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
Achieving 20kg Weight Loss: UK Medication Options
Losing 20kg is a significant health goal that many UK adults pursue. If you've been struggling with weight management through lifestyle changes alone, prescription weight loss medication may be an option worth exploring with your GP. Several medications are now available through the NHS and private prescribing in the UK, designed to support sustained weight loss when combined with dietary changes and increased physical activity. This guide explains which prescription options are available, who may be eligible, and how to access them through both NHS and private routes.
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 Medications
Three prescription-only medications are currently licensed in the UK for weight management and may help you lose around 20kg (3 stone 2lbs) or more, though individual results vary significantly.
Mounjaro (tirzepatide) is a once-weekly injection that works as 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 — equivalent to roughly 13–18kg for someone starting at 90kg. Wegovy (semaglutide) is also a once-weekly injection (GLP-1 only) with average weight loss of around 15% of body weight in trials. Saxenda (liraglutide) is a daily injection with slightly lower average weight loss but remains an option for some patients.
All three medications work by signalling to your brain that you feel fuller, reducing appetite and food cravings. They also slow gastric emptying, helping you feel satisfied with smaller meals. Use our GLP-1 weight loss calculator to estimate potential results at different doses based on your starting weight.
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 Access vs Private Prescribing
NHS route: Prescription weight loss medications are available through specialist tier 3 weight management services, but you'll need a referral from your GP. NHS availability varies significantly by region, and waiting lists can be lengthy in many areas — sometimes 6–12 months or longer.
Private route: You can access these medications through GPhC-registered online pharmacies following a clinical assessment with a UK-licensed prescriber, without waiting for NHS referral. This typically costs between £129–£320 per month depending on dose.
Either way, a prescriber will assess your BMI, health history, and weight-related conditions to determine eligibility. Check your BMI using our free BMI calculator and discuss your options with your GP as a starting point.
- 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 a realistic timeframe to lose 20kg on prescription weight loss medication?
Based on clinical trials, losing 20kg typically takes 12–18 months. Most medications require a gradual dose increase over 16–20 weeks, then continued treatment at maintenance dose. Weight loss accelerates after the first 4–8 weeks as your body adjusts. Always combine medication with a reduced-calorie diet and regular physical activity for best results.
Am I eligible for Mounjaro or Wegovy on the NHS?
You may be eligible if you have a BMI of 30 or above, or a BMI of 27–29.9 with weight-related health conditions like type 2 diabetes, high blood pressure, or sleep apnoea. South Asian, Chinese, Black African, and Black Caribbean adults may have lower BMI thresholds. Eligibility is assessed individually by your GP or specialist service.
What happens after I reach my 20kg weight loss goal?
You'll continue on your maintenance dose to prevent weight regain. Stopping medication often leads to weight rebound — typically 30–50% of lost weight returns within a year. Your prescriber will work with you on long-term management strategies, including ongoing lifestyle support and possibly continued medication.
Can I switch between Mounjaro and Wegovy if one isn't working?
Yes, switching is possible under medical supervision, though there's no official conversion chart. A prescriber may recommend switching if you're not seeing expected results or experiencing significant side effects. Any switch requires a clinical assessment and must be managed carefully to avoid gaps in treatment.
Are weight loss injections covered by the NHS for everyone?
No — NHS access is limited to specialist tier 3 weight management services and varies by region. Eligibility criteria are strict, and availability depends on your local ICB (Integrated Care Board) budget and commissioning decisions. Your GP can advise whether your area offers NHS prescribing and whether you might qualify.
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.