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 Diarrhoea And Sulphur Burps — How It Compares To Wegovy
Diarrhoea and unusual digestive symptoms are among the most commonly reported side effects when starting tirzepatide (Mounjaro) or semaglutide (Wegovy) in the UK. Both weight loss injections work on similar appetite-regulating pathways, so both can cause these effects — but the frequency and severity may differ. Understanding how these medications compare can help you make an informed decision with your prescriber about which might suit you better.
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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 Digestive Side Effects
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are GLP-1 receptor agonists — Mounjaro is technically a dual GIP/GLP-1 agonist, meaning it activates two hormone pathways instead of one. Both medications slow gastric emptying, the speed at which food leaves your stomach, which can trigger nausea, diarrhoea, constipation, and changes in digestion. Sulphur burps — burps with an unpleasant rotten egg smell — are linked to slower digestion and can occur with either medication, though they're more commonly reported with Mounjaro in clinical practice and patient reports.
Clinical trial data: In the SURMOUNT trials (Mounjaro), diarrhoea affected approximately 23% of participants at the 15mg dose. In the STEP trials (Wegovy), diarrhoea affected approximately 20% at the 2.4mg maximum dose. Sulphur burps were not formally measured in trials but are documented in post-launch safety data. These digestive effects typically improve over weeks as your body adjusts to the medication and dose titration slows.
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Which Is More Likely To Cause These Symptoms?
Evidence suggests Mounjaro may cause slightly more frequent gastrointestinal upset than Wegovy, particularly at higher doses, because it has a dual mechanism activating both GIP and GLP-1 pathways. However, individual responses vary significantly — some people experience minimal symptoms with Mounjaro whilst others struggle with Wegovy. Your prescriber will assess your medical history and may recommend one over the other based on your individual suitability.
Both medications are prescription-only and require a clinical assessment before being prescribed. Your GP or licensed prescriber can discuss these potential side effects with you and help decide which weight loss injection, if either, is appropriate for your circumstances. If you experience severe or persistent diarrhoea, stomach pain, or concerning symptoms, contact your GP or prescriber immediately.
- 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 worse with Mounjaro than Wegovy?
Clinical trials suggest Mounjaro may cause slightly more frequent diarrhoea, particularly at higher doses, because it activates two hormone pathways rather than one. However, individual responses vary — some people tolerate Mounjaro well whilst experiencing more issues with Wegovy. Your prescriber can help assess which is more suitable for you.
What causes sulphur burps on these injections?
Sulphur burps occur because both medications slow stomach emptying, allowing food to sit longer in your digestive system. Slower digestion can produce hydrogen sulphide gas, creating that rotten egg smell. These typically improve as your body adjusts, and are usually temporary.
How long do these side effects last?
Digestive side effects, including diarrhoea and sulphur burps, often improve within 2–4 weeks of starting a new dose or as your body adjusts to the medication. Staying hydrated, eating smaller meals, and avoiding fatty or spicy foods can help. If symptoms persist beyond 4 weeks, discuss with your prescriber.
Can I reduce my dose if side effects are too severe?
Yes — your prescriber can adjust your dose or slow your titration schedule if side effects are intolerable. Both Mounjaro and Wegovy use a gradual dose escalation plan, and staying at a lower dose for longer may reduce gastrointestinal symptoms whilst still providing weight loss benefit.
Should I choose Wegovy over Mounjaro to avoid these symptoms?
Not necessarily — whilst Mounjaro may cause slightly more frequent diarrhoea in trials, individual responses are unpredictable. Some people experience minimal symptoms with Mounjaro but significant symptoms with Wegovy. Your prescriber will assess your health and recommend the most suitable option based on your individual circumstances and medical history.
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