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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Diarrhea With Mounjaro — What You Need to Know
Diarrhoea is one of the most common side effects reported by people taking Mounjaro (tirzepatide) in the UK. It affects the digestive system as the medication works, and it's usually temporary. This NHS-aligned guide explains why diarrhoea happens with Mounjaro, how long it typically lasts, practical management strategies, and when you should contact your GP. Understanding what to expect helps you manage the side effect confidently while your body adjusts to the medication.
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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 Mounjaro Causes Diarrhoea and How Long It Lasts
Mounjaro works as a dual GIP and GLP-1 receptor agonist, which means it affects how your digestive system processes food. The medication slows gastric emptying—how quickly food moves through your stomach—and this can trigger loose stools or diarrhoea in the first few weeks of treatment or after dose increases. Diarrhoea is classified as a "very common" side effect, affecting more than 1 in 10 people taking Mounjaro.
In most cases, diarrhoea improves within 2–4 weeks as your body adjusts to the medication. However, some people experience it for longer, particularly if they increase their tirzepatide dose. The severity varies—some notice mild looseness, while others have more frequent episodes. Keeping a symptom diary can help you identify patterns and share this information with your GP or prescriber.
- Usually appears in the first 1–2 weeks after starting or increasing dose
- Typically resolves within 2–4 weeks with continued use
- May return temporarily after dose escalation
- Severity varies between individuals
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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 Diarrhoea While Taking Mounjaro
Practical steps to manage diarrhoea while taking Mounjaro include dietary adjustments, hydration, and knowing when to seek medical advice. Eat smaller, more frequent meals rather than large portions, as smaller amounts are easier to digest. Avoid high-fat, greasy, or very fibrous foods temporarily—these can worsen loose stools. Stay well hydrated by drinking plenty of water and sugar-free fluids, as diarrhoea can lead to fluid loss. If diarrhoea is severe, persistent beyond 4 weeks, or accompanied by severe abdominal pain, contact your GP immediately. Your prescriber may adjust your dose timing or provide anti-diarrhoeal guidance. You can also use our free GLP-1 weight loss calculator to track your progress and discuss results with your healthcare team.
- 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.
How common is diarrhoea with Mounjaro?
Diarrhoea is very common with Mounjaro, affecting more than 1 in 10 people. It's one of the most frequently reported gastrointestinal side effects and typically improves within 2–4 weeks as your body adjusts to the medication and the tirzepatide dose reaches a stable level.
When should I contact my GP about diarrhoea with Mounjaro?
Contact your GP if diarrhoea is severe, lasts longer than 4 weeks, causes significant dehydration, is accompanied by severe abdominal pain spreading to your back, or if you're unable to take other medicines by mouth due to the digestive disruption. Your GP can advise on symptom management or dose adjustment.
What's the difference between diarrhoea from Mounjaro and other GLP-1 injections like Wegovy?
Both Mounjaro and Wegovy can cause diarrhoea because both affect gut hormone pathways, but Mounjaro uses a dual GIP and GLP-1 mechanism while Wegovy is GLP-1 only. Clinical trials suggest diarrhoea frequency is similar between them, though individual experiences vary. Your prescriber can discuss which medication suits you best.
Can I take anti-diarrhoeal medicine while on Mounjaro?
Some anti-diarrhoeal medicines can be used, but always check with your GP or prescriber first. They need to ensure any additional medication doesn't interact with Mounjaro or affect its absorption. Your GP can recommend safe options suitable for your specific situation.
What's the NHS position on managing Mounjaro side effects?
The NHS recommends discussing any side effects with your GP or the specialist weight management service (tier 3) where Mounjaro is prescribed. They can provide personalised management advice, monitor your response, and adjust your treatment plan if needed. MHRA monitoring of Mounjaro includes ongoing safety assessments of gastrointestinal side effects.
Ready to take control of your weight loss journey?
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