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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Mounjaro Gastritis vs Wegovy — What's the Difference?
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) can cause stomach discomfort, including gastritis-like symptoms, as GLP-1 and dual GIP/GLP-1 medications slow gastric emptying. This comparison looks at the clinical evidence, mechanism differences, and how to decide which might suit you better. Individual experiences vary significantly, and only a prescriber can assess your personal suitability following a clinical assessment.
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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.
Clinical Trial Data and Mechanism Differences
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist. SURMOUNT clinical trial data (up to 72 weeks) reported nausea in over 25% of participants at higher doses, with stomach pain and indigestion in 10–20% of cases. The dual mechanism may cause stronger initial gastric effects, though these often reduce over time as the body adapts.
Wegovy (semaglutide) is a GLP-1-only agonist. STEP trial data (68 weeks) showed nausea in approximately 25% of participants, with similar rates of stomach discomfort. Because Wegovy acts on a single pathway, some prescribers believe side effects may be slightly more predictable, though individual variation is substantial.
- Both slow gastric emptying — this is how they signal fullness
- Both commonly cause nausea, especially during dose escalation
- Stomach symptoms typically peak in weeks 1–4, then often improve
- Individual tolerance varies widely — genetics, food choices, and dose speed all matter
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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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Which Is Right for You? NHS and Prescriber Considerations
Neither medication is universally "better" for gastritis risk — choice depends on your individual health profile, medical history, and prescriber assessment. Both require a UK-licensed prescriber to evaluate your eligibility and tolerance.
- Your GP or specialist will consider your baseline stomach health, other medications, and any history of gastritis or reflux
- Slower dose titration (starting at the lowest dose and escalating gradually) reduces gastrointestinal symptoms for both medicines
- If stomach problems develop, your prescriber may adjust your dose or switch you to the other medication
- Use our free GLP-1 weight loss calculator to explore potential outcomes with either medication — then discuss results with your prescriber
Access in the UK is through NHS specialist weight management services or private clinical assessment with a licensed prescriber. Both pathways require evidence of suitability.
- 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.
Does Mounjaro cause more gastritis than Wegovy?
Clinical trial data shows similar rates of stomach discomfort for both (25%+ nausea at higher doses). Mounjaro's dual mechanism may cause slightly stronger initial effects, but individual variation is enormous. Some people tolerate one better than the other — this is a prescriber decision based on your personal health history.
What should I do if I develop gastritis symptoms on either medication?
Contact your prescriber immediately if you develop persistent stomach pain, vomiting, or reflux. They may slow your dose escalation, adjust your eating pattern, recommend antacids, or consider switching you to the other medication. Never stop the injection without guidance.
Can I take antacids or stomach medication alongside Mounjaro or Wegovy?
Many people take antacids or proton pump inhibitors (PPIs) alongside these medications, but you must inform your prescriber first. Some medications may interact or reduce absorption. Your GP will advise on what's safe for your specific situation.
Which medication has fewer side effects — Mounjaro or Wegovy?
Neither is universally "fewer" side effects. SURMOUNT and STEP trials show similar gastrointestinal symptom rates. Individual tolerance is highly personal — genetics, dose speed, food choices, and baseline stomach health all play a role. Only a prescriber can advise based on your profile.
How long do gastritis symptoms last on Mounjaro or Wegovy?
Most gastrointestinal symptoms peak in the first 4 weeks, then improve as your body adapts. By weeks 8–12, many people report significant improvement. If symptoms persist beyond 12 weeks, inform your prescriber — they may adjust your approach or consider switching medications.
Ready to take control of your weight loss journey?
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