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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Diarrhea On Mounjaro vs Wegovy — What You Need To Know
Diarrhoea is one of the most common side effects reported by people taking Mounjaro or Wegovy for weight management in the UK. Both medications can cause digestive changes, but the reasons differ slightly — Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy is a GLP-1 agonist only. Understanding how these medications work, why diarrhoea occurs, and how the side effect profiles compare can help you and your prescriber decide which option might be more suitable for your individual circumstances.
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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 Cause Diarrhoea — Mechanism Differences
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) affect how your digestive system works, but through slightly different mechanisms. Mounjaro activates both GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 pathways, while Wegovy activates GLP-1 only. Both slow gastric emptying — how quickly food moves from your stomach — which can cause digestive upset including diarrhoea. This side effect is typically most common when starting the medication or increasing the dose. According to SURMOUNT-1 trial data for Mounjaro and STEP-1 trial data for Wegovy, diarrhoea was reported in approximately 20–25% of participants on the maximum doses. The severity and frequency often decrease over time as your body adapts, though some people experience it throughout their treatment.
- Mounjaro: Dual mechanism may affect gastric motility differently; SURMOUNT trial showed diarrhoea in ~23% at 15mg dose
- Wegovy: Single GLP-1 pathway; STEP trial showed diarrhoea in ~21% at 2.4mg dose
- Timing: Most common in weeks 2–8 after starting or increasing dose; may improve by week 12–16
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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.
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Managing Diarrhoea And Choosing Between Mounjaro And Wegovy
If you experience diarrhoea on either medication, practical steps include staying well-hydrated, eating smaller meals, avoiding high-fat foods, and increasing soluble fibre intake gradually. Always inform your prescriber — they may suggest slowing your dose titration or adjusting meal patterns. The choice between Mounjaro and Wegovy depends on your individual tolerance, not just diarrhoea risk. Both are prescription-only medicines requiring a UK clinical assessment. Your prescriber will consider your full medical history, other medications (especially oral contraceptives — both medications may reduce absorption), and preference. If diarrhoea is a significant concern, discuss this openly during your consultation. Some people tolerate one medication better than the other, and your GP or specialist can help you make an informed decision based on your personal health needs, not on superiority claims.
- 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 on Mounjaro or Wegovy?
Clinical trials show similar rates — approximately 20–25% of people on maximum doses experience diarrhoea with either medication. Individual tolerance varies significantly. Some people experience it on one medication but not the other, which is why prescriber assessment is important.
How long does diarrhoea last on Mounjaro?
Diarrhoea is most common in the first 2–8 weeks after starting or increasing your dose. For many people, it improves by weeks 12–16 as your body adjusts. If it persists beyond this or worsens, contact your prescriber for guidance.
Can I prevent diarrhoea when taking these weight loss injections?
There's no assured prevention, but staying hydrated, eating smaller meals, avoiding high-fat foods, and increasing soluble fibre gradually may help. Some people find slower dose titration (taking longer to reach their target dose) reduces digestive symptoms.
Is diarrhoea a sign I should stop taking Mounjaro or Wegovy?
Not necessarily — it's a common, usually temporary side effect that often improves. However, if it's severe, persistent, or affecting your quality of life, discuss it with your prescriber. They can adjust your approach rather than stopping treatment completely.
Does my GP need to know about diarrhoea if I'm taking these medications privately?
Yes — always inform your GP about any medications you're taking and any side effects you experience. This helps your GP support your overall health and ensures they have a complete picture of your treatment, especially if you're on other medicines.
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.