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: Managing Diarrhea and Other Side Effects
Diarrhea is a common side effect of both Mounjaro (tirzepatide) and Wegovy (semaglutide), but the frequency and severity can differ between individuals. Both medications are prescription-only GLP-1 and GIP/GLP-1 receptor agonists used for weight management in the UK. Understanding how they compare, what clinical trials show, and how diarrhea can be managed may help you make an informed decision alongside your prescriber. This guide compares the evidence and practical management strategies for both treatments.
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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 Compare for Diarrhea
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) list diarrhoea as a very common side effect in UK clinical trials and patient data. In the SURMOUNT-1 trial for Mounjaro, diarrhoea occurred in approximately 23% of participants on the highest dose. In the STEP trials for Wegovy, diarrhoea occurred in approximately 20% of participants. The key difference lies in their mechanism: Mounjaro activates both GIP and GLP-1 receptors (dual agonist), while Wegovy activates only the GLP-1 receptor. This dual mechanism in Mounjaro may contribute slightly different side effect profiles, though individual responses vary enormously. Diarrhoea typically improves over time as the body adjusts to the medication. Severity ranges from mild and manageable to more pronounced, and not everyone experiences it. Your prescriber can discuss what to expect and how to manage symptoms if they occur.
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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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Managing Diarrhea: Practical Strategies for Both Medications
If diarrhoea develops with either Mounjaro or Wegovy, several practical steps may help: increase fibre intake gradually (soluble fibre like oats and beans can help firm stool), stay well hydrated (drink water regularly throughout the day), eat smaller, more frequent meals rather than large ones, and avoid high-fat and spicy foods which can trigger loose stools. Over-the-counter remedies like loperamide (Imodium) can provide short-term relief, though discuss this with your pharmacist first. If diarrhoea is severe or persistent, contact your GP or prescriber—dose adjustment or a temporary pause may be needed. Most people find symptoms settle within a few weeks as their digestive system adapts. Using our GLP-1 weight loss calculator can help you understand projected outcomes at different doses, which may help inform discussions with your prescriber about managing side effects.
- 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.
Which causes more diarrhea, Mounjaro or Wegovy?
Clinical trial data shows diarrhoea rates are similar (approximately 20–23% at highest doses). Individual experiences vary greatly. Some people experience no diarrhoea, while others find it significant. Your prescriber can discuss your personal risk factors and help choose the most suitable medication.
Does diarrhea from Mounjaro or Wegovy go away?
Yes, diarrhoea typically improves as your body adjusts to the medication—usually within 2–4 weeks. Some people find it resolves completely, while others experience occasional mild symptoms. Staying hydrated and eating smaller meals helps. If it persists beyond several weeks, contact your GP.
Can I take anti-diarrhea medicine with Mounjaro or Wegovy?
Over-the-counter remedies like loperamide can provide short-term relief. However, discuss this with your pharmacist or GP first, as these medications may slow digestion further. Your prescriber should be informed if diarrhoea is affecting your quality of life.
What's the difference between Mounjaro and Wegovy mechanisms?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist, while Wegovy (semaglutide) is a GLP-1 agonist only. The dual mechanism in Mounjaro may produce slightly different effects on digestion, though both commonly cause diarrhoea during the adjustment period.
How do I know which medication is right for me?
Your GP or prescriber will assess your health history, BMI, and any existing conditions to recommend the most suitable option. Both are prescription-only UK-licensed medications and require a clinical assessment. Eligibility depends on BMI and weight-related health factors, which you can check using our BMI calculator. ```
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