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
Mounjaro and Diarrhoea vs Wegovy — What's the Difference?
Diarrhoea is one of the most commonly reported side effects when starting weight loss injections like Mounjaro (tirzepatide) or Wegovy (semaglutide). Both medications can cause digestive changes, but the frequency and severity may differ between them. Understanding how each works and what clinical trials showed can help you and your GP or prescriber make an informed decision about which might be suitable for you. Both are prescription-only medicines requiring clinical assessment before use.
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.
Clinical Trial Data — Diarrhoea Rates Compared
In the SURMOUNT-1 clinical trial for Mounjaro (tirzepatide), diarrhoea was reported in approximately 22–23% of participants taking the highest dose (15 mg). In the STEP-1 trial for Wegovy (semaglutide), diarrhoea was reported in approximately 20–21% of participants at the maximum dose (2.4 mg). Both GLP-1 and dual GIP+GLP-1 mechanisms can slow gastric emptying—how quickly food moves through the stomach—which contributes to digestive side effects. The key difference: Mounjaro activates two hormone pathways (GIP and GLP-1), whilst Wegovy activates GLP-1 only. This dual mechanism in Mounjaro may theoretically increase gastrointestinal effects, though individual responses vary considerably. Diarrhoea typically improved or resolved during dose titration or once the body adjusted to the medication, with many participants experiencing resolution within weeks to months.
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.
Which Is More Likely to Cause Diarrhoea — Mounjaro or Wegovy?
The clinical evidence suggests Mounjaro may cause diarrhoea slightly more frequently than Wegovy, though the difference is modest and both can affect digestion. However, individual experiences vary significantly—some people report no digestive issues with either medication, whilst others experience symptoms at lower doses. Diarrhoea is often dose-dependent: it may worsen as doses increase during titration. Your GP or prescriber will assess which medication is more suitable for your individual health profile, medical history, and risk factors. If diarrhoea becomes severe or prolonged, your prescriber may adjust your dose, recommend dietary changes, or suggest supportive measures. Both Mounjaro (KwikPen) and Wegovy require a UK prescription and clinical assessment—neither can be recommended to individuals without professional evaluation.
- 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 common with weight loss injections?
Yes, diarrhoea is one of the most common side effects of both Mounjaro and Wegovy. Clinical trials showed rates of 20–23% at highest doses. It's usually temporary, often improving within weeks as your body adjusts. Most people find it manageable with dietary modifications.
Does Mounjaro cause more diarrhoea than Wegovy?
Clinical trials suggest Mounjaro may cause diarrhoea slightly more often than Wegovy, but the difference is small and varies by individual. Both can cause digestive changes. Your prescriber will assess which is more appropriate for you based on your health profile and eligibility.
What should I do if I get diarrhoea on Mounjaro or Wegovy?
Contact your prescriber for guidance. They may recommend dietary changes (soft foods, increased hydration, soluble fibre), suggest over-the-counter remedies, or adjust your dose. Many people find symptoms resolve naturally over time as their body adapts.
Can I prevent diarrhoea when starting these injections?
Staying well hydrated, eating smaller, gentler meals, and avoiding fatty or high-fibre foods during the initial weeks may help. Starting at the lowest dose and titrating gradually (as recommended) also reduces the risk of severe symptoms.
How long does diarrhoea last with weight loss injections?
For most people, diarrhoea is temporary and often improves within 2–4 weeks of starting or during dose titration. Some people experience no symptoms, whilst others may have persistent mild diarrhoea that improves with dietary adjustments or medication dose optimisation.
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.