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 Causing Diarrhoea — How It Compares to Wegovy
Diarrhoea is one of the most frequently reported side effects of Mounjaro (tirzepatide), affecting many people during the first few weeks of treatment. If you're considering Mounjaro or comparing it to Wegovy (semaglutide), understanding how these medications differ in terms of gastrointestinal side effects is important. Both are prescription-only GLP-1-based weight management injections available in the UK, but their mechanisms and tolerability profiles aren't identical. This guide compares the two medications to help you understand what to expect.
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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 Side Effects
Mounjaro is a dual GIP and GLP-1 receptor agonist, meaning it activates two separate hormone pathways in the body. This dual mechanism may explain why some people experience more gastrointestinal effects with Mounjaro compared to single-mechanism medications. Wegovy contains semaglutide, a GLP-1-only receptor agonist — it works on just one pathway.
In clinical trials, diarrhoea rates were significant for both medications:
- Mounjaro (SURMOUNT trial): Diarrhoea reported in approximately 23% of people on the highest dose (15 mg)
- Wegovy (STEP trial): Diarrhoea reported in approximately 20% of people on the highest dose (2.4 mg)
The difference is modest, but Mounjaro's dual mechanism may cause slightly more frequent or pronounced gastric effects during the titration phase. Both medications slow gastric emptying — how quickly food leaves your stomach — which typically decreases over time as your body adjusts. Your GLP-1 weight loss calculator can help estimate potential outcomes, but side effect tolerance varies significantly between individuals.
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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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Which Medication Might Be Right for You
Choosing between Mounjaro and Wegovy depends on your individual health profile, not just side effects. A licensed prescriber will assess factors including your BMI, medical history, and any previous medication tolerability when deciding which is most suitable for you. Your eligibility for either medication under NHS NICE guidance requires a BMI of 30 or above, or 27 or above with weight-related health conditions.
If you experience diarrhoea with either medication, managing it often involves staying hydrated, eating smaller meals, and allowing your body time to adjust — most people find symptoms improve after the first few weeks. Your prescriber may also adjust your dose titration schedule. Use our free BMI calculator to check your starting point, and discuss any concerns about side effects openly with your healthcare provider before starting treatment.
- 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 with Mounjaro than Wegovy?
Clinical trials show similar rates — approximately 23% with Mounjaro and 20% with Wegovy at maximum doses. Individual experience varies significantly. Mounjaro's dual GIP+GLP-1 mechanism may cause slightly more pronounced gastric effects during dose escalation, but most people adjust within 2–3 weeks.
How long does diarrhoea last when taking Mounjaro?
Diarrhoea often improves after the first 2–3 weeks as your body adapts. Some people experience it intermittently throughout treatment. If it persists or worsens, contact your prescriber — they may adjust your dose titration or recommend dietary changes like smaller meals and increased hydration.
Can I take Mounjaro if I have a sensitive stomach?
If you have a history of gastrointestinal problems like IBS or inflammatory bowel disease, inform your prescriber before starting. They'll assess whether Mounjaro is suitable for you, consider Wegovy as an alternative, or manage your specific situation with a tailored approach.
What's the difference between Mounjaro and Wegovy UK prescriptions?
Both are prescription-only and available through NHS specialist tier 3 services or private prescribers following clinical assessment. Mounjaro is a tirzepatide dual agonist; Wegovy is semaglutide (GLP-1 only). Eligibility and side effect profiles differ slightly, so your GP or prescriber will recommend the most suitable option for your health.
Will diarrhoea stop if I stop taking Mounjaro?
Yes, diarrhoea typically resolves within a few days to a week after stopping Mounjaro. However, stopping weight management treatment may affect your weight loss progress. Discuss any concerns about tolerability with your prescriber rather than stopping without guidance.
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