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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Explosive Diarrhoea with Mounjaro vs Wegovy — What You Need to Know
Gastrointestinal side effects, including diarrhoea, are common when starting tirzepatide (Mounjaro) or semaglutide (Wegovy). Both weight loss injections can cause digestive upset, but the frequency, severity, and timing differ between the two medications. This guide compares how these prescription treatments affect your digestive system, what clinical trials showed, and when to seek help from your GP. Understanding what to expect helps you manage side effects more effectively.
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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.
Mounjaro and Wegovy Diarrhoea — Clinical Trial Evidence
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist. In the SURMOUNT-1 clinical trial, diarrhoea was reported as a very common side effect, affecting more than 1 in 10 participants. Diarrhoea typically occurs in the first few weeks as your body adjusts to the medication, particularly during dose escalation phases (every 4 weeks).
Wegovy (semaglutide) is a GLP-1 receptor agonist only. In the STEP-1 clinical trial, diarrhoea was also very common, affecting similar proportions of users. However, some evidence suggests the timing and intensity may differ slightly due to Wegovy's single-mechanism action versus Mounjaro's dual mechanism.
- Both medications slow gastric emptying (how quickly food leaves your stomach), which contributes to digestive changes
- Diarrhoea often improves after 4–8 weeks as your digestive system adapts
- Dose titration (gradual increase every 4 weeks) can worsen symptoms temporarily
- Individual responses vary significantly — some users experience mild symptoms, others more severe
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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 and Choosing the Right Medication for You
If you experience explosive or persistent diarrhoea, speak to your GP or prescriber immediately. They may suggest staying on your current dose longer before increasing, switching to a lower dose, or adjusting other medications. Use our free GLP-1 weight loss calculator to understand potential benefits alongside side effect considerations.
Neither medication is "better" for diarrhoea — suitability depends on your individual health, medical history, and how your body responds. A prescriber will assess which tirzepatide (Mounjaro) or semaglutide (Wegovy) UK prescription is appropriate after a clinical assessment. Both require ongoing monitoring. Staying hydrated, eating smaller meals, and keeping a food diary can help manage GI symptoms while your body adjusts.
- 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 with Mounjaro worse than with Wegovy?
Clinical trials show both medications cause diarrhoea at similar rates (very common, affecting >1 in 10 users). Mounjaro's dual GIP+GLP-1 mechanism may cause slightly different timing of symptoms compared to Wegovy's GLP-1-only action, but individual experience varies significantly. Your prescriber can advise which suits you better.
How long does explosive diarrhoea last with Mounjaro?
Diarrhoea typically peaks in the first 2–4 weeks and often improves by week 8 as your body adjusts. However, it can recur temporarily with each dose increase during the mounjaro dose titration phase. If diarrhoea persists beyond 8 weeks or is severe, contact your GP.
Can I stop Mounjaro if diarrhoea is too severe?
Never stop abruptly without medical guidance. Speak to your prescriber or GP immediately if diarrhoea is severe or affecting your quality of life. They may adjust your dose, pause escalation, or suggest management strategies. Stopping suddenly should only be done on medical advice.
What dietary changes help with Mounjaro diarrhoea?
Eat smaller, more frequent meals. Avoid high-fat foods, excess fibre, and spicy foods temporarily. Stay well hydrated and consider soluble fibre foods like oats. Keep a food diary to identify triggers. Your GP or a dietitian can provide personalised advice tailored to your situation.
Should I choose Wegovy instead of Mounjaro to avoid diarrhoea?
Both medications carry similar diarrhoea risk. The choice between tirzepatide and semaglutide depends on your overall health, eligibility, and what a prescriber assesses as safest for you after clinical evaluation. Diarrhoea alone should not be the deciding factor — speak to your GP about your concerns.
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