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 and Diarrhea vs Wegovy — What's the Difference?
Diarrhea is a common side effect of both Mounjaro (tirzepatide) and Wegovy (semaglutide), but the frequency and severity can differ between the two medications. Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy is a GLP-1 receptor agonist only. Understanding how each medication affects your digestive system can help you make an informed decision alongside your prescriber. This guide compares the clinical trial data and practical considerations for both weekly weight loss injections available in the UK.
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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.
Clinical Trial Data — Diarrhea Rates in SURMOUNT vs STEP
In the SURMOUNT-1 clinical trial for Mounjaro (tirzepatide), diarrhea was reported as a very common side effect, affecting more than 1 in 10 participants. The frequency tended to decrease over time as the body adjusted to the medication, particularly after dose stabilisation. Similarly, in the STEP-1 clinical trial for Wegovy (semaglutide), diarrhea was also very common. Research suggests that both medications slow gastric emptying (how quickly food moves through your stomach), which can initially trigger loose stools. However, direct head-to-head comparison data is limited — most evidence comes from separate trials with different participant groups. Individual tolerance varies significantly; some people experience mild symptoms whilst others report more severe diarrhea. If diarrhoea persists or becomes severe, contact your GP or prescriber for guidance on dose adjustment or management strategies.
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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.
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Mechanism Differences — Why GLP-1 vs Dual GIP+GLP-1 Matters
Wegovy acts on the GLP-1 receptor only, whereas Mounjaro targets both GIP and GLP-1 receptors simultaneously. This dual mechanism in Mounjaro may affect digestive function differently — the additional GIP receptor activation can influence appetite signalling and glucose metabolism. Some evidence suggests this dual pathway might produce different gastrointestinal side effect profiles, though individual responses vary considerably. Diarrhea with GLP-1 medications is typically dose-dependent; starting at a lower dose (such as Mounjaro's 2.5 mg or Wegovy's 0.25 mg) and gradually increasing every 4 weeks may help your body adjust and reduce digestive upset. Your prescriber will personalise your dose escalation based on how well you tolerate the medication.
- 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 diarrhea worse with Mounjaro than Wegovy?
Clinical trials show both medications cause diarrhea in more than 1 in 10 people, but direct comparisons are limited. Individual experiences vary significantly — some people tolerate one better than the other. Only your prescriber can assess which is more suitable for you based on your medical history and previous medication responses.
How long does Mounjaro diarrhea last?
Diarrhea often improves within the first few weeks as your body adjusts. For many people, symptoms decrease by weeks 4–8. If diarrhea persists beyond this period or worsens, inform your GP or prescriber — dose adjustment or dietary changes may help manage it.
Can I prevent diarrhea on Mounjaro or Wegovy?
There's no assured prevention, but staying well hydrated, eating smaller meals, and gradually adjusting to lower-fibre foods may help. Avoid high-fat and high-sugar foods, as these can worsen symptoms. Always discuss dietary strategies with your prescriber or a dietitian.
Should I stop Mounjaro or Wegovy if I get diarrhea?
Do not stop medication without medical advice. Contact your GP or prescriber first — they may suggest dose adjustment, timing changes, or dietary modifications. Diarrhea often improves naturally; stopping suddenly can affect your weight loss progress and may require restarting at a lower dose.
Which injection is right for me — Mounjaro or Wegovy?
Your prescriber will assess your individual health, medical history, and suitability for each medication. Both require a UK prescription and clinical assessment. Use our free GLP-1 weight loss calculator to estimate potential outcomes, and discuss side effect tolerability with your prescriber when deciding.
Ready to take control of your weight loss journey?
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