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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How to Stop Diarrhoea on Mounjaro
Diarrhoea is one of the most common side effects when starting Mounjaro (tirzepatide), but there are practical steps you can take to manage it. This guide covers evidence-based strategies used in UK clinical practice, when to contact your GP, and how to determine whether the side effect typically improves over time. Managing digestive side effects is important for staying on your prescription safely and effectively.
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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.
Practical Strategies to Manage Diarrhoea on Mounjaro
Diarrhoea on Mounjaro often improves within the first few weeks as your body adjusts to the medication. In the meantime, several practical steps can help:
- Stay hydrated — drink plenty of water, herbal teas, and electrolyte drinks to prevent dehydration, which is especially important with diarrhoea
- Eat smaller, more frequent meals — rather than large meals, which can trigger or worsen digestive symptoms
- Choose bland, easy-to-digest foods — plain rice, toast, boiled potatoes, lean chicken, and bananas are often better tolerated
- Avoid trigger foods — fatty, spicy, high-fibre, or high-sugar foods may worsen diarrhoea; keep a food diary to identify your triggers
- Take your Mounjaro injection consistently — skipping doses can cause unpredictable side effects; consistency helps your body adjust
- Avoid alcohol and caffeine temporarily — both can irritate the digestive system and worsen diarrhoea
- Consider over-the-counter remedies — loperamide (Imodium) or bismuth subsalicylate may help, but check with your GP or pharmacist first as some can interact with medications
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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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When to Contact Your GP and NHS Support
Contact your GP or NHS 111 if you experience severe or persistent diarrhoea lasting more than a few days, signs of dehydration (dark urine, dizziness, dry mouth), bloody stools, or severe stomach pain. Your GP can assess whether you need to pause your dose titration, adjust your Mounjaro KwikPen dose, or explore alternative medications. Most patients find diarrhoea improves significantly within 2–4 weeks as their body adjusts; however, if it remains severe after a month, your prescriber may recommend a lower maintenance dose or switching to an alternative weight management option. Keep your healthcare team informed about all side effects — they can provide personalised guidance based on your specific situation.
- 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 does diarrhoea last when starting Mounjaro?
For most people, diarrhoea improves within 2–4 weeks as your body adjusts to tirzepatide. However, some people experience it longer. If diarrhoea persists beyond 4 weeks, contact your GP — your prescriber may adjust your dose or suggest alternative strategies tailored to your situation.
Can I take anti-diarrhoea medication while on Mounjaro?
Over-the-counter anti-diarrhoea remedies like loperamide can be used, but always check with your GP or pharmacist first. Some medications can interact with Mounjaro or mask underlying issues. Your prescriber can recommend what's safe alongside your UK prescription.
What foods should I avoid to reduce diarrhoea on Mounjaro?
Avoid fatty, spicy, high-fibre, and high-sugar foods initially. Stick to bland options like rice, toast, boiled potatoes, and lean proteins. Caffeine and alcohol can worsen symptoms. Keep a food diary to identify your personal triggers and adjust accordingly.
Is diarrhoea a sign Mounjaro isn't working for me?
No. Diarrhoea is a common, temporary side effect, not an indicator of efficacy. Most people experience diarrhoea as their body adjusts, but it doesn't mean the medication won't help with weight management. Continue as directed unless advised otherwise by your GP.
Should I stop taking Mounjaro if diarrhoea is severe?
Do not stop without consulting your prescriber. Contact your GP or NHS 111 immediately if diarrhoea is severe, persistent, or accompanied by dehydration, bloody stools, or severe stomach pain. Your GP can advise whether to pause, adjust your dose, or continue with additional support strategies.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.