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
Free tracker included with every download this week
Explosive Diarrhoea from Mounjaro — What You Need to Know
Diarrhoea is one of the most common side effects of Mounjaro (tirzepatide), affecting many people starting this prescription weight management injection. In clinical trials, gastrointestinal symptoms including diarrhoea occurred in over 20% of users. If you're experiencing explosive diarrhoea while taking Mounjaro, you're not alone — and there are practical ways to manage it. This guide explains why it happens, how long it typically lasts, and when to contact your GP for support. Use our BMI calculator to understand your current health context while managing your treatment.
10,000+ calculations today · NHS-aligned thresholds · 100% free
Calculate Your BMI Instantly
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 Explosive Diarrhoea?
Mounjaro works as a dual GIP and GLP-1 receptor agonist, which affects how your digestive system processes food. The medication slows gastric emptying — meaning food moves through your stomach and intestines more slowly than usual. This can irritate the gut lining and increase bowel movements, particularly when you first start treatment or when your dose increases (dose titration).
Additionally, Mounjaro may change how your body absorbs nutrients and processes bile, leading to looser stools. The severity often depends on your individual sensitivity, diet, and how quickly your dose increases. Unlike some side effects that improve with time, gastrointestinal symptoms can persist, though many people report improvement after 4–8 weeks as their body adjusts.
- Onset: Usually begins within days of starting or after a dose increase
- Severity: Ranges from mild loose stools to urgent, explosive diarrhoea
- Duration: Often improves after initial adjustment period, but varies by individual
- Diet impact: High-fat foods and large meals tend to worsen symptoms
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Managing Explosive Diarrhoea While on Mounjaro
Immediate strategies: Eat smaller, frequent meals rather than three large ones. Avoid high-fat foods, spicy dishes, and excessive fibre, which can worsen diarrhoea. Stay well hydrated — you're losing fluids rapidly. Over-the-counter loperamide (Imodium) can help, but speak to your GP before using it regularly, as it may mask more serious issues.
If explosive diarrhoea is severely impacting your quality of life, contact your GP or prescriber. They may suggest slowing your dose titration, pausing dose increases temporarily, or switching to Wegovy (semaglutide) if your body tolerates GLP-1-only medications better. Keep a food diary to identify trigger foods. Many people find symptoms ease significantly after 6–12 weeks as their body adapts to the medication.
- 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 explosive diarrhoea from Mounjaro last?
For most people, gastrointestinal symptoms peak in the first 2–4 weeks and gradually improve over 4–12 weeks as the body adjusts. However, some individuals experience persistent symptoms throughout treatment. If diarrhoea doesn't improve after 8 weeks, contact your GP — your prescriber may adjust your treatment plan.
Can I stop taking Mounjaro if the diarrhoea is too severe?
Do not stop suddenly without medical advice. Contact your prescriber or GP first. They can assess whether to slow your dose increase, pause treatment temporarily, or explore alternatives. Stopping abruptly may affect your weight management plan and could cause other complications.
Are there foods I should avoid while on Mounjaro?
Yes. High-fat foods, fried meals, spicy dishes, and excess fibre tend to trigger or worsen diarrhoea. Stick to bland, low-fat options: plain chicken, rice, toast, bananas, and boiled vegetables. Eat smaller portions more frequently. Hydration is critical — drink plenty of water and electrolyte drinks to replace fluids.
Is explosive diarrhoea a sign something is seriously wrong?
Usually, no — it's a common adjustment side effect. However, seek urgent medical attention if you experience severe abdominal pain, persistent vomiting, blood in stools, signs of dehydration, or diarrhoea lasting more than 2 weeks without improvement. These could indicate complications requiring medical review.
Should I switch from Mounjaro to Wegovy if I have severe diarrhoea?
Possibly. Wegovy (semaglutide) is also a GLP-1 medication and may cause similar side effects, but some people tolerate it better than Mounjaro's dual GIP/GLP-1 action. Your prescriber can discuss this option. Any switch must be managed through a clinical assessment — never switch medications without professional guidance.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.