhoea |
Data sourced from SURMOUNT-1 and STEP 1 clinical trials. Costs are approximate and vary by provider. Always consult a prescriber.
Am I Eligible for Weight Loss Medication?
In the UK, eligibility for GLP-1 weight loss medications like Mounjaro and Wegovy is based on your BMI and overall health profile.
- BMI of 30 or above (obese)
- Age 18 or over
- No contraindicated conditions
- Committed to lifestyle changes
- BMI of 27 or above
- Plus type 2 diabetes
- Or high blood pressure
- Or obstructive sleep apnoea
Use our BMI calculator to check whether your BMI falls within the eligible range.
How We Built These Tools
Every calculator and guide on myweightcheck is built with accuracy and transparency in mind. Our BMI calculator uses the standard WHO formula with NHS-aligned category thresholds. Our GLP-1 projections are modelled on published clinical trial data from SURMOUNT and STEP studies.
All data stays on your device. We don't require sign-ups, and we don't store your health information on any server. Your progress tracker saves data locally in your browser using localStorage, so only you can see it.
We're based in the UK and our tools are designed specifically for UK users — supporting stone & lbs by default, referencing NHS guidelines, using NICE-approved medication data, and linking to GPhC-registered pharmacies.
Frequently Asked Questions
Common questions about BMI, weight loss medications, and our free UK tools.
Is constipation a serious side effect of Mounjaro?
Constipation is common but not usually serious. It typically improves within 1–2 weeks as your body adjusts. However, if you experience severe abdominal pain alongside constipation, contact your GP immediately, as this could indicate a more serious condition like pancreatitis or bowel obstruction.
How long does Mounjaro constipation last?
Most people find constipation improves after the first 2–4 weeks. It may recur temporarily when your dose increases, but generally resolves within days to a week with hydration and fibre. If constipation persists beyond 2 weeks despite lifestyle changes, speak to your GP or prescriber.
Can I take laxatives while on Mounjaro?
Yes, over-the-counter laxatives are generally safe to use alongside Mounjaro. Stool softeners (docusate) and osmotic laxatives (lactulose, polyethylene glycol) are good first-line options. Avoid stimulant laxatives unless advised by your GP, as these can cause cramping and may interact with Mounjaro's slowed digestion.
Will drinking more water really help with Mounjaro constipation?
Yes. Dehydration is a major cause of constipation, and Mounjaro users often eat less, which naturally reduces fluid intake. Drinking 6–8 glasses of water daily, especially with meals and during exercise, helps soften stool and supports regular bowel movements. Add herbal tea or warm lemon water if plain water feels monotonous.
Should I stop Mounjaro if I get constipation?
No — constipation alone is not a reason to stop Mounjaro. It's a manageable side effect that often improves with hydration, fibre, and activity. However, if constipation is severe, persistent, or accompanied by pain, contact your GP or prescriber. They can adjust your approach or, in rare cases, discuss whether continuing treatment is right for you. Compare Mounjaro with alternatives like Wegovy using our free GLP-1 weight loss calculator to understand your options.
Ready to take control of your weight loss journey?
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