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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Can Mounjaro Cause Gastroparesis?
Mounjaro (tirzepatide) is a prescription-only GLP-1 receptor agonist licensed for weight management in the UK. One concern some patients raise is whether Mounjaro can cause gastroparesis — a condition where the stomach empties more slowly than normal. The evidence shows that while Mounjaro does slow gastric emptying as part of how it works, true gastroparesis is rare. NHS guidance and clinical trial data support its safety when prescribed appropriately through NHS specialist services or following a clinical assessment with a UK-licensed 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.
How Mounjaro Affects the Stomach
Mounjaro slows gastric emptying as part of its intended mechanism. This means food moves through your stomach more slowly, which helps you feel fuller for longer and supports weight loss. However, this is different from gastroparesis — a pathological condition where the stomach cannot empty properly at all.
In clinical trials, gastrointestinal side effects were common but typically mild and temporary:
- Nausea (very common, especially when starting or increasing doses)
- Vomiting (less common, usually early in treatment)
- Constipation and diarrhoea (depending on individual response)
- Stomach pain or discomfort
Most patients find these effects reduce over time as their body adjusts. True gastroparesis — where the stomach cannot empty at all — was not reported as a clinical safety concern in the SURMOUNT trials. NHS prescribers are trained to monitor for genuine gastroparesis symptoms and would stop treatment if this developed.
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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.
Free Weight Loss Tools & Guides
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NHS Safety Monitoring and Prescribing
Mounjaro is prescribed through NHS tier 3 specialist weight management services or via private clinical assessment with a UK-licensed prescriber. All prescribers assess contraindications before prescribing, including a personal or family history of severe gastroparesis or severe gastrointestinal disease.
If you have a history of stomach problems — such as gastroparesis, severe reflux, or pancreatitis — inform your prescriber immediately. They will determine whether Mounjaro is safe for you. While on Mounjaro, contact your GP if you experience severe, persistent stomach pain spreading to the back, as this could indicate pancreatitis (a rare but serious side effect requiring urgent medical attention). Use our GLP-1 weight loss calculator to explore weight loss projections, and discuss all individual concerns with your healthcare provider before starting treatment.
- 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 definitely cause gastroparesis?
No. While Mounjaro slows gastric emptying as part of how it works, true gastroparesis (a pathological inability to empty the stomach) is not a recognised side effect in clinical trials. Mild nausea, vomiting, and stomach discomfort are common early side effects, but these usually improve as your body adjusts to the medication.
What's the difference between Mounjaro's side effects and gastroparesis?
Mounjaro's gastrointestinal side effects are normal, temporary, and usually dose-dependent. Gastroparesis is a serious condition where the stomach cannot empty at all. The slow gastric emptying caused by Mounjaro is intentional and reversible — it stops when you stop the medication. Gastroparesis is a chronic disorder requiring ongoing medical management.
Who should not take Mounjaro due to stomach problems?
People with a history of severe gastroparesis, severe gastrointestinal disease, or severe pancreatitis should not take Mounjaro. Always inform your prescriber of any past stomach issues. Your GP or NHS tier 3 service will assess your individual medical history before prescribing.
What stomach symptoms should prompt me to stop Mounjaro?
Seek urgent medical attention if you experience severe, persistent stomach pain that spreads to your back (possible pancreatitis), persistent vomiting that prevents you eating or drinking, or symptoms suggesting severe gastric obstruction. Contact your GP or NHS 111 immediately if unsure.
Is Mounjaro safer than older weight loss treatments in terms of stomach effects?
Mounjaro has been extensively studied in clinical trials. Its safety profile for gastrointestinal side effects is comparable to other GLP-1 medicines like Wegovy (semaglutide). Side effects are typically mild, temporary, and manageable. Your prescriber will monitor your response and adjust your mounjaro dose titration if needed to minimise discomfort.
Ready to take control of your weight loss journey?
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