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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Severe Diarrhea on Mounjaro — UK Health Guide
Diarrhoea is one of the most common side effects reported by people taking Mounjaro (tirzepatide) in the UK. While most cases are mild to moderate, some individuals experience severe diarrhoea that affects daily life. Understanding what's happening, why it occurs, and when to contact your GP is essential for safe, informed management of this medication. This guide explains what you need to know.
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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 Diarrhoea Occurs on Mounjaro
Mounjaro is a dual GIP and GLP-1 receptor agonist — it activates two natural hormone pathways that regulate appetite and blood sugar. One key effect is that it slows gastric emptying, meaning food moves through your stomach and digestive system more slowly than usual. This delayed movement can trigger diarrhoea, constipation, or alternating bowel patterns as your digestive system adapts.
Additionally, Mounjaro affects gut motility and fluid secretion in the intestines, which may cause loose stools or increased bowel frequency. Most people find these effects improve within 4–8 weeks as their body adjusts to the medication. However, diarrhoea can persist or intensify, especially when dose titration occurs — each increase from 2.5 mg up to 15 mg may trigger temporary digestive disturbance. The severity also varies based on individual sensitivity and diet.
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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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Managing and When to Seek Help
For mild to moderate diarrhoea, simple dietary adjustments often help: eat smaller, more frequent meals, stay well hydrated (especially important on Mounjaro), avoid high-fat and high-fibre foods temporarily, and consider gentle foods like rice, toast, chicken, or plain fish. Speak to your GP or prescriber if diarrhoea is severe, persistent (lasting more than a week), or accompanied by signs of dehydration such as dark urine, dizziness, or extreme thirst.
Your GP may recommend over-the-counter remedies like loperamide or suggest adjusting your Mounjaro dose. In rare cases, severe diarrhoea may require temporary dose reduction or stopping the medication. Never adjust your dose without guidance. Use our GLP-1 weight loss calculator to track whether benefits justify managing side effects, and monitor your progress with our free progress tracker.
- 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 on Mounjaro dangerous?
Mild to moderate diarrhoea is a common, usually temporary side effect. However, severe or prolonged diarrhoea can lead to dehydration, electrolyte imbalance, and malnutrition. Contact your GP if diarrhoea is severe, lasts more than a week, or if you experience signs of dehydration like dizziness or dark urine.
When does diarrhoea from Mounjaro stop?
Most people experience improvement within 4–8 weeks as their body adapts. Diarrhoea may worsen temporarily during dose titration (dose increases), but typically settles within days of stabilising at each new dose. If it persists beyond 8 weeks, discuss this with your prescriber.
Can I take anti-diarrhoea medication with Mounjaro?
Yes, over-the-counter remedies like loperamide (Imodium) are generally safe alongside Mounjaro. However, always inform your GP or pharmacist of all medications. Avoid high-dose or prolonged use without medical guidance, as this may mask underlying issues.
What's the difference between Mounjaro and Wegovy side effects?
Mounjaro (tirzepatide) is a dual GIP+GLP-1 agonist, while Wegovy (semaglutide) is a single GLP-1 agonist. Both cause diarrhoea commonly, but Mounjaro's dual mechanism may produce different patterns of digestive effects. Clinical trials show both have similar gastrointestinal side effect rates overall.
Should I stop taking Mounjaro if I have severe diarrhoea?
Never stop your Mounjaro prescription without guidance from your GP or prescriber. Severe diarrhoea requires medical assessment — it may warrant temporary dose reduction, dietary changes, or other support rather than stopping entirely. Your prescriber will advise what's best for your situation.
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