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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Diarrhea After Mounjaro — Common Questions Answered
Diarrhoea is one of the most frequently reported side effects when starting Mounjaro (tirzepatide), a prescription-only weight management injection. It typically occurs as your body adjusts to the medication and usually improves over time. This guide explains why diarrhoea happens with Mounjaro, how long you can expect it to last, and when you should contact your GP. Understanding what to expect helps you manage the adjustment period more confidently.
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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 hormone pathways that regulate appetite and blood sugar. One effect of these pathways is slowing gastric emptying — how quickly food moves through your stomach and digestive system. This can initially cause gastrointestinal changes including diarrhoea, constipation, nausea, and indigestion.
The diarrhoea is not a sign that Mounjaro is harming you — it's a normal physiological response as your digestive system adjusts. Most people experience improvement within the first few weeks as their body adapts. Additionally, if you're making dietary changes (eating less food or changing what you eat) as part of your weight loss journey, this can contribute to temporary digestive changes.
- Digestive adjustments typically reduce after 2–4 weeks
- Staying hydrated is particularly important during this period
- Dietary changes alongside the medication can also affect digestion
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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
If you experience diarrhoea after starting Mounjaro, stay well hydrated by drinking plenty of water and consider electrolyte drinks if loss is significant. Eat bland, easily digestible foods where possible, and avoid high-fat or spicy meals which may worsen symptoms. Most cases resolve without intervention, but if diarrhoea persists beyond 2–3 weeks, becomes severe, or is accompanied by severe abdominal pain, fever, or bloody stools, contact your GP immediately.
Your prescriber may also adjust your dose or provide additional guidance. Never stop Mounjaro without medical advice. Use our GLP-1 weight loss calculator above to understand expected outcomes — managing side effects is part of the weight loss journey with these medications.
- 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 a common side effect of Mounjaro?
Yes. Clinical data shows diarrhoea affects more than 1 in 10 people taking Mounjaro. It's one of the most frequently reported gastrointestinal side effects, particularly when starting the medication or increasing the dose. It typically improves within the first few weeks as your body adjusts.
How long does diarrhoea last after starting Mounjaro?
Most people experience improvement within 2–4 weeks as their digestive system adapts to the medication. However, individual timelines vary. If diarrhoea persists beyond 3–4 weeks or becomes severe, contact your GP. They may recommend dietary adjustments or other supportive measures.
What's the difference between Mounjaro diarrhoea and Wegovy diarrhoea?
Both Mounjaro and Wegovy (semaglutide) can cause diarrhoea, but Mounjaro is a dual GIP and GLP-1 agonist whereas Wegovy is a GLP-1 agonist only. The mechanisms are slightly different, but gastrointestinal side effects are common with both. Your prescriber can discuss which medication might suit you best.
Can I take anything to stop diarrhoea while on Mounjaro?
Before taking any medication, speak to your GP or pharmacist. Some over-the-counter remedies may interact with Mounjaro or mask important symptoms. Your GP can advise on safe options. Dietary changes and hydration are usually recommended first.
Should I stop taking Mounjaro if I get diarrhoea?
No. Never stop Mounjaro without medical advice, even if you experience diarrhoea. Contact your prescriber or GP instead. Diarrhoea is usually temporary and manageable. Your prescriber can offer practical advice or, if necessary, adjust your dose to help manage symptoms while you continue treatment.
Ready to take control of your weight loss journey?
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