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, Wegovy, and Gastroparesis — What You Need to Know
Gastroparesis — a condition affecting how quickly food moves through the stomach — is a potential concern for anyone considering Mounjaro or Wegovy. Both medications slow gastric emptying as part of how they work, which is why understanding the difference between them matters. This guide compares the two treatments in the context of gastroparesis risk, looking at clinical evidence, mechanisms, and what prescribers consider in the UK when assessing suitability for individual patients.
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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 and Wegovy Affect Stomach Function
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) work partly by slowing gastric emptying — the rate at which food leaves the stomach. This effect is intentional and helps you feel fuller after eating smaller meals, contributing to reduced appetite and weight loss. However, slowing gastric emptying can also cause nausea, constipation, and in rare cases, may worsen existing gastroparesis.
Mounjaro is a dual GIP and GLP-1 receptor agonist, activating two hormone pathways. Wegovy is a GLP-1 receptor agonist only, activating one pathway. The clinical trials — SURMOUNT for Mounjaro and STEP for Wegovy — showed similar rates of gastrointestinal side effects overall. Both reported nausea, vomiting, and constipation as very common side effects. The difference in mechanism does not appear to translate to significantly different gastroparesis risk in clinical practice, though individual responses vary considerably.
- Both slow gastric emptying as part of their mechanism
- Gastrointestinal side effects reported in both SURMOUNT and STEP trials
- Risk is considered low in people without existing stomach disorders
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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.
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Prescriber Assessment and Individual Suitability
When assessing whether Mounjaro or Wegovy is suitable, a UK prescriber will review your medical history for any existing digestive problems, including gastroparesis, severe reflux, or previous stomach surgery. If you have a diagnosis of gastroparesis, you will typically not be prescribed either medication, as both slow gastric function. The choice between Mounjaro and Wegovy, where both are options, depends on multiple factors: your BMI, weight-related health conditions, other medications, allergies, and individual risk factors.
Neither medication is "safer" regarding gastroparesis — the risk profile is comparable. Your prescriber will decide which is more appropriate based on your full clinical picture. Use our GLP-1 weight loss calculator to explore potential outcomes, but remember that the final decision always involves a clinical assessment by a licensed prescriber who understands your personal health context.
If you have existing digestive issues: Always inform your GP or prescriber of any history of gastroparesis, stomach ulcers, severe reflux, or abdominal surgery before pursuing either medication. Some people may benefit from dietary adjustments or other approaches. If you experience severe, persistent stomach pain spreading to your back during treatment, seek urgent medical attention — this can indicate pancreatitis.
- 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.
Can I take Mounjaro if I have gastroparesis?
Mounjaro is not recommended if you have diagnosed gastroparesis, as it slows stomach emptying and could worsen the condition. Always inform your prescriber of any existing gastroparesis diagnosis or symptoms before any GLP-1 medication is considered. Your GP can discuss alternative weight management approaches suited to your situation.
Is Wegovy safer than Mounjaro for gastroparesis risk?
No — both medications slow gastric emptying similarly. Clinical trial data shows comparable rates of stomach-related side effects. Neither is inherently safer regarding gastroparesis risk. Your prescriber will assess your individual health factors to decide if either is suitable for you.
What's the difference between Mounjaro and Wegovy in how they work?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, while Wegovy (semaglutide) is a GLP-1 agonist only. Despite this mechanistic difference, both produce similar rates of gastrointestinal side effects and comparable weight loss outcomes in clinical trials.
What stomach symptoms should I watch for on these medications?
Common side effects include nausea, vomiting, and constipation — usually mild and temporary. Seek urgent medical attention if you experience severe, persistent abdominal pain, especially pain radiating to the back, which could indicate pancreatitis or a serious complication.
How does a prescriber decide between Mounjaro and Wegovy?
Prescribers assess your BMI, weight-related health conditions, medical history (including any digestive issues), current medications, and allergies. Neither medication treats gastroparesis, so if you have existing stomach problems, alternative approaches may be recommended. A clinical assessment is always required before prescription.
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