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 Diarrhea vs Wegovy — What You Need to Know
Diarrhoea is a common side effect of both Mounjaro (tirzepatide) and Wegovy (semaglutide), but rates and severity differ between these two prescription weight loss injections. This guide compares how often diarrhoea occurs with each medication, explains why, and helps you understand which might be more suitable for you—though only a prescriber can assess your individual eligibility and tolerance after a full clinical assessment.
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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.
Diarrhoea Rates: Clinical Trial Data and Mechanisms
Both medications cause diarrhoea through different mechanisms. Mounjaro (tirzepatide) activates dual GIP and GLP-1 receptors, slowing gastric emptying and affecting bile acid handling, which increases diarrhoea risk. Clinical trials show diarrhoea occurs in approximately 20–23% of people on the maximum 15mg dose. Wegovy (semaglutide) activates GLP-1 receptors only, with a slightly lower reported diarrhoea incidence of around 20% on the 2.4mg maximum dose. The difference is modest—both are common side effects. Diarrhoea typically develops early during dose titration and often improves over time as your body adapts. Starting at lower doses (Mounjaro 2.5mg, Wegovy 0.25mg) and increasing gradually helps minimise gastrointestinal upset.
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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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Suitability, Individual Tolerance, and Prescriber Assessment
Whether Mounjaro or Wegovy suits you better depends on many factors: your medical history, other medications, dose tolerance, and individual response patterns. Some people experience mild diarrhoea that resolves; others find it bothersome. Neither medication is universally "better"—both require a clinical assessment from a UK-licensed prescriber before prescription. Your GP or a private prescriber will review your health, ask about digestive issues, and help decide the most appropriate option. Use our GLP-1 weight loss calculator and BMI calculator above to understand your baseline health context, then discuss results with your prescriber who can personalise advice based on your needs.
- 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 than Wegovy?
Clinical data shows rates are similar (around 20–23% on Mounjaro 15mg vs ~20% on Wegovy 2.4mg). Individual experience varies widely. Some people tolerate one better than the other. Your prescriber will assess your personal risk based on your medical history and other medications.
How long does diarrhoea last with these injections?
Diarrhoea often appears during the first few weeks of treatment and frequently improves as your body adapts over weeks to months. Gradual dose increases help minimise severity. If it persists, inform your prescriber—dietary adjustments or medication changes may help.
Can I reduce my Mounjaro or Wegovy dose if diarrhoea is severe?
Yes. If diarrhoea is troublesome, your prescriber may recommend staying on your current dose longer before increasing, or reducing to a lower dose. Never stop or adjust medication without guidance from your prescriber.
Are there other side effects that differ between Mounjaro and Wegovy?
Both share common side effects (nausea, vomiting, constipation, stomach pain). Mounjaro's dual mechanism may cause slightly different symptom patterns, but individual tolerance varies. Your prescriber will discuss all potential side effects before prescription.
Which injection is available on the NHS in the UK?
Both Mounjaro and Wegovy are licensed in the UK and available through specialist NHS tier 3 weight management services and private prescribing. NHS access varies by region and waiting times can be long. Your GP can refer you to NHS specialist services if eligible.
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