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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How Mounjaro Works — The Science Behind the Injection
Mounjaro is a prescription-only weight management injection that works differently from other slimming injections available in the UK. It's a dual GIP and GLP-1 receptor agonist — meaning it targets two natural hormone pathways in your body to help regulate appetite and support weight loss. Understanding how Mounjaro works can help you decide if it might be right for you, alongside lifestyle changes. This guide explains the mechanism, clinical evidence, and what you can realistically expect from this tirzepatide injection.
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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.
The Dual Hormone Mechanism — What Makes Mounjaro Different
Mounjaro's key difference is that it activates two hormone pathways, not just one. Unlike Wegovy (semaglutide), which is a GLP-1 only agonist, Mounjaro stimulates both GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) receptors.
This dual action helps your body:
- Reduce appetite and food cravings by signalling fullness to the brain
- Feel satisfied after eating smaller portions
- Slow gastric emptying (how quickly food leaves your stomach) — though this effect typically decreases over time
- Support better blood sugar control and fat utilisation
Both GIP and GLP-1 are natural hormones your gut produces after eating. Mounjaro amplifies these signals, creating a stronger appetite-suppressing effect than single-pathway medications. Clinical trials showed this dual mechanism may support greater average weight loss compared to GLP-1-only injections, though individual results vary significantly.
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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.
Clinical Evidence and Expected Weight Loss in UK Trials
The SURMOUNT-1 clinical trial tested tirzepatide in adults across multiple countries. At the highest dose of 15 mg once weekly, participants lost an average of approximately 20.9% of their body weight over 72 weeks — alongside a reduced-calorie diet and increased physical activity. At 5 mg, average loss was around 15%. However, these are clinical averages: some people lost more, others less. Your GP or prescriber will assess whether Mounjaro is suitable and monitor your progress. Use our free GLP-1 weight loss calculator to estimate potential outcomes at different doses based on your current weight.
- 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 work for everyone?
No — individual results vary significantly. While clinical trials showed average weight loss of 15–21%, some people respond better than others. Your genes, diet, exercise, metabolism, and adherence all affect outcomes. Your prescriber will monitor your progress and adjust your dose if needed.
How long does it take for Mounjaro to work?
Most people notice appetite reduction within the first 1–2 weeks of starting or increasing their dose. Measurable weight loss typically appears within 4–8 weeks. Full effects at each dose level take about 4 weeks to stabilise, which is why prescribers space dose increases at least 4 weeks apart.
Is Mounjaro the same as Wegovy?
No. Both are prescription-only UK weight management injections, but Mounjaro (tirzepatide) has a dual GIP+GLP-1 mechanism, while Wegovy (semaglutide) is GLP-1 only. Clinical trial data suggests Mounjaro may support slightly greater average weight loss, but results vary between individuals.
Can I get Mounjaro on the NHS in the UK?
NICE has approved tirzepatide for NHS use within specialist tier 3/4 weight management services. Access requires referral from your GP and varies significantly by region. Waiting lists can be very long. Private access is available through online prescribers following a clinical assessment.
What happens if I stop taking Mounjaro?
Weight typically returns gradually once you stop the injections — often within weeks to months. This is why Mounjaro works best alongside permanent lifestyle changes: reduced-calorie eating and regular physical activity. Your GP or prescriber can advise on stopping and maintaining weight loss long-term.
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.