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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Does Mounjaro Cause Diarrhoea? Comparing Mounjaro and Wegovy
Yes, diarrhoea is a very common side effect of Mounjaro (tirzepatide), affecting more than 1 in 10 people in clinical trials. Wegovy (semaglutide) also causes diarrhoea at similar rates. Both medications are GLP-1 receptor agonists — Mounjaro is a dual GIP/GLP-1 agonist — and both work by slowing how food moves through your digestive system, which commonly causes loose stools or diarrhoea, especially when starting treatment. This guide compares the diarrhoea risk between these two UK-prescribed weight management injections and explains what you can expect.
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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.
Mounjaro vs Wegovy: Diarrhoea in Clinical Trials
Mounjaro (tirzepatide) caused diarrhoea in clinical trials at these rates:
- Very common: more than 1 in 10 people experienced diarrhoea
- Diarrhoea often appeared in the first 4–8 weeks of treatment
- In SURMOUNT-1 trial, diarrhoea was reported across all dose levels (5 mg to 15 mg)
- Many participants reported improvement after the first month as their body adapted
Wegovy (semaglutide) showed similar diarrhoea rates in clinical trials:
- Very common: more than 1 in 10 people experienced diarrhoea
- STEP-1 trial reported diarrhoea as one of the most frequent gastrointestinal side effects
- Both medications slow gastric emptying — the rate at which food leaves your stomach
- This mechanism, whilst helping with appetite control, often causes loose stools
Neither medication is definitively "worse" for diarrhoea — individual experiences vary significantly. The KwikPen (Mounjaro's injection device) and Wegovy's auto-injector work differently, but the active ingredients' effects on the digestive system are comparable.
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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 Diarrhoea: Individual Factors and Prescriber Guidance
Diarrhoea risk depends on your individual factors: dose level (higher doses carry higher risk), how quickly your dose increases, dietary choices, and how your body responds. Many people find diarrhoea improves after 2–4 weeks as their digestive system adapts. Your prescriber can help by adjusting your dose titration schedule if diarrhoea is severe. You can also manage symptoms through dietary changes: eating smaller, more frequent meals, staying hydrated, limiting high-fat foods, and gradually introducing fibre. If you experience severe or persistent diarrhoea, contact your GP — they may recommend over-the-counter remedies or pause your dose escalation. Only your healthcare team can assess whether Mounjaro or Wegovy is more suitable for you based on your medical history and tolerability.
- 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 permanent?
No. Diarrhoea is typically temporary and often improves after 2–4 weeks as your body adapts to the medication. Most people experience it most severely in the first month. If it persists beyond 4–6 weeks or is severe, speak to your GP or prescriber — they may adjust your dose or suggest dietary changes.
Why do both Mounjaro and Wegovy cause diarrhoea?
Both medications are GLP-1 receptor agonists (Mounjaro also activates GIP receptors). They slow gastric emptying — the rate at which food moves through your stomach and digestive system. This slower movement commonly causes loose stools or diarrhoea, particularly when starting treatment or increasing your dose. The effect usually lessens over time.
Which causes more diarrhoea — Mounjaro or Wegovy?
Clinical trial data shows similar diarrhoea rates for both. SURMOUNT-1 (Mounjaro) and STEP-1 (Wegovy) both reported diarrhoea as very common (>1 in 10 people). Individual experiences vary greatly — some people tolerate one better than the other. Only a prescriber can assess which is more suitable for you following clinical assessment.
How can I manage diarrhoea while on Mounjaro or Wegovy?
Eat smaller, more frequent meals; stay well hydrated; limit high-fat and high-fibre foods initially; avoid caffeine and artificial sweeteners. Some people find anti-diarrhoeal medications helpful (ask your GP). Most importantly, speak to your prescriber if diarrhoea is severe — they can adjust your dose titration or recommend dietary strategies.
Should I stop my injection if I get diarrhoea?
No — do not stop without speaking to your prescriber. Diarrhoea usually improves as your body adapts. However, if it's severe or affecting your daily life, contact your GP immediately. They may pause your dose increase, suggest dietary changes, or recommend remedies. Your prescriber will help you decide whether to continue, adjust, or switch medications.
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