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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Sulphur Burps and Diarrhoea on Mounjaro — What You Need to Know
Sulphur burps (belching with an unpleasant rotten egg smell) and diarrhoea are among the most commonly reported side effects of Mounjaro (tirzepatide) in the UK. These digestive effects occur because Mounjaro slows how quickly food moves through your stomach, which can lead to gas build-up and changes in bowel habits. Understanding what to expect, how long these effects typically last, and when to seek medical advice can help you manage your Mounjaro prescription more confidently. Most people find these side effects improve within the first few weeks of treatment.
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Enter your height and weight below. Supports both imperial (stone & lbs) and metric (kg). Results include NHS category, healthy weight range, and ethnicity adjustments.
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 Causes Sulphur Burps and Diarrhoea
Mounjaro works by activating GIP and GLP-1 receptors in your gut, which slows gastric emptying — the rate at which food leaves your stomach. This delay in digestion can cause several effects:
- Gas accumulation: Food spends longer in your stomach, giving bacteria more time to produce gas, which escapes as burps with a sulphurous smell
- Altered gut motility: Changes to how your intestines contract can lead to diarrhoea, constipation, or alternating bowel habits
- Bacterial fermentation: Slower digestion means more fermentation of undigested food, producing hydrogen sulphide — the characteristic rotten egg odour
These effects are usually most pronounced in the first 4–8 weeks as your body adjusts to the medication. When you increase your tirzepatide dose (following the standard dose titration schedule), you may temporarily experience these symptoms again as your digestive system adapts.
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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 Sulphur Burps and Diarrhoea — Practical Tips
Most people find symptoms improve significantly after 2–4 weeks as their body adjusts. In the meantime, try eating smaller, more frequent meals rather than large portions — this reduces the amount of food your stomach has to process slowly. Avoid high-fat and spicy foods, which can worsen both symptoms. Drinking plenty of water throughout the day supports digestion and helps prevent dehydration from diarrhoea.
Taking Mounjaro from a pre-filled KwikPen as prescribed is important — never adjust your dose to try to reduce side effects. If sulphur burps or diarrhoea are severe, persistent, or accompanied by stomach pain, contact your GP. Your prescriber may suggest temporary dietary adjustments or, rarely, a pause in dose escalation while your body acclimatises.
Track how your symptoms change over time using our free progress tracker — logging your side effects alongside your weight helps you and your healthcare team spot patterns and see when symptoms typically improve. This information is useful to share with your GP at follow-up appointments.
Remember: sulphur burps and diarrhoea are very common on Mounjaro and do not mean the medication is not working. In fact, these effects confirm that Mounjaro is active in your system. However, every person responds differently, so your experience may be milder or resolve faster than others.
- 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.
How long do sulphur burps last on Mounjaro?
Most people experience sulphur burps most intensely in the first 2–4 weeks of starting Mounjaro or increasing their dose. For many, the symptom reduces significantly or disappears by week 6–8 as the body adjusts to the medication. If burps persist beyond 8 weeks, discuss this with your GP, as dietary changes or other management strategies may help.
Is diarrhoea a normal side effect of Mounjaro in the UK?
Yes, diarrhoea is very common — it is listed as affecting more than 1 in 10 people taking Mounjaro in UK prescribing information. It typically occurs because Mounjaro slows digestion. Most people find diarrhoea improves within the first few weeks, but if it is severe or persistent, contact your GP for advice.
What foods should I avoid if I have sulphur burps on Mounjaro?
Avoid high-fat foods, spicy dishes, and foods high in sulphur (eggs, cruciferous vegetables, dairy). Eat smaller, more frequent meals instead of large portions. Stay well-hydrated and eat slowly. Keep a food diary to identify triggers specific to you — your GP or a dietitian can help.
When should I contact my GP about these side effects?
Contact your GP if sulphur burps or diarrhoea are severe, persistent beyond 8 weeks, accompanied by stomach pain, or if diarrhoea leads to dehydration. Never stop taking Mounjaro without medical advice — your GP can adjust your management plan or dose timing if needed.
Does sulphur burps mean Mounjaro is working?
Sulphur burps indicate Mounjaro is active in your system and affecting your digestion — which is how it helps with weight management. However, the presence or absence of this side effect does not necessarily predict weight loss success. Your GP will monitor your progress through weight checks and health assessments, which is what actually matters.
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.