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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Managing Bowel Changes While Taking Mounjaro
Mounjaro (tirzepatide) commonly affects digestion, with constipation and diarrhoea being among the most frequently reported side effects. If you're taking Mounjaro or considering it, understanding how to manage these changes is important for your comfort and wellbeing. This guide covers practical strategies to support healthy digestion whilst on treatment, based on NHS guidance and clinical experience.
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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.
Understanding Digestion Changes on Mounjaro
Mounjaro is a dual GIP and GLP-1 receptor agonist that slows how quickly food moves through your stomach—a process called gastric emptying. This is how it helps reduce appetite, but it can also lead to constipation, particularly when starting treatment or increasing doses. Diarrhoea is also common, especially in the first few weeks as your body adjusts. These side effects typically improve over time as your body adapts to the medication.
Managing constipation: Increase fibre intake gradually (sudden increases can worsen bloating), drink 2–3 litres of water daily, stay physically active, and consider a fibre supplement if needed. Prune juice or a stool softener may help if symptoms persist.
Managing diarrhoea: Eat smaller, more frequent meals, avoid high-fat foods, stay hydrated with electrolyte drinks, and keep a food diary to identify trigger foods. Most people find diarrhoea settles within a few weeks as dose titration progresses.
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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.
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NHS Support and When to Contact Your GP
If bowel changes are severe, persistent, or affecting your quality of life, contact your GP or the prescriber who issued your Mounjaro KwikPen. Severe abdominal pain spreading to the back is rare but requires urgent medical attention—it may indicate pancreatitis and requires immediate assessment.
Your prescriber may adjust your dose, pause treatment briefly, or recommend specific dietary changes. Keep a symptom log to share with your GP, noting what you've eaten, your hydration, activity level, and any medications you're taking—some oral medicines are less well absorbed when taking Mounjaro, which can affect their effectiveness.
Remember that Mounjaro eligibility and prescribing in the UK is determined after clinical assessment, and your healthcare team can provide personalised support based on your individual circumstances.
- 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.
How long do bowel changes last on Mounjaro?
Most people experience constipation or diarrhoea most noticeably in the first 2–4 weeks. Symptoms typically improve significantly within 4–8 weeks as your body adjusts. However, some people experience ongoing mild changes. This is why keeping a symptom diary and communicating with your prescriber is helpful.
Can I take laxatives or anti-diarrhoea medication with Mounjaro?
Generally yes, but check with your GP or pharmacist first. Laxatives can interact with how quickly Mounjaro moves through your system. For diarrhoea, simple measures like hydration and dietary changes often work first. Avoid very strong over-the-counter remedies without medical guidance.
What foods should I avoid on Mounjaro?
High-fat, greasy, and heavily spiced foods often worsen both constipation and diarrhoea. Alcohol can also trigger digestion issues. Focus on lean proteins, vegetables, whole grains, and regular meals. Smaller portions help too, since Mounjaro slows stomach emptying—large meals can cause discomfort.
Does dose titration affect bowel symptoms?
Yes. When you increase your Mounjaro dose, bowel changes may temporarily worsen before improving again. This is why the standard dose titration schedule—starting at 2.5mg and increasing every 4 weeks—gives your body time to adapt. If symptoms become unmanageable, speak to your prescriber about your dosing schedule.
Is severe constipation or diarrhoea a reason to stop Mounjaro?
Not necessarily, but discuss it with your prescriber. Minor changes are common and manageable, but if bowel symptoms significantly impact your life, your GP can adjust your dose, pause treatment, or recommend specific support. Never stop Mounjaro suddenly without medical guidance.
Ready to take control of your weight loss journey?
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