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
Weight Loss Difference Calculator — Understanding Your UK Options
Working out the difference between weight loss methods helps you make informed decisions about managing your weight. In the UK, prescription weight loss medications like Mounjaro and Wegovy can support significant results when combined with lifestyle changes, but eligibility depends on your BMI and health status. This guide explores the prescription weight management options available to UK adults, how they compare, and how to find out if you're eligible. Use our free BMI calculator to check your current status, and our GLP-1 weight loss calculator to see projected outcomes at different doses.
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.
UK-Licensed Prescription Weight Loss Options
Three prescription medications are currently licensed in the UK for weight management:
- Mounjaro (tirzepatide) — A dual GIP and GLP-1 receptor agonist. In clinical trials, participants lost an average of 15–21% of their body weight over 72 weeks. Starting dose 2.5 mg, maximum 15 mg once weekly.
- Wegovy (semaglutide) — A GLP-1 receptor agonist. Clinical data showed average weight loss of around 15% of body weight. Doses range from 0.25 mg to 2.4 mg once weekly.
- Saxenda (liraglutide) — A GLP-1 receptor agonist administered as a daily injection. Maximum dose 3 mg per day.
All three are prescription-only medicines requiring clinical assessment. Individual results vary significantly. Weight loss medication works best alongside a reduced-calorie diet and increased physical activity as part of overall weight management in the UK.
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.
NHS vs Private Access and Eligibility in the UK
NHS access: Available through specialist tier 3 weight management services, typically via GP referral. Availability and waiting times vary significantly by region across the UK. Private access: Available through UK-licensed online prescribers following a clinical assessment, without needing NHS referral.
Eligibility criteria: You may be eligible if you have a BMI of 30 or above, or 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or sleep apnoea. For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, lower BMI thresholds apply. Start by calculating your BMI using our free tool above, then speak to your GP about prescription weight loss options suitable for you.
- 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.
What's the difference between Mounjaro and Wegovy for weight loss?
Mounjaro works on two hormone pathways (GIP and GLP-1), while Wegovy targets only GLP-1. Clinical trials suggest Mounjaro may produce slightly higher average weight loss (15–21% vs 15%), but individual results vary significantly. Both are prescription-only once-weekly injections available in the UK. Your prescriber will assess which suits your health profile.
How much weight can I expect to lose with prescription medication?
Clinical trials show average weight loss of 15–21% of body weight with Mounjaro and around 15% with Wegovy over several months. However, individual results vary widely depending on dose, adherence, diet, and exercise. Weight loss medication works best alongside lifestyle changes. Always discuss realistic expectations with your prescriber.
Am I eligible for prescription weight loss medication on the NHS?
Generally, you may be eligible if your BMI is 30 or above, or 27–29.9 with weight-related health conditions. Some regions have additional criteria. Lower BMI thresholds apply for certain ethnic groups. Your GP can assess your eligibility and refer you to a specialist weight management service if appropriate.
Can I get weight loss injections privately in the UK without NHS referral?
Yes. Private prescribing services in the UK offer prescription weight loss medication following an online clinical assessment. You don't need NHS referral, but you must still be assessed by a UK-licensed prescriber. Costs vary by dose and provider. Always use a GPhC-registered pharmacy.
How long does it take to see weight loss results with medication?
Most people begin to notice changes within 4–8 weeks, though results vary. Medication is typically titrated (gradually increased) over several weeks or months. Weight loss continues as your dose increases and is usually most noticeable after 12–16 weeks. Combine medication with reduced-calorie diet and physical activity for best results.
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.