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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Gastroparesis and Mounjaro — What UK Patients Need to Know
Gastroparesis is a rare but serious condition where the stomach empties food more slowly than normal. Because Mounjaro (tirzepatide) slows gastric emptying as part of how it works, there is a theoretical risk of gastroparesis developing or worsening in patients taking this medication. The UK medicines regulator (MHRA) and NHS services take this risk seriously. This guide explains what gastroparesis is, who is at risk, warning signs to watch for, and what to do if you're concerned whilst taking Mounjaro.
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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.
NHS Safety Monitoring and Gastroparesis Risk
How Mounjaro affects the stomach: Mounjaro (tirzepatide) works as a dual GIP and GLP-1 receptor agonist. Part of its mechanism involves slowing gastric emptying—how quickly food moves from your stomach into the small intestine. In clinical trials, this effect typically reduces over time and is usually mild. However, in rare cases, prolonged delayed gastric emptying can contribute to gastroparesis.
MHRA monitoring status: Mounjaro carries a Black Triangle (▼) designation in the UK, meaning it is subject to additional monitoring by the MHRA for safety signals. Gastroparesis is listed as a rare side effect in the Product Information, though actual incidence rates in clinical practice remain very low.
Pre-existing conditions that increase risk: You should not take Mounjaro if you have a history of severe gastroparesis or severe digestion problems. If you have mild to moderate gastroparesis, your GP or prescriber must assess whether Mounjaro is safe for you individually. Patients with type 2 diabetes (particularly those on insulin) may already have some degree of delayed gastric emptying and should be assessed carefully.
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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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Recognising Symptoms and Seeking Help
Warning signs of gastroparesis to watch for: Symptoms include persistent nausea, vomiting (especially undigested food hours after eating), bloating, early fullness when eating, stomach pain, and unexplained weight loss despite eating. If you experience any of these symptoms whilst taking Mounjaro, contact your GP immediately—do not wait.
What to do if you're concerned: Call your GP surgery or NHS 111 if symptoms develop. Be clear that you are taking Mounjaro and describe your symptoms in detail. Your GP may refer you for further investigation (stomach scan or gastric emptying study). If gastroparesis is confirmed, your prescriber will discuss whether to continue, pause, or stop Mounjaro based on severity and your individual health picture.
- 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 Mounjaro cause gastroparesis?
Gastroparesis is a rare side effect listed by the MHRA. Mounjaro slows gastric emptying as part of how it works, but this effect usually reduces over time. True gastroparesis (severe, persistent delayed emptying) is uncommon. However, if you develop symptoms like persistent nausea, vomiting, or bloating, seek medical advice immediately.
Who is most at risk of gastroparesis with Mounjaro?
People with pre-existing severe gastroparesis or severe digestion problems should not take Mounjaro. Those with mild gastroparesis, type 2 diabetes with insulin use, or previous stomach surgery require careful individual assessment by their prescriber before starting treatment.
How does Mounjaro slow stomach emptying?
Mounjaro activates GIP and GLP-1 receptors, which naturally slow how quickly food leaves the stomach into the small intestine. This is part of how it reduces appetite and supports weight loss. This effect is usually temporary and mild, but in rare cases can contribute to gastroparesis risk.
What should I do if I develop gastroparesis symptoms on Mounjaro?
Contact your GP immediately. Describe symptoms like persistent nausea, vomiting, bloating, or early fullness. Your GP may refer you for investigation and will work with your prescriber to decide whether to continue, pause, or stop Mounjaro based on severity and your individual circumstances.
How is gastroparesis diagnosed in patients taking Mounjaro?
Diagnosis typically involves a gastric emptying scan (nuclear medicine test) where you eat a small amount of radioactive food and your doctor tracks how quickly it leaves your stomach. Your GP can refer you for this test on the NHS if gastroparesis is suspected during treatment with tirzepatide.
Ready to take control of your weight loss journey?
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