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 Does Mounjaro Injections Work — And How It Differs From Wegovy
Mounjaro (tirzepatide) is a prescription-only injection that works differently from Wegovy — it uses a dual mechanism to help regulate appetite and blood sugar. This guide explains how Mounjaro works in your body, compares it to other GLP-1 medications like Wegovy, and helps you understand what clinical trial data shows about effectiveness. Whether you're exploring weight management options or trying to understand how this tirzepatide injection functions, this page covers the mechanism, UK eligibility, and how to access it through the NHS or private prescription.
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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 Works — Dual Mechanism vs Single GLP-1
Mounjaro (tirzepatide) activates two hormone pathways simultaneously, which sets it apart from other weight management injections. It's a dual GIP and GLP-1 receptor agonist — meaning it mimics two natural hormones your gut produces after eating.
- GLP-1 activation: Reduces appetite, increases feelings of fullness, and slows how quickly food leaves your stomach
- GIP activation: Helps regulate fat storage and utilisation, working alongside GLP-1 to support appetite control
In contrast, Wegovy (semaglutide) activates only the GLP-1 pathway — a single mechanism. Clinical trials show this dual-action approach may help some people achieve greater average weight loss. In the SURMOUNT-1 trial, participants on the maximum 15mg Mounjaro dose lost approximately 20.9% of their body weight over 72 weeks, though individual results vary significantly.
You receive Mounjaro as a once-weekly KwikPen injection, starting at 2.5mg and increasing gradually to your target dose — a process called dose titration. Your prescriber will monitor your response at each stage.
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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.
Mounjaro vs Wegovy — Which Mechanism Might Suit You?
Both Mounjaro and Wegovy are prescription-only medicines requiring a clinical assessment before you can access them. The choice between them depends on individual factors your GP or prescriber will evaluate — not on which is "better", as both work effectively for many people.
- Mounjaro eligibility: Adults with BMI ≥30, or ≥27 with weight-related health conditions (type 2 diabetes, hypertension, cardiovascular disease). Lower thresholds apply for South Asian, Black African, and Black Caribbean adults.
- Wegovy eligibility: Same criteria apply. Access is through NHS specialist weight management services or private prescription.
Use our free GLP-1 weight loss calculator to explore potential weight loss estimates at different dose levels. Your prescriber will personalise your treatment based on your health profile, not trial averages. Both medications work best alongside a reduced-calorie diet and increased physical activity.
- 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.
Is Mounjaro a GLP-1 injection?
Mounjaro contains tirzepatide, which is a dual GIP and GLP-1 receptor agonist — not a GLP-1-only medication. While it activates the GLP-1 pathway like Wegovy does, it also activates GIP, which is a second hormone pathway. This dual mechanism is what distinguishes Mounjaro from other weight management injections available in the UK.
How much weight can you lose on Mounjaro?
In the SURMOUNT-1 clinical trial, average weight loss ranged from approximately 15% at the 5mg dose to 20.9% at the maximum 15mg dose over 72 weeks — alongside a reduced-calorie diet and exercise. Individual results vary significantly based on genetics, adherence, diet, and lifestyle. Your GP cannot predict your personal outcome.
Why does Mounjaro work better than Wegovy for some people?
Mounjaro's dual GIP+GLP-1 mechanism may provide additional appetite suppression and fat regulation benefits compared to Wegovy's GLP-1-only approach. However, "better" is individual — Wegovy works effectively for many people. Suitability depends on your personal health profile, which your prescriber will assess during a clinical consultation.
How long does Mounjaro take to work?
Many people notice reduced appetite and fullness within the first 1–2 weeks, though some take longer. Measurable weight loss typically appears within 4–8 weeks. Mounjaro's effects build gradually as your dose increases during the titration phase. Full effects are usually seen after reaching your target dose.
Can I switch from Wegovy to Mounjaro?
Yes, switching between medications is possible and should be managed by your prescriber. While there are approximate dose equivalences (e.g., 1.7mg Wegovy ≈ 10mg Mounjaro), there is no official conversion chart. Your prescriber will guide any transition to ensure safe, effective treatment tailored to your response.
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.