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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Mounjaro Diarrhoea — What You Need to Know
Diarrhoea is one of the most common side effects of Mounjaro (tirzepatide), a prescription-only weight management injection used in the UK. Many people taking Mounjaro experience loose stools or diarrhoea, particularly in the first few weeks after starting or increasing their dose. This guide explains why this happens, what to expect, and when to seek advice from your GP. Most cases are mild and improve over time, but it's important to understand how to manage it safely.
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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
Mounjaro is a dual GIP and GLP-1 receptor agonist — it works by activating two natural hormone pathways in your body that help regulate appetite and blood sugar control. One of the effects of this mechanism is that it slows how quickly food passes through your stomach (gastric emptying). However, as your digestive system adjusts to the medication, this can initially cause changes in bowel movements, including diarrhoea.
Additionally, Mounjaro may affect how your gut processes dietary fats and other nutrients, which can contribute to loose stools. This is particularly common when you first start the medication or when your dose is increased. The good news is that for most people, diarrhoea improves significantly after the first 4–8 weeks as your body adapts.
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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 While Taking Mounjaro
Dietary adjustments can help manage diarrhoea effectively. Eat smaller, more frequent meals rather than large ones; avoid high-fat and overly processed foods; stay well hydrated with water and electrolyte drinks; and increase soluble fibre gradually (oats, bananas, rice). Keep a food diary to identify which foods trigger worse symptoms.
If diarrhoea is severe or persistent beyond 8 weeks, contact your GP or the prescriber who issued your Mounjaro prescription. They may recommend over-the-counter anti-diarrhoea medication or adjust your dose. Never stop taking Mounjaro without medical guidance. Use our GLP-1 weight loss calculator to understand how dose adjustments might affect your treatment plan.
- 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, affecting more than 1 in 10 people taking Mounjaro. Clinical trial data shows it's one of the most frequently reported side effects, especially at higher doses (10mg and above). Most cases are mild to moderate and improve within the first month as your body adjusts.
How long does Mounjaro diarrhoea last?
For most people, diarrhoea peaks in the first 1–2 weeks after starting or increasing a dose, then improves significantly over 4–8 weeks as your digestive system adapts. However, some people experience ongoing mild loose stools. If it persists beyond 8 weeks, consult your GP.
Is Mounjaro diarrhoea dangerous?
Mild diarrhoea is not dangerous, but severe or prolonged diarrhoea can lead to dehydration and nutrient deficiency. This is why staying hydrated and eating balanced meals is important. If you experience severe diarrhoea, abdominal pain, or signs of dehydration (dizziness, dark urine), contact your GP immediately.
Can I take anti-diarrhoea medication with Mounjaro?
Over-the-counter anti-diarrhoea medications like loperamide (Imodium) can be used safely with Mounjaro for short-term relief. However, always inform your prescriber or GP before using them, as they may recommend specific products or dosing. Avoid long-term use without medical guidance.
What's the difference between Mounjaro and Wegovy diarrhoea?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) commonly cause diarrhoea. Mounjaro is a dual GIP and GLP-1 agonist, while Wegovy is a GLP-1-only medication — some people report similar diarrhoea rates, though individual experiences vary. Your GP or prescriber can discuss which medication may suit you best following a clinical assessment.
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