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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What Causes Sulphur Burps on Mounjaro?
Sulphur burps (rotten egg-smelling belches) are a known side effect reported by some people taking Mounjaro, though they are not listed as a common side effect in the official information. These burps occur due to how tirzepatide slows gastric emptying — the process by which food moves from your stomach into your small intestine. When food sits longer in your stomach, bacterial fermentation can produce hydrogen sulphide gas, which creates that distinctive odour. While unpleasant, sulphur burps are typically temporary and tend to improve as your body adjusts to the medication.
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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 Affects Your Digestion
Mounjaro is a dual GIP and GLP-1 receptor agonist — meaning it activates two natural hormone pathways that regulate appetite and digestion. One of its key mechanisms is slowing gastric emptying, which helps you feel fuller for longer. However, this delayed movement of food through your stomach creates the ideal environment for bacterial fermentation. When food remains in the stomach longer than usual, naturally occurring bacteria break down undigested food particles, producing gases including hydrogen sulphide — the compound responsible for the characteristic "rotten egg" smell.
This effect is particularly common in the early weeks of Mounjaro treatment, especially as your dose increases during titration. Your digestive system is adapting to a new medication, and gastric emptying normalises somewhat over time — which is why sulphur burps typically become less frequent or disappear entirely.
It's worth noting that sulphur burps can also occur with other GLP-1 medications like Wegovy (semaglutide), and the mechanisms are similar across this medication class.
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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 — Practical Steps
If you experience sulphur burps on Mounjaro, several practical strategies may help:
- Eat smaller, more frequent meals — reduces the volume of food fermenting in your stomach
- Chew food thoroughly — aids digestion and reduces fermentation
- Avoid foods that worsen burps — fatty, fried, or high-sulphur foods (eggs, meat, cruciferous vegetables) may increase gas production
- Stay hydrated — drink plenty of water throughout the day
- Avoid carbonated drinks — they add additional gas to your system
- Try ginger or peppermint tea — may help with digestion and gas relief
Most people find sulphur burps resolve within a few weeks to months as their body adjusts to tirzepatide. If they persist or worsen, discuss this with your prescriber — they may adjust your dose or timing.
- 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.
Are sulphur burps a sign that Mounjaro isn't working?
No. Sulphur burps are a side effect of how tirzepatide slows digestion, not an indicator of effectiveness. Many people experience excellent weight loss results while having temporary sulphur burps. They typically improve over time as your body adjusts to the medication.
How long do sulphur burps last on Mounjaro?
Most people report that sulphur burps decrease significantly within 2–8 weeks as their body adapts to tirzepatide. They may persist longer in some cases, but they usually resolve by 3 months. Individual experiences vary, so discuss persistent symptoms with your GP or prescriber.
Can I prevent sulphur burps before starting Mounjaro?
Not entirely, but dietary changes before and during Mounjaro treatment may reduce their intensity. Eating smaller meals, chewing thoroughly, avoiding fried foods, and staying hydrated are helpful preventative strategies from the first dose onwards.
Should I stop taking Mounjaro if I get sulphur burps?
Sulphur burps alone are not a reason to stop treatment — they are typically temporary and manageable. However, if they significantly affect your quality of life or are accompanied by severe digestive symptoms, contact your prescriber. They can advise whether continuing is appropriate or if dose adjustment might help.
Do sulphur burps happen with other weight loss injections like Wegovy?
Yes. Both Mounjaro and Wegovy (semaglutide) slow gastric emptying, so sulphur burps can occur with either medication. Wegovy may produce slightly different digestive side effects since it's a GLP-1-only agonist, but sulphur burps are reported by users of both medications during titration.
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