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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Mounjaro Constant Diarrhoea vs Wegovy — What's the Difference?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections used in the UK, and diarrhoea is a common side effect of both. However, they work differently — Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy is a GLP-1 agonist only. This difference in mechanism may affect side effect profiles, including digestive symptoms. Understanding how each medication works can help you and your prescriber decide which might be more suitable for you. Both require clinical assessment before prescription.
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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.
How Mounjaro and Wegovy Differ in Mechanism and Side Effects
Mounjaro (tirzepatide) activates two natural hormone pathways — GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 — giving it a dual mechanism. Wegovy (semaglutide) activates GLP-1 only. This difference can influence side effect patterns. In SURMOUNT clinical trials, Mounjaro showed diarrhoea in approximately 20–22% of participants at the highest dose. In STEP trials, Wegovy showed diarrhoea in approximately 21% of participants. Both are very common side effects. The dual mechanism of Mounjaro may affect gastric emptying and fat utilisation differently, potentially leading to varied digestive experiences between individuals. Diarrhoea typically improves over time as the body adjusts, though some people experience persistent symptoms. Your prescriber may recommend dietary adjustments, increased hydration, or dose modifications if diarrhoea is severe. Both medications require a UK prescription and clinical assessment to determine suitability for your individual health profile.
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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.
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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Individual Suitability and Prescriber Assessment
Neither Mounjaro nor Wegovy is universally "better" — suitability depends on your individual health, medical history, and how your body responds. Some people tolerate Mounjaro better; others find Wegovy suits them more. UK prescribers assess eligibility based on BMI (30 or above, or 27+ with weight-related health conditions), existing medications, and any contraindications. If you experience constant diarrhoea on one medication, your prescriber may suggest switching to the other, adjusting the dose, or exploring alternative approaches. You can use our free GLP-1 weight loss calculator to understand potential outcomes with either medication, and our BMI calculator to check your current eligibility status. Open communication with your prescriber is essential — report persistent side effects promptly so adjustments can be made.
- 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 more common with Mounjaro or Wegovy?
Clinical trial data shows diarrhoea rates are similar — approximately 20–22% with Mounjaro and 21% with Wegovy. Individual experiences vary widely. Some people experience no diarrhoea; others find it significant. Your prescriber can discuss your personal risk based on your health profile.
Why do both medications cause diarrhoea?
Both Mounjaro and Wegovy slow gastric emptying (how quickly food leaves the stomach) and affect gut hormone signalling. This can increase bowel motility and change stool consistency. Side effects often improve as your body adapts over weeks or months.
Can I switch from Mounjaro to Wegovy if I have constant diarrhoea?
Yes, switching is possible — but only with your prescriber's guidance. There is no official conversion chart, but approximate equivalence exists (e.g., Mounjaro 10 mg ≈ Wegovy 1.7 mg). Your prescriber will manage the transition safely.
What can I do to manage diarrhoea whilst taking either medication?
Increase water intake, eat small frequent meals with soluble fibre, and avoid high-fat foods initially. Over-the-counter remedies like loperamide may help, but check with your GP first. Most people find symptoms improve within 4–8 weeks as tolerance builds.
How do I know which medication is right for me if both cause similar side effects?
Your prescriber will assess your full health picture — existing conditions, other medications, and personal preferences. Some people prioritise efficacy (Mounjaro showed slightly higher average weight loss), whilst others prefer the simpler single-mechanism approach of Wegovy. Both are UK prescription-only and require clinical assessment.
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