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
Free tracker included with every download this week
Mounjaro Belly Ache vs Wegovy: What's the Difference?
Stomach pain and belly ache are common side effects reported with both Mounjaro and Wegovy in the UK. Both are prescription-only weight loss injections, but they work differently — Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, while Wegovy (semaglutide) is a GLP-1-only medication. This comparison explains the differences in how they affect your stomach, what clinical trials showed, and how to discuss side effects with your prescriber. Both require a clinical assessment before prescription, and individual experiences vary significantly.
10,000+ calculations today · NHS-aligned thresholds · 100% free
Calculate Your BMI Instantly
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: Stomach Discomfort in SURMOUNT vs STEP
Gastrointestinal side effects, including nausea, vomiting, constipation, and stomach pain, were reported in clinical trials for both medications. In the SURMOUNT-1 trial (Mounjaro), nausea affected more than 1 in 10 participants, with stomach pain reported as common. The STEP-1 trial (Wegovy) showed similar patterns — nausea was very common, and abdominal discomfort was frequently reported.
The key difference is mechanism: Mounjaro's dual GIP and GLP-1 action may produce different gastrointestinal effects than Wegovy's single GLP-1 pathway. However, both slow gastric emptying (how quickly food leaves your stomach), which can cause bloating, cramping, and belly ache — especially when first starting or increasing doses.
Individual responses vary greatly. Some people experience mild nausea that settles within weeks; others report more persistent stomach discomfort. Your UK-licensed prescriber will monitor your side effects and can adjust your dose or suggest management strategies like eating smaller, lower-fat meals.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Which Medication Suits You? A Prescriber Decision
Whether Mounjaro or Wegovy is more suitable depends on your personal health history, stomach sensitivity, and how you respond to the medication — not the side effect profile alone. Both are available in the UK through NHS specialist weight management services or private prescription following clinical assessment.
Your prescriber will consider:
- Your existing digestive health
- Other medications you're taking
- Previous reactions to GLP-1 medications
- Your BMI and weight-related health conditions
- Your personal weight loss goals
If you experience significant belly ache on either medication, contact your prescriber immediately — they may reduce your dose, pause treatment, or recommend an alternative. Never stop taking the injection without guidance. Use our free GLP-1 weight loss calculator to explore what results might be possible, and discuss side effect management openly with your GP or prescriber.
- 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 belly ache with Mounjaro worse than Wegovy?
Both medications commonly cause stomach pain in clinical trials. Individual experiences vary significantly — some people tolerate one better than the other, but this is unpredictable. Your prescriber can help monitor side effects and adjust treatment if needed. There's no definitive data showing one causes worse belly ache than the other.
How long does Mounjaro belly ache last?
For most people, stomach discomfort from Mounjaro improves within a few weeks as your body adjusts. However, some experience ongoing mild discomfort, especially at higher doses. If pain persists beyond 4 weeks or is severe, contact your prescriber — you may need a dose adjustment or a medication review.
Can you switch from Mounjaro to Wegovy if you have stomach pain?
Possibly, but switching requires your prescriber's guidance. There's no official established conversion between the two doses, so your prescriber will manage any transition carefully. Some people experience fewer GI side effects with Wegovy, but outcomes are individual. Discuss switching options with your GP.
What should I eat if Mounjaro causes belly ache?
When experiencing stomach discomfort with Mounjaro, eat smaller, frequent meals of low-fat, bland foods — lean proteins, rice, vegetables, and plain carbohydrates. Avoid fatty, spicy, or high-fibre foods temporarily. Staying hydrated is important. If pain worsens or becomes severe, seek advice from your prescriber or GP immediately.
Is severe stomach pain a sign to stop Mounjaro?
Severe, persistent stomach pain — especially pain spreading to your back — could indicate a serious condition like pancreatitis and requires urgent medical attention. Contact your GP or call NHS 111 immediately. Do not stop taking your injection without medical guidance, but do seek urgent advice if pain is severe or unusual.
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.