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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Diarrhoea With Mounjaro: What You Need to Know
Diarrhoea is one of the most frequently reported side effects when taking Mounjaro (tirzepatide) in the UK. It affects many people during the initial weeks of treatment and often improves over time. Understanding why it happens, how to manage it, and when to seek help from your GP is essential for staying on track with your weight management plan. This guide covers everything you need to know about diarrhoea with Mounjaro under NHS guidance.
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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 Diarrhoea Happens With Mounjaro
Mounjaro works by slowing how quickly food moves through your stomach and digestive system — a process called gastric emptying. This is one of the mechanisms that helps you feel fuller and eat less. However, this slowed movement can cause digestive changes, including diarrhoea, constipation, or loose stools.
Diarrhoea is classified as a very common side effect — meaning it may affect more than 1 in 10 people taking Mounjaro. It typically occurs during the first few weeks of treatment or when your dose increases. As your body adjusts to the medication, diarrhoea usually settles within 2–4 weeks, though some people experience it longer.
The dual GIP and GLP-1 mechanism of tirzepatide affects how your gut processes food, which can also trigger temporary looseness. Dehydration and dietary changes often make diarrhoea worse, so staying hydrated and eating bland foods during this adjustment period is important.
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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.
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Managing Diarrhoea: Practical NHS-Aligned Steps
Hydration is critical. Diarrhoea causes fluid loss, so drink plenty of water, oral rehydration solutions (such as Dioralyte), herbal teas, and broths throughout the day. Avoid caffeine and high-sugar drinks, which can worsen symptoms.
Dietary adjustments: Eat bland, low-fat, easily digestible foods like rice, boiled potatoes, plain chicken, toast, and bananas. Avoid high-fat foods, spicy meals, high-fibre foods, dairy, and alcohol until symptoms improve. Keep a food diary to identify triggers.
Pace your meals. Eat smaller, more frequent meals rather than large ones — this reduces stress on your digestive system. Your GP or NHS dietitian can provide personalised dietary guidance if diarrhoea persists beyond 4 weeks.
- 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?
For most people, diarrhoea improves within 2–4 weeks as your body adjusts to the medication. Some people experience it for longer, particularly when increasing doses. If diarrhoea persists beyond 4 weeks, contact your GP for advice — they may recommend dietary changes or temporary dose adjustments.
Should I stop taking Mounjaro if I have diarrhoea?
No — do not stop Mounjaro without speaking to your GP first. Diarrhoea is a known, usually temporary side effect and not a reason to discontinue treatment. Your prescriber can advise on managing symptoms while continuing your medication safely.
When should I contact my GP about Mounjaro-related diarrhoea?
Contact your GP if diarrhoea is severe, lasts longer than 4 weeks, or is causing dehydration (signs include dark urine, dizziness, extreme thirst). Also contact them if diarrhoea is affecting your ability to take other medications — Mounjaro may reduce how well oral medications are absorbed.
Can I take antidiarrhoeal medication like loperamide with Mounjaro?
Some over-the-counter antidiarrhoeals can be used, but always check with your GP or pharmacist first, as certain medications may interact with Mounjaro or mask signs of other issues. Your GP can recommend the safest option for your individual situation.
Is diarrhoea with Mounjaro different from diarrhoea with Wegovy?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) commonly cause diarrhoea as a side effect because they both slow gastric emptying. Mounjaro's dual GIP and GLP-1 mechanism may affect some people differently, but management strategies are broadly similar — hydration, bland foods, and GP support.
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