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
Free tracker included with every download this week
Does Mounjaro Give You Diarrhoea?
Yes, diarrhoea is one of the most common side effects of Mounjaro (tirzepatide). In clinical trials, diarrhoea affected a significant proportion of users, particularly during dose escalation. It's usually temporary and manageable, but understanding what to expect helps you prepare. This guide explains why Mounjaro causes diarrhoea, how long it typically lasts, and practical strategies to manage it while using this prescription weight management injection in the UK.
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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 Does Mounjaro Cause Diarrhoea?
Mounjaro works by activating GIP and GLP-1 receptor pathways in your gut, which slows how quickly food leaves your stomach and signals fullness to your brain. This change in gastric motility—combined with increased gut sensitivity to food—commonly triggers loose stools or diarrhoea, especially when you first start or increase your dose. The effect is dose-dependent, meaning higher doses of tirzepatide may cause more pronounced symptoms. Diarrhoea typically emerges within the first few weeks of starting Mounjaro or after a dose escalation, and often improves as your body adjusts. The mechanism also affects water absorption in the intestines, contributing to softer stools. Most users find diarrhoea is temporary, but if it persists or becomes severe, your prescriber should be informed—they can advise whether to pause escalation or offer management strategies.
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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 Cost/Access in the UK
Managing Mounjaro-related diarrhoea involves staying hydrated, eating bland foods (rice, plain chicken, toast), and avoiding high-fat or spicy meals that can worsen symptoms. Some users find soluble fibre helpful, and oral rehydration solutions support fluid balance. Always speak to your GP or prescriber if diarrhoea is severe or persistent—they may adjust your dose timing or offer medication to help. When accessing Mounjaro in the UK, whether through NHS specialist weight management services or private prescription, your prescriber will discuss side effects including diarrhoea before you start. Private online consultations typically cost £20–£50, and a month's supply of Mounjaro (a KwikPen containing four weekly doses) ranges from approximately £150–£250 depending on your dose level. Use our free GLP-1 weight loss calculator to explore expected outcomes alongside understanding potential 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.
How long does Mounjaro diarrhoea last?
Diarrhoea typically improves within 2–4 weeks as your body adjusts to the medication. However, some users experience it intermittently throughout treatment. If it persists beyond a month or is severe, contact your prescriber—they may recommend dose adjustment or additional support.
Is diarrhoea a sign Mounjaro isn't working?
No. Diarrhoea is a common, expected side effect unrelated to whether Mounjaro is helping with weight loss. It occurs because of how the medication affects your gut. Weight loss results depend on many factors including diet, activity, and individual response—not on whether you experience side effects.
Can I take anti-diarrhoea medication with Mounjaro?
Some over-the-counter remedies (like loperamide) may be used short-term, but always check with your prescriber first. They'll advise what's safe alongside tirzepatide, as some medications can interact or mask important symptoms. Your GP or online prescriber can recommend safe options.
Does diarrhoea mean I should stop Mounjaro?
Not necessarily. Diarrhoea is very common and usually manageable. However, if it's severe, affecting your daily life, or accompanied by other concerning symptoms (severe pain, dehydration), contact your prescriber immediately. They'll assess whether to pause, continue, or adjust your dose.
How is Mounjaro accessed in the UK, and is cost covered by the NHS?
NHS access is through specialist tier 3 weight management services via GP referral—availability varies by region and waiting lists can be long. Private prescription is available through online consultations with UK-licensed prescribers; a month's supply typically costs £150–£250 depending on dose. Your prescriber will assess eligibility based on BMI and health conditions during consultation.
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.