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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Why Does Mounjaro Cause Diarrhoea?
Diarrhoea is one of the most common side effects reported by people taking Mounjaro (tirzepatide) for weight management in the UK. It happens because of how the medication works in your gut, and while it can be uncomfortable, it's typically temporary. Understanding why this occurs and what you can do about it may help you manage the side effect more confidently if you're prescribed this 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.
How Mounjaro Works in Your Digestive System
Mounjaro is a dual GIP and GLP-1 receptor agonist—it activates two hormone pathways that regulate appetite and blood sugar. One key effect is that it slows gastric emptying, meaning food moves through your stomach more slowly. This slower movement can increase stool frequency and water content, leading to diarrhoea.
Additionally, Mounjaro affects how your gut processes nutrients and fluid. When you first start Mounjaro or increase your dose during the titration process, your digestive system needs time to adjust. The most common gastrointestinal side effects include nausea, vomiting, constipation, and diarrhoea—though not everyone experiences all of them.
- Diarrhoea typically occurs within the first few weeks of starting or increasing your Mounjaro KwikPen dose
- It tends to improve as your body adapts to the medication
- Diarrhoea is more common at higher doses (10mg, 15mg) than at starter doses (2.5mg, 5mg)
If you're considering Mounjaro or have been prescribed it, your GP or prescriber should have discussed these potential effects with you during your clinical assessment.
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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 and When to Seek Help
Most people find diarrhoea settles within 4–8 weeks as their body adjusts. In the meantime, staying well hydrated is essential—drink water regularly throughout the day. Eating bland, easy-to-digest foods (rice, plain chicken, toast) and avoiding high-fat or spicy foods may help.
Contact your GP if diarrhoea is severe, persistent beyond 8 weeks, or accompanied by abdominal pain, fever, or signs of dehydration. Diarrhoea can affect your electrolyte balance if it's very frequent. Your prescriber may pause your dose increase, reduce your dose temporarily, or suggest medication to manage symptoms. Never stop taking Mounjaro without medical guidance—your GP can advise the safest approach for your situation.
- 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 diarrhoea last when taking Mounjaro?
Most people find diarrhoea improves within 4–8 weeks as their body adjusts. However, some individuals experience it longer. Staying hydrated and eating bland foods can help. If it persists beyond 8 weeks or is severe, contact your GP for guidance on managing the side effect.
Can I prevent diarrhoea when starting Mounjaro?
You can't prevent it entirely, but you can minimise it by starting at the lowest dose (2.5mg) and following the recommended dose titration schedule carefully. Eating smaller, bland meals and avoiding high-fat foods while your body adjusts may also help reduce symptoms.
Is diarrhoea a reason to stop taking Mounjaro?
Diarrhoea alone is not usually a reason to stop Mounjaro, as it typically settles with time. However, if it's severe, persistent, or affecting your quality of life, your GP or prescriber can adjust your dose, pause your titration, or recommend strategies to manage it. Always consult your healthcare professional before stopping.
What's the difference between Mounjaro and Wegovy side effects?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) can cause diarrhoea and other gastrointestinal effects. Mounjaro is a dual GIP and GLP-1 agonist, while Wegovy is a single GLP-1 agonist. Some people tolerate one better than the other, but individual responses vary. Your prescriber can discuss which is more suitable for you.
Should I contact my GP about diarrhoea from Mounjaro?
Contact your GP if diarrhoea is severe, persistent beyond 8 weeks, or accompanied by abdominal pain, fever, or dehydration symptoms. Your GP can advise on dose adjustments, symptom management, or alternative options. Always report side effects during your follow-up appointments—your healthcare team is there to support you.
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