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 Sulphur Burps and Diarrhoea vs Wegovy
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only GLP-1 based weight loss injections available in the UK, and both commonly cause gastrointestinal side effects including sulphur burps and diarrhoea. However, they work through different mechanisms — Mounjaro is a dual GIP and GLP-1 receptor agonist, whilst Wegovy is a GLP-1 agonist only. This difference may influence how often and severely these side effects occur, though individual responses vary significantly. Understanding how they compare can help you and your prescriber decide which medication might suit you best.
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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 These Side Effects
Mounjaro (tirzepatide) activates two hormone pathways — GIP and GLP-1 — which both slow gastric emptying and affect appetite regulation. The dual mechanism may intensify gastrointestinal effects in some people, including sulphur burps and diarrhoea, particularly during dose titration. SURMOUNT clinical trials showed very common gastrointestinal side effects, with diarrhoea and nausea affecting more than 1 in 10 participants.
Wegovy (semaglutide) activates only the GLP-1 pathway, which is a single mechanism. STEP clinical trials similarly reported very common diarrhoea and nausea. The key difference is mechanistic — Mounjaro's dual action may produce more pronounced or prolonged gastrointestinal symptoms in some individuals, though both medications frequently cause these effects.
- Mounjaro: dual GIP+GLP-1 action — potentially stronger gastric effects
- Wegovy: single GLP-1 action — may cause less intense gut symptoms for some
- Both: sulphur burps and diarrhoea usually improve after 4–8 weeks as the body adjusts
- Both: timing of doses and diet quality can influence symptom severity
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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.
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Which Medication Might Be Better for You?
The choice between Mounjaro and Wegovy depends on your individual health profile, not just side effect comparisons. If you've experienced severe gastrointestinal symptoms in the past or have a sensitive digestive system, your GP or prescriber may lean towards Wegovy's single mechanism. Conversely, if you have type 2 diabetes or prediabetes alongside weight management goals, Mounjaro's dual GIP+GLP-1 action may offer additional metabolic benefits despite potentially stronger initial side effects.
Both medications require a UK-licensed prescriber's assessment and are available through NHS specialist tier 3 services or private online prescribing. Use our GLP-1 weight loss calculator to estimate potential weight loss with either medication, and discuss both options with your prescriber — they will assess which is most suitable for your health status, medical history, and medication interactions.
- 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.
Do both Mounjaro and Wegovy cause sulphur burps?
Yes, both medications commonly cause sulphur burps (belching with a rotten egg odour) as a very common side effect. This occurs because both slow gastric emptying and alter gut bacteria fermentation. Mounjaro's dual mechanism may intensify the effect in some individuals, though both injections typically cause this symptom, especially during the first 4–8 weeks.
Is diarrhoea worse with Mounjaro than Wegovy?
Clinical trial data shows both medications cause diarrhoea very commonly. Mounjaro's dual GIP+GLP-1 mechanism may produce more pronounced gastrointestinal effects in some people compared to Wegovy's single GLP-1 action, but individual responses vary. Your prescriber can discuss this when assessing suitability.
How long do these side effects last with either medication?
Sulphur burps and diarrhoea typically improve significantly after 4–8 weeks as your digestive system adjusts. Staying hydrated, eating smaller meals, and avoiding high-fat foods can reduce symptom severity. If symptoms persist beyond 8 weeks, contact your prescriber — they may adjust your dose or suggest alternative support.
Can I switch from Mounjaro to Wegovy if side effects are bad?
Potentially yes, but any switch between medications must be managed by your prescriber. There is no official conversion chart, but approximate equivalence exists (e.g., Mounjaro 10 mg ≈ Wegovy 1.7 mg). Your prescriber will assess whether switching is appropriate based on your symptoms and health.
Are these side effects serious?
Sulphur burps and diarrhoea are very common but typically not serious — they are uncomfortable rather than dangerous. However, persistent diarrhoea can lead to dehydration or nutrient loss. Seek urgent medical attention only if you experience severe abdominal pain (possible pancreatitis) or signs of serious dehydration.
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