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 vs Wegovy: Understanding Diarrhoea and Other Side Effects
Diarrhoea is one of the most common side effects reported with both Mounjaro (tirzepatide) and Wegovy (semaglutide) weight loss injections. If you're considering either medication, understanding how they compare—including their mechanisms, trial data, and individual risk factors—can help you make an informed decision with your GP or prescriber. Both are prescription-only medicines that work differently, and diarrhoea frequency varies between individuals and doses.
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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 Trial Data
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist—meaning it activates two hormone pathways in the gut. Wegovy (semaglutide) activates GLP-1 only. This mechanistic difference may influence side effect profiles. In the SURMOUNT-1 trial, Mounjaro at 15 mg caused diarrhoea in approximately 22% of participants. The STEP-1 trial for Wegovy at 2.4 mg reported diarrhoea in around 20% of participants. Both figures represent "common" side effects (affecting up to 1 in 10 people). However, diarrhoea severity and duration vary widely—some experience mild, brief symptoms during dose escalation, whilst others report persistent loose stools. Dose titration schedules differ: Mounjaro starts at 2.5 mg and escalates over weeks, whilst Wegovy begins at 0.25 mg. Slower escalation may reduce gastrointestinal upset initially, though both medications require gradual dose increases managed by your prescriber.
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Which Is More Suitable for You? Prescriber Assessment Matters
Neither Mounjaro nor Wegovy is universally "better"—suitability depends entirely on your individual health profile, existing conditions, and how your body responds to the medication. Your GP or prescriber will assess your medical history, including any prior digestive issues, to determine which is appropriate. If you have a history of inflammatory bowel disease, severe gastroparesis, or chronic diarrhoea, your prescriber may recommend one over the other or advise caution. Both medications are UK prescription-only and require a clinical assessment before dispensing. Managing diarrhoea involves dietary adjustments (soluble fibre, adequate hydration), slower dose escalation, and close communication with your prescriber. Use our free GLP-1 weight loss calculator to explore projected outcomes, and discuss side effect tolerability openly during your consultation.
- 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 worse with Mounjaro or Wegovy?
Clinical trial data shows both cause diarrhoea in roughly 20–22% of participants. Individual experiences vary significantly—some tolerate one better than the other. Your prescriber can assess your personal risk based on your medical history and previous medication responses.
How long does Mounjaro diarrhoea last?
For many people, diarrhoea improves as the body adjusts, often within 1–2 weeks of starting a new dose. If symptoms persist beyond this, inform your GP or prescriber. Dietary changes like increasing soluble fibre and hydration can help manage symptoms.
Can I switch from Mounjaro to Wegovy if I develop diarrhoea?
Switching between medications is a clinical decision made by your prescriber, not something to do independently. Discuss persistent side effects with your GP—sometimes dose adjustment or lifestyle changes help before switching is needed.
Are there diarrhoea management strategies whilst on Mounjaro or Wegovy?
Yes. Stay hydrated, eat soluble fibre (oats, lentils), avoid high-fat foods, and eat slowly. Anti-diarrhoeal medication like loperamide may help temporarily, but always consult your prescriber before taking additional medicines.
Does diarrhoea mean the medication is working?
Side effects don't indicate effectiveness. Diarrhoea is a known side effect of GLP-1 and dual GIP/GLP-1 medications but isn't necessary for weight loss. Many people experience significant weight loss with minimal digestive side effects.
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