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 Severe Diarrhoea While Taking Mounjaro
Diarrhoea is one of the most common side effects experienced by people taking Mounjaro (tirzepatide) in the UK, affecting more than 1 in 10 users. While it's usually mild to moderate and tends to improve over time as your body adjusts, severe diarrhoea can be distressing and may affect your quality of life. This guide explains why diarrhoea occurs with Mounjaro, when it requires medical attention, practical steps to manage it, and how to access support through NHS GP services or private prescribing routes. Understanding what to expect and knowing when to contact your healthcare provider is essential for safe, sustainable weight management with this medication.
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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 Does Mounjaro Cause Diarrhoea and Managing Severe Symptoms
Mounjaro works as a dual GIP and GLP-1 receptor agonist, which means it activates two natural hormone pathways in your body. One key effect is that it slows gastric emptying — how quickly food moves through your stomach into your digestive system. This slowdown, combined with increased fluid secretion in the bowel, can trigger diarrhoea. For most people, diarrhoea is mild or moderate and often resolves within a few weeks as your body adapts during the dose titration process.
Severe diarrhoea — defined as frequent, watery stools that disrupt daily life or cause dehydration — is less common but can occur, particularly in the first 4–8 weeks or during dose increases. If diarrhoea is severe or persistent beyond 2 weeks, contact your GP immediately. Severe, prolonged diarrhoea can lead to dehydration and electrolyte imbalances, which require medical evaluation. Your prescriber may adjust your dose, pause treatment, or recommend dietary modifications. Never stop Mounjaro suddenly without medical guidance — always speak to the healthcare professional who prescribed it.
- Stay hydrated with water, electrolyte drinks, and broths
- Eat bland, low-fibre foods (white rice, plain chicken, toast)
- Avoid high-fat, high-fibre, and dairy-heavy meals
- Keep a symptom diary to discuss patterns with your GP
Clinically BackedWeight Loss
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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.
Accessing Support: NHS and Private Prescription Routes in the UK
If you're experiencing severe diarrhoea, accessing timely support depends on your prescription route. NHS route: Contact your GP or specialist weight management service immediately — they can assess your symptoms, check for dehydration, and adjust your dose or medication if needed. Private route: If you obtained Mounjaro through a private online prescriber, contact them directly with your symptoms — they will advise whether to continue, pause, or modify your dose. Both routes require clinical assessment; never self-adjust a Mounjaro KwikPen dose. Your prescriber will help you decide whether to continue at your current dose during the next injection, wait before the next dose, or switch to a different medication. Our free GLP-1 weight loss calculator can help you understand projected outcomes, but your healthcare provider's guidance on managing side effects always takes priority.
- 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 severe diarrhoea on Mounjaro dangerous?
Mild to moderate diarrhoea is common and usually resolves within 2–4 weeks. Severe diarrhoea is less common but can lead to dehydration and electrolyte imbalances, which require medical attention. Contact your GP if diarrhoea is severe, persistent beyond 2 weeks, or accompanied by dizziness, weakness, or dark urine — these signs suggest dehydration.
How long does diarrhoea last on Mounjaro?
For most people, diarrhoea improves within 2–4 weeks as your body adjusts during dose titration. Some experience it only during dose increases. However, if diarrhoea persists beyond 2–3 weeks or is severe, speak to your prescriber — they may adjust your dose or recommend dietary changes to help manage it.
Can I stop taking Mounjaro if the diarrhoea is too severe?
Do not stop Mounjaro suddenly without medical guidance. Contact your GP or the healthcare professional who prescribed it immediately. They can assess your symptoms, recommend management strategies, adjust your dose, pause treatment temporarily, or explore alternative medications. Stopping abruptly without clinical support is not safe.
What foods should I eat to help with Mounjaro-related diarrhoea?
Eat bland, low-fibre, low-fat foods: white rice, plain chicken, toast, bananas, and boiled potatoes. Avoid high-fibre foods (beans, whole grains), dairy, fatty foods, and high-sugar items. Drink plenty of water and electrolyte drinks. Keep a food diary to identify triggering foods and discuss patterns with your GP.
Is severe diarrhoea a reason to switch from Mounjaro to Wegovy?
Possibly. Wegovy (semaglutide), a GLP-1 agonist, has a different mechanism than Mounjaro's dual GIP/GLP-1 action and may cause less diarrhoea in some people, though diarrhoea remains a common side effect of all GLP-1 medications. Only your prescriber can advise whether switching is appropriate for you based on your individual health profile and symptom severity.
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.