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 and Gastroparesis — UK Safety Guide
Mounjaro (tirzepatide) is generally not recommended for people with severe gastroparesis, as it slows how quickly food leaves the stomach — worsening existing digestive problems. However, eligibility depends on the severity of your condition and individual health factors. If you have gastroparesis or suspect you might, your GP or prescriber must assess your suitability before starting Mounjaro. This guide covers NHS guidance, how Mounjaro affects digestion, and what to discuss with your healthcare team.
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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 Digestion and Gastroparesis Risk
Mounjaro works partly by slowing gastric emptying — the rate at which food moves from your stomach into your small intestine. This effect helps create a feeling of fullness and reduces appetite, which supports weight loss. However, this mechanism becomes problematic if you already have gastroparesis (delayed gastric emptying due to nerve or muscle dysfunction).
- Gastroparesis causes food to remain in the stomach longer than normal, leading to nausea, vomiting, bloating, and abdominal pain
- Adding Mounjaro to severe gastroparesis could intensify these symptoms significantly
- For mild gastroparesis, individual assessment by your prescriber is essential — some patients may tolerate Mounjaro with close monitoring
- The gastrointestinal slowing effect of Mounjaro typically reduces over time as your body adapts
This is why gastroparesis is listed as a contraindication in the Mounjaro prescribing information — your prescriber must know about any history of digestive disorders before treatment begins.
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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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Gastroparesis and Eligibility for Mounjaro on the NHS
The NHS tier 3 weight management services assess all patients for contraindications before prescribing Mounjaro. If you have documented gastroparesis, you will typically be deemed ineligible for Mounjaro and offered alternative approaches instead.
- Your GP should refer you to a specialist weight management service if you're interested in prescription weight loss support
- At assessment, be completely honest about any digestive symptoms, stomach conditions, or previous diagnoses
- If you have mild or borderline symptoms, request a gastroenterologist review before your Mounjaro assessment
- Alternative GLP-1 medications or non-pharmacological weight loss methods may be recommended instead
Always discuss existing health conditions openly with your healthcare team — this ensures you receive safe, appropriate 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.
Can I take Mounjaro if I have gastroparesis?
Severe gastroparesis is a contraindication to Mounjaro — meaning it's not recommended. Mounjaro slows stomach emptying, which worsens gastroparesis symptoms. If you have mild symptoms, individual assessment by your prescriber is necessary. Always disclose any digestive disorders before starting treatment.
What is gastroparesis and how does Mounjaro cause it?
Gastroparesis is delayed stomach emptying caused by nerve or muscle dysfunction. Mounjaro doesn't cause gastroparesis but intentionally slows gastric emptying as part of its weight loss mechanism. This effect is helpful for appetite control but harmful if gastroparesis already exists. The slowing typically reduces over time.
What symptoms suggest I might have gastroparesis?
Common symptoms include persistent nausea, vomiting, early fullness after eating, bloating, abdominal pain, and unexplained weight loss. If you experience these regularly, see your GP before starting any weight loss medication. Diagnosis usually involves imaging or specialist testing.
Will my NHS GP assess my suitability for Mounjaro?
Your GP can do an initial assessment and refer you to a tier 3 specialist weight management service. The specialist team will evaluate your medical history, including any digestive conditions. They have access to full prescribing guidance and can make a final decision on eligibility.
What are the alternatives to Mounjaro if I have gastroparesis?
Alternatives include structured dietary programmes, exercise support, behaviour change therapy, and possibly other weight loss medications assessed as safer for your condition. Your specialist team can discuss options tailored to your individual health profile and needs.
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.