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 Give You Diarrhoea?
Yes, diarrhoea is a common side effect of Mounjaro (tirzepatide) in the UK. Clinical trials showed that gastrointestinal side effects, including diarrhoea, occur in a significant number of people taking this prescription weight loss injection. The good news is that these effects often improve over time as your body adjusts to the medication. Understanding what to expect helps you decide whether Mounjaro is right for you and how to manage symptoms if they occur.
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Calculate Your BMI Instantly
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.
How Common Is Diarrhoea With Mounjaro?
Diarrhoea is one of the most frequently reported side effects of Mounjaro (tirzepatide). In the SURMOUNT clinical trials — which tested Mounjaro in adults across different doses — diarrhoea affected a significant proportion of participants. According to the trial data, diarrhoea may affect up to 1 in 3 people taking Mounjaro, making it very common.
The risk increases slightly with higher doses of the tirzepatide KwikPen injection. Diarrhoea typically appears within the first few weeks of starting treatment or when your dose is increased during dose titration. Most people find that symptoms settle down after a few weeks as their body adapts to the medication. The NHS recognises diarrhoea as an expected side effect when prescribing Mounjaro through tier 3 specialist weight management services.
- Very common (affects more than 1 in 10 people)
- Usually mild to moderate in severity
- Often resolves within 2–4 weeks
- More likely during dose escalation phases
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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.
Managing Diarrhoea and When to Seek Advice
If you experience diarrhoea while taking Mounjaro, practical steps can help manage symptoms. Stay well hydrated by drinking plenty of water throughout the day, eat bland foods like toast and rice, and avoid high-fat or spicy meals which may worsen symptoms. Most people find that eating smaller, more frequent meals helps. If diarrhoea is severe, persistent for more than a week, or accompanied by signs of dehydration — such as dark urine, dizziness, or extreme thirst — contact your GP or the NHS 111 service.
Your prescriber will monitor you during follow-up appointments and can adjust your dose if gastrointestinal side effects are significantly affecting your quality of life. Never stop taking Mounjaro without medical advice, as your prescriber needs to manage any dose changes carefully. Use our free BMI calculator above to track your health progress while managing side effects.
- 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 dangerous when taking Mounjaro?
Mild to moderate diarrhoea is not dangerous, but severe or prolonged diarrhoea can lead to dehydration. This is rare with Mounjaro. Watch for signs of dehydration like dark urine, dizziness, or extreme thirst, and contact your GP or NHS 111 if concerned. Most people manage diarrhoea successfully with hydration and dietary adjustments.
How long does Mounjaro diarrhoea last?
Diarrhoea typically appears in the first few weeks of starting Mounjaro or after a dose increase during titration. For most people, symptoms settle within 2–4 weeks as the body adjusts. If diarrhoea persists beyond this period, discuss it with your prescriber — they may adjust your dose or recommend additional strategies.
Can I prevent diarrhoea when starting Mounjaro?
You cannot completely prevent it, as it's a common biological response to tirzepatide. However, you can reduce severity by starting with a low dose (2.5 mg), avoiding high-fat foods, staying hydrated, and eating smaller meals. Your prescriber will guide you through dose titration to minimise side effects.
Should I stop taking Mounjaro if I get diarrhoea?
No — do not stop without medical advice. Contact your GP or prescriber instead. They can assess whether your symptoms need managing differently, whether dose adjustment is necessary, or whether Mounjaro remains appropriate for you. Stopping suddenly without guidance is not recommended.
What's the difference between Mounjaro and Wegovy diarrhoea?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) commonly cause diarrhoea. Mounjaro uses a dual GIP and GLP-1 mechanism, while Wegovy uses GLP-1 alone, but both affect gut function similarly. Individual responses vary significantly — some people experience more diarrhoea with one than the other, which is why your prescriber assesses what suits you best.
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.