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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Managing Mounjaro Diarrhea: An NHS-Aligned UK Guide
Diarrhoea is one of the most common side effects of Mounjaro (tirzepatide), affecting a significant proportion of people taking this prescription weight management injection. If you're experiencing loose stools or frequent bowel movements while on Mounjaro, you're not alone. This guide explains what causes it, how long it typically lasts, practical management strategies, and when to contact your GP for medical advice. Understanding this side effect helps you stay informed and make informed decisions about your treatment with your healthcare provider.
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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 your gut, which slows how quickly food moves through your digestive system and affects how your intestines absorb and process nutrients. This mechanism is essential for the medication's weight loss effect, but it can trigger diarrhoea in some people.
How it develops: Diarrhoea typically appears within the first 1–2 weeks after starting Mounjaro or increasing your dose. The NHS notes that this side effect often improves over time as your body adjusts, though the timeline varies between individuals. Approximately 21% of people taking Mounjaro report diarrhoea during clinical trials, making it a very common (but not universal) side effect.
The diarrhoea is usually mild to moderate and tends to resolve within 2–4 weeks of dose adjustment or continued use. However, if it persists or becomes severe, your GP or prescriber can advise on dose timing, dietary modifications, or whether continuing treatment is appropriate for your individual situation.
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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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Practical Management and When to Seek Help
Self-management strategies: Staying hydrated is essential—diarrhoea increases fluid loss. Drink water throughout the day and consider electrolyte drinks if you're having frequent loose stools. Eating small, frequent meals rather than large ones, avoiding high-fat and high-fibre foods initially, and taking Mounjaro at a consistent time each week may help your digestive system adapt.
When to contact your GP: Seek medical advice if diarrhoea is severe, persists beyond 4 weeks, causes you to feel faint or dizzy (signs of dehydration), or interferes significantly with your daily life. Your GP can check for other causes, advise on management, or discuss whether a dose adjustment might help. Diarrhoea alone does not mean you must stop Mounjaro—many people find it improves with patience and supportive care.
- 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?
In most cases, diarrhoea improves within 2–4 weeks as your body adjusts to the medication. Some people experience it for only a few days, while others may take longer to adapt. If diarrhoea persists beyond 4 weeks or worsens, speak to your GP or prescriber for personalised guidance.
Can I take anti-diarrhoea medication with Mounjaro?
Over-the-counter remedies like loperamide may help, but always inform your GP or pharmacist before using them. Some anti-diarrhoea medicines can interact with how Mounjaro works or mask symptoms of a more serious issue. Your prescriber can recommend safe options for your situation.
Is diarrhoea a reason to stop taking Mounjaro?
Diarrhoea alone is not usually a reason to discontinue Mounjaro, as it often improves with time and self-care. However, if it's severe, causes dehydration, or significantly affects your quality of life, speak to your GP. They can discuss dose timing adjustments, dietary changes, or whether to pause treatment temporarily.
What should I eat if I have diarrhoea while on Mounjaro?
Eat small, frequent meals of bland, easily digestible foods—rice, toast, bananas, plain chicken, and soup are good choices. Avoid high-fat, high-fibre, and spicy foods initially, as these can worsen diarrhoea. Increase fibre intake gradually once symptoms improve. Stay hydrated with water and electrolyte drinks.
Is Mounjaro diarrhoea the same as the diarrhoea caused by Wegovy?
Both are GLP-1 related medications, so diarrhoea is a common side effect with both. However, Mounjaro is a dual GIP and GLP-1 agonist, while Wegovy is a GLP-1 agonist only. Individual responses vary. Your GP can discuss which medication might be better tolerated for your body if side effects are a concern.
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