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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Farting and Gas on Mounjaro vs Wegovy
Gastrointestinal side effects like flatulence, bloating, and stomach discomfort are common when starting Mounjaro (tirzepatide) or Wegovy (semaglutide) in the UK. Both medications slow how quickly food leaves your stomach and change gut bacteria activity, which can increase gas production. While these effects are usually temporary and often improve within a few weeks, understanding the differences between these two prescription-only medications can help you decide which might be more suitable for you alongside lifestyle changes. This guide compares the side effect profiles and clinical evidence to help inform conversations with your prescriber.
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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.
Clinical Trial Data and Digestive Side Effects
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways, which may contribute to its side effect profile. In the SURMOUNT-1 clinical trial, nausea was reported in approximately 25% of participants at the 15mg dose, with diarrhoea and constipation also frequently reported. Flatulence and abdominal discomfort are common but less frequently documented in trial summaries than nausea.
Wegovy (semaglutide) is a GLP-1 receptor agonist only — it activates a single hormone pathway. In the STEP-1 trial, nausea occurred in approximately 25% of participants at the 2.4mg dose, with similar rates of diarrhoea and constipation. Digestive side effects including gas and bloating are part of the expected side effect profile.
The key difference: Mounjaro's dual mechanism may theoretically produce a slightly different digestive effect, though individual variation is significant. Both medications slow gastric emptying (stomach to intestine movement), which increases bacterial fermentation and gas production. These effects typically diminish over 4–8 weeks as your gut 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 Digestive Side Effects and Individual Suitability
If you experience flatulence or bloating on either medication, practical strategies include eating smaller, more frequent meals; increasing water intake; reducing high-fibre foods temporarily; and taking time to rest after meals. Over-the-counter antacids like simethicone may help reduce bloating. Most users report these effects decrease significantly after the first month.
Which medication causes more gas? The honest answer is: individual variation is far greater than medication differences. Your gut bacteria, diet, baseline digestive sensitivity, and dose all matter more than which GLP-1 you choose. Your prescriber will assess your personal health history, any existing digestive conditions, and other medications before recommending either option. If you have a history of irritable bowel syndrome (IBS) or gastroparesis, this influences suitability. Both medications require a clinical assessment and prescription in the UK.
- 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 farting on Mounjaro normal?
Yes. Flatulence and gas are common side effects when starting any GLP-1 medication, including Mounjaro. They occur because the medication slows stomach emptying and changes how your gut bacteria ferment food. This is a normal, temporary effect that typically improves within 2–4 weeks as your digestive system adapts. If symptoms persist or worsen, contact your prescriber.
Does Wegovy cause more bloating than Mounjaro?
Clinical trials don't directly compare bloating rates between the two. Both can cause digestive side effects including flatulence and bloating. Individual experiences vary significantly — some people find one medication causes fewer digestive issues than the other, but this is highly personal. Your prescriber can help choose based on your medical history.
How long does gas last on Mounjaro?
Most users report that flatulence and bloating peak in the first 1–2 weeks and improve substantially by week 4–8. However, timing varies. Some people experience relief within days; others take longer. Staying hydrated, eating slowly, and reducing processed foods can help. If digestive issues persist beyond 8 weeks, speak to your prescriber.
Should I stop Mounjaro if I have bad bloating?
Do not stop without medical advice. Contact your GP or prescriber first. They may suggest dose adjustments, temporary dietary changes, or management strategies. Bloating usually improves with time. If you have severe or worsening symptoms, your prescriber might recommend pausing the next dose or switching medications — but this decision should be made clinically.
Does the Mounjaro KwikPen dose affect flatulence levels?
Possibly. Flatulence may be dose-dependent — starting at a lower dose (2.5mg) typically causes fewer digestive side effects than jumping to higher doses. The Mounjaro KwikPen is dose-titrated over weeks, meaning you gradually increase doses to minimise initial side effects. This gradual approach often reduces early bloating compared to starting at a higher dose.
Ready to take control of your weight loss journey?
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