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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How Long Does Mounjaro Diarrhoea Last?
Diarrhoea is one of the most common side effects when starting Mounjaro (tirzepatide), but it's usually temporary. Most people experience diarrhoea during the first few weeks of treatment or after each dose increase, with symptoms typically improving within 3–7 days. However, the duration varies between individuals, and some people may experience it for longer. If diarrhoea persists beyond a week or significantly impacts your daily life, contact your GP for advice and support managing this side effect.
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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 receptors in the gut, which affects how quickly food moves through your digestive system. This is called delayed gastric emptying—your stomach takes longer to empty its contents into the small intestine. As your body adjusts to this change, diarrhoea can occur alongside other digestive side effects like nausea, constipation, and stomach discomfort.
Diarrhoea is listed as very common in the UK product information, meaning it affects more than 1 in 10 people taking Mounjaro. It's usually most noticeable during the first 4 weeks on each new dose, as your digestive system adapts. The good news is that for most people, symptoms settle as their body adjusts—typically within the first week of each dose change.
If you're experiencing persistent diarrhoea, your GP may suggest dietary adjustments or over-the-counter remedies to help manage symptoms whilst your body adapts to the medication.
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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
Several practical steps can help manage diarrhoea whilst taking Mounjaro. Stay well hydrated by drinking plenty of water, especially if experiencing loose stools, as dehydration is a key concern. Eat small, regular meals rather than large portions, and focus on bland, easily digestible foods like plain rice, toast, and boiled vegetables. Avoid high-fat and spicy foods, which can worsen diarrhoea.
Keep a food diary to identify any triggering foods specific to you. Over-the-counter remedies like loperamide (Imodium) may help, though always check with your GP or pharmacist before using these alongside Mounjaro. If diarrhoea persists beyond 7–10 days, worsens, or causes severe dehydration symptoms like dizziness or dark urine, contact your GP. They may recommend dose adjustments or temporary use of anti-diarrhoeal medication.
- 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 from Mounjaro dangerous?
Diarrhoea itself isn't dangerous, but dehydration is a concern if it's severe or prolonged. Drink plenty of fluids and contact your GP if you experience dizziness, dark urine, or symptoms lasting more than a week. Most people experience mild, short-term diarrhoea that improves within days.
Why do some people get diarrhoea and others don't?
Individual digestive systems respond differently to GLP-1 and GIP receptor activation. Genetics, existing gut health, diet, and medication dose all influence whether you experience diarrhoea. You may experience it at one dose but not another—responses vary throughout treatment.
Does diarrhoea mean Mounjaro isn't working?
No. Diarrhoea is a side effect of how Mounjaro works, not a sign that the medication isn't effective. You can experience diarrhoea and still benefit from weight loss and metabolic improvements. Side effects and effectiveness are separate outcomes.
What's the difference between Mounjaro diarrhoea and other causes?
Mounjaro-related diarrhoea typically appears within days of starting or increasing your dose and improves within a week. It's often accompanied by other Mounjaro side effects like nausea or appetite changes. If diarrhoea appears suddenly without dose changes, consider other causes and contact your GP.
Should I stop taking Mounjaro if I have severe diarrhoea?
Don't stop without medical advice—contact your GP first. They can assess whether diarrhoea is manageable with dietary changes or medication adjustments, or whether you need to pause treatment temporarily. Your prescriber should guide any changes to your dosing schedule.
Ready to take control of your weight loss journey?
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