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
Free tracker included with every download this week
Gastroparesis and Mounjaro: What You Need to Know
Gastroparesis — a condition where food moves too slowly through the stomach — is a known side effect consideration for Mounjaro (tirzepatide), the GLP-1 and GIP receptor agonist used for weight management in the UK. While most people tolerate Mounjaro well, understanding how it affects stomach function and recognising warning signs is important. This guide explains the relationship between tirzepatide and gastroparesis, who may be at higher risk, and what to do if you experience symptoms while using this prescription medication.
10,000+ calculations today · NHS-aligned thresholds · 100% free
Calculate Your BMI Instantly
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 Stomach Function
Mounjaro works by activating GLP-1 and GIP receptors, which naturally slow gastric emptying — the rate at which food leaves your stomach. This mechanism helps create the feeling of fullness that supports weight loss, which is why slowed digestion is a common and usually temporary side effect during the first few weeks of tirzepatide treatment.
Gastroparesis is different: it's a persistent condition where the stomach muscles fail to contract properly, leaving food in the stomach much longer than normal. It can cause severe nausea, vomiting, bloating, and stomach pain. While some people experience temporary delayed gastric emptying on Mounjaro (especially at higher doses like the full tirzepatide dose titration), true gastroparesis is rare but serious.
Key distinction: temporary nausea and constipation during dose adjustment is normal; persistent, severe stomach symptoms lasting weeks may indicate gastroparesis and require immediate GP assessment.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Who Is at Higher Risk and When to Seek Help
You may have higher risk of gastroparesis complications if you already have type 2 diabetes, delayed stomach emptying history, or severe digestive problems before starting Mounjaro. These should be discussed with your prescriber during your clinical assessment before tirzepatide is prescribed.
Seek urgent medical attention immediately if you experience: severe, persistent stomach pain spreading to your back (possible pancreatitis or gastroparesis); uncontrollable vomiting lasting more than a few hours; inability to keep food or water down; or severe bloating and distension. Contact your GP or NHS 111 — do not wait. If gastroparesis is suspected, your prescriber may recommend pausing or stopping Mounjaro whilst this is investigated.
- 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?
True gastroparesis from Mounjaro is rare, but temporary slowed stomach emptying is common during the first few weeks of treatment. Most people experience mild nausea and constipation that resolve within 4–8 weeks. If symptoms persist or worsen — especially severe stomach pain, uncontrollable vomiting, or inability to eat — contact your GP immediately.
What's the difference between normal Mounjaro side effects and gastroparesis?
Normal side effects: mild to moderate nausea, constipation, or indigestion lasting days to weeks, improving over time. Gastroparesis symptoms: severe, persistent stomach pain, vomiting unrelated to food intake, bloating lasting weeks, and difficulty eating solid foods. True gastroparesis requires medical evaluation.
Should I stop Mounjaro if I develop stomach problems?
Do not stop tirzepatide without medical advice, but contact your GP urgently if you experience severe or persistent stomach symptoms. Your prescriber will assess whether to pause treatment, adjust your dose, or investigate further. Never self-stop prescription medication.
Who should not take Mounjaro due to gastroparesis risk?
People with a history of severe gastroparesis or other severe stomach conditions should discuss this with their prescriber before starting Mounjaro. Those with type 2 diabetes already at risk of gastroparesis need careful assessment. Your GP or weight management specialist will evaluate whether tirzepatide is suitable for you.
Is gastroparesis more likely at higher Mounjaro doses?
Delayed stomach emptying is more common at higher doses during the dose titration phase, but true gastroparesis remains rare. Most people progress safely through the standard weekly dose increases (2.5 mg to 15 mg maximum). Individual risk varies — your prescriber monitors for symptoms at each dose step.
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.