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 Diarrhoea — What You Need to Know
Diarrhoea is one of the most common side effects experienced by people taking Mounjaro (tirzepatide) in the UK. It typically occurs early in treatment or when your dose increases, and in most cases improves as your body adjusts. This NHS-aligned guide explains why it happens, when to seek medical advice, and practical ways to manage it while using this prescription weight management injection.
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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 Diarrhoea and How to Manage It
Mounjaro works by activating GLP-1 and GIP receptors in your gut, which slows how quickly food moves through your digestive system and helps reduce appetite. This dual mechanism can temporarily alter your digestion, causing diarrhoea as your body adjusts. The NHS notes that gastrointestinal side effects like diarrhoea are very common (affecting more than 1 in 10 people) during the first few weeks of treatment or after each dose increase.
Most cases resolve within 1–2 weeks as your system adapts. To help manage diarrhoea while taking Mounjaro:
- Stay well hydrated — drink plenty of water, herbal tea, or electrolyte drinks to replace lost fluids
- Eat bland, easily digestible foods (rice, toast, boiled chicken, bananas)
- Avoid high-fat, high-fibre, and spicy foods temporarily
- Take your Mounjaro injection on a day when you can monitor your response
- Speak to your prescriber if diarrhoea is severe or persists beyond 2 weeks
Try using our GLP-1 weight loss calculator to understand how tirzepatide works and what to expect during dose titration.
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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 Seek Medical Advice from Your GP
Contact your GP or prescriber immediately if diarrhoea is severe, bloody, or accompanied by abdominal pain — these could indicate a more serious condition. If diarrhoea persists for more than 2 weeks despite home management, or if you're becoming dehydrated (signs include dark urine, dizziness, or extreme thirst), speak to your healthcare team. Never stop taking Mounjaro without medical guidance. Your prescriber may adjust your dose timing, suggest dietary changes, or recommend over-the-counter remedies. Remember that experiencing diarrhoea does not mean Mounjaro is unsuitable — it's a normal adjustment period for most people. Use our BMI calculator to track your progress and discuss your overall health outcomes with your GP at your next appointment.
- 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 diarrhoea with Mounjaro dangerous?
Mild to moderate diarrhoea is a common side effect but not dangerous. However, severe diarrhoea leading to dehydration requires medical attention. Contact your GP if it's severe, bloody, or persistent beyond 2 weeks, or if you experience signs of dehydration like extreme thirst or dizziness.
How long does Mounjaro diarrhoea typically last?
Most people experience diarrhoea for 1–2 weeks when starting Mounjaro or increasing their dose. It usually resolves as your body adjusts to the medication. If it continues beyond 2 weeks, contact your prescriber.
Can I take anti-diarrhoea medication while on Mounjaro?
Over-the-counter anti-diarrhoea remedies like loperamide (Imodium) can be used, but discuss this with your pharmacist or GP first. They'll check it won't interact with Mounjaro or mask a more serious issue. Always follow their guidance.
What foods should I avoid to reduce diarrhoea on Mounjaro?
Temporarily avoid high-fat foods, spicy meals, high-fibre foods, caffeine, and artificial sweeteners. Stick to bland options like rice, toast, plain chicken, and bananas. Gradually reintroduce normal foods as symptoms improve.
Does diarrhoea mean Mounjaro isn't working?
No. Gastrointestinal side effects are separate from how well Mounjaro is helping with weight management. Diarrhoea is a temporary adjustment reaction, not a sign of treatment failure. Most people find their weight loss results are positive despite initial digestive symptoms.
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.