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
Does Your Stomach Shrink On Mounjaro?
No, Mounjaro (tirzepatide) does not physically shrink your stomach. Instead, it works by slowing how quickly food leaves your stomach — a process called gastric emptying. This means you feel fuller for longer after eating smaller meals. Mounjaro is a prescription-only dual GIP and GLP-1 receptor agonist injection, licensed in the UK for weight management in eligible adults. Understanding how it actually works can help you know what to expect when using this medication.
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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 Your Stomach
Mounjaro works through two main mechanisms: gastric emptying and appetite regulation. When you eat, Mounjaro slows the rate at which food moves from your stomach into your small intestine. This delayed gastric emptying creates a prolonged feeling of fullness — meaning you feel satisfied after eating less food, which naturally supports a calorie deficit for weight loss.
The medication also acts on appetite centres in the brain via GIP and GLP-1 receptor pathways, reducing hunger signals and food cravings. However, this gastric effect typically reduces over time as your body adapts to the medication. The appetite-regulating effects tend to persist longer. Your stomach itself remains anatomically unchanged — the drug modulates how your digestive system functions, not your stomach's physical structure.
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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
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
What to Expect When Starting Mounjaro
When starting a Mounjaro KwikPen at the initial dose of 2.5 mg, most people experience increased feelings of fullness and reduced appetite within days. This can make eating feel uncomfortable if you attempt large meals — many people naturally eat smaller portions without consciously restricting calories.
As you progress through dose titration (increasing every 4 weeks from 2.5 mg up to 15 mg), these effects may strengthen initially, then stabilise. The delayed stomach-emptying sensation may feel like heaviness or fullness that lasts several hours after eating. If you experience severe nausea or persistent vomiting, contact your GP — these side effects should be managed with your prescriber, not tolerated. Use our GLP-1 weight loss calculator to estimate potential results with Mounjaro at different doses.
- 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 permanently shrink your stomach?
No. Mounjaro does not permanently change your stomach size. It temporarily slows gastric emptying, making you feel fuller. If you stop taking Mounjaro, your stomach function returns to normal over time. Your stomach's physical structure is not altered by the medication.
Why do I feel so full on Mounjaro?
Mounjaro slows how quickly food leaves your stomach (gastric emptying) and reduces appetite signals in your brain via GIP and GLP-1 pathways. Together, these effects create prolonged fullness after eating smaller meals. This is the intended mechanism for supporting weight loss.
Does the stomach-fullness effect last forever on Mounjaro?
The gastric emptying effect may reduce over time as your body adapts to the medication, typically within weeks to months. However, the appetite-suppressing effects via GLP-1 and GIP receptors tend to persist. Individual experiences vary — your prescriber can adjust your dose if effects change.
Is feeling extremely full on Mounjaro normal?
Mild to moderate fullness is normal and expected. Severe, persistent discomfort, nausea, or vomiting warrants a conversation with your GP or prescriber. They may suggest dose adjustments, meal timing changes, or dietary modifications to make the medication more tolerable.
What is the difference between Mounjaro and Wegovy for stomach effects?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist. Wegovy (semaglutide) is a GLP-1-only agonist. Both slow gastric emptying, but Mounjaro's dual mechanism may produce stronger initial appetite suppression. Clinical evidence shows Mounjaro achieved approximately 20.9% average weight loss versus Wegovy's 14.9% in trials — though individual results vary significantly and require prescription and clinical assessment.
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.