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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Sulfur Burps and Diarrhoea on Mounjaro — What You Need to Know
Sulfur burps and diarrhoea are among the most commonly reported side effects of Mounjaro (tirzepatide) in UK users. These gastrointestinal symptoms typically occur during the first few weeks of treatment or after dose increases on your KwikPen. Whilst they can be uncomfortable, they are not dangerous and often improve as your body adjusts. Understanding what causes them and how to manage them can help you stay on track with your 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.
Why Mounjaro Causes Sulfur Burps and Diarrhoea
Mounjaro works by slowing gastric emptying — the speed at which food moves from your stomach into your small intestine. This slowdown is how the medication helps you feel fuller for longer. When food stays in your stomach longer, bacteria naturally present in your gut ferment undigested food, producing gases including hydrogen sulphide — the chemical that creates that rotten egg or sulphurous smell.
Diarrhoea occurs because Mounjaro's mechanism also affects how your intestines process and absorb food and water. As your digestive system adjusts to the medication, looser stools are common. These side effects are listed as "very common" in the UK prescribing information, affecting more than 1 in 10 users during dose titration.
- Slowed stomach emptying = longer food fermentation = sulphur-smelling burps
- Altered intestinal water absorption = loose stools or diarrhoea
- Most common in weeks 1–4 and after each dose increase
- Usually improve within 2–4 weeks as your body 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.
Free Weight Loss Tools & Guides
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Managing Sulfur Burps and Diarrhoea — Practical Tips
Dietary changes often help significantly. Avoid foods that are hard to digest or high in sulphur: processed foods, cruciferous vegetables (broccoli, cabbage), eggs, red meat, and high-fat foods. Eat smaller, more frequent meals rather than large portions. Stay hydrated — diarrhoea depletes water and electrolytes, so drink plenty of water, electrolyte drinks, or herbal tea.
Consider keeping a food diary to identify which foods trigger worse symptoms. If symptoms persist beyond 4 weeks, or if diarrhoea is severe enough to affect daily life, contact your GP. They may suggest medication adjustments, additional support, or dietary recommendations from a specialist. Most users find symptoms ease significantly after the first month of Mounjaro 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.
How long do sulfur burps and diarrhoea last on Mounjaro?
For most users, these symptoms improve within 2–4 weeks as your body adjusts to the medication. They may return briefly after each dose increase on your KwikPen, but typically settle again within a week or two. If symptoms persist beyond 4 weeks, contact your GP.
Can I take anti-diarrhoea medication whilst on Mounjaro?
Whilst over-the-counter anti-diarrhoea medication (like loperamide) is available, always check with your GP or pharmacist before using it with Mounjaro. Some medications can interact or mask important symptoms. Your GP may recommend dietary changes or hydration strategies first.
Are sulfur burps and diarrhoea a sign Mounjaro isn't working?
No — these are common, expected side effects, not a sign of problems. They actually indicate your body is responding to the medication's mechanism. Side effects and weight loss effectiveness are separate. Many users experience both side effects and good weight loss results.
Should I stop taking Mounjaro if I have severe diarrhoea?
Do not stop suddenly without speaking to your GP. If you're experiencing severe diarrhoea affecting your daily life, contact your prescriber — they may adjust your dose, pause treatment temporarily, or suggest additional support. Never stop a prescription medication without professional guidance.
Does diet really help with these side effects?
Yes — significantly. Avoiding processed foods, high-fat meals, and sulphur-rich foods (eggs, cruciferous vegetables, red meat) often reduces both sulfur burps and diarrhoea. Eating smaller meals more frequently and staying well hydrated also helps your digestive system adjust more comfortably to Mounjaro.
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.