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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Achieving a 40 Kilo Weight Loss: UK Medication Options
Losing 40 kilograms is a significant health goal that typically takes 12–24 months at a safe, sustainable rate of 0.5–1 kg per week. Prescription weight management medications are now available in the UK through both NHS specialist services and private online prescribing. If you're considering losing 40 kilo and have a BMI of 30 or above, or 27 or above with weight-related health conditions, you may be eligible for prescription support. This guide covers the medication options available, how they work, and how to access them.
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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.
UK-Licensed Prescription Weight Loss Medications
Three prescription medications are currently licensed in the UK for weight management:
- Mounjaro (tirzepatide) — A once-weekly injection with dual GIP and GLP-1 action. Clinical trials showed average weight loss of around 15–21% of body weight over 72 weeks, though individual results vary.
- Wegovy (semaglutide) — A once-weekly GLP-1 injection. Average weight loss in clinical trials was approximately 15% of body weight at the maximum 2.4 mg dose.
- Saxenda (liraglutide) — A daily GLP-1 injection with a maximum dose of 3 mg daily. Produces modest weight loss when combined with diet and exercise.
All three are prescription-only medicines requiring clinical assessment. For a 40 kilo target, most people use either Mounjaro or Wegovy. You can estimate potential outcomes using our GLP-1 weight loss calculator, though real-world results depend on lifestyle changes, consistency, and individual metabolism.
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 in the UK
NHS access: NICE has approved tirzepatide (Mounjaro) and semaglutide (Wegovy) for specialist weight management services. Ask your GP about referral to your local tier 3 weight management service — access varies by region and waiting lists can be lengthy. Private access: Available through GPhC-registered online prescribing services following a clinical assessment. Faster access but costs typically range from £129–£320 per month depending on dose and provider.
Before pursuing any medication, calculate your BMI using our free BMI calculator to confirm eligibility. Most prescribers require a BMI of 30+ or 27+ with weight-related conditions like type 2 diabetes, high blood pressure, or sleep apnoea. Your GP can advise whether prescription treatment is appropriate for your individual circumstances and support you through the process.
- 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.
How long would it take to lose 40 kilograms?
At the NHS-recommended safe rate of 0.5–1 kg per week, losing 40 kg would take approximately 40–80 weeks (9–18 months). With prescription weight loss medication alongside diet and exercise changes, some people achieve faster initial loss. Real timescales depend on starting weight, metabolism, medication dose, and adherence to lifestyle changes.
What is a healthy weight loss rate?
The NHS recommends losing no more than 0.5–1 kg per week. This rate minimises muscle loss and supports sustainable, long-term results. A calorie deficit of around 500 kcal per day typically achieves this loss. Faster weight loss is possible with medical support but carries higher risks of nutrient deficiency and muscle loss.
Am I eligible for prescription weight loss medication in the UK?
You may be eligible if you have a BMI of 30 kg/m² or above, or 27 kg/m² or above with a weight-related health condition such as type 2 diabetes, high blood pressure, or sleep apnoea. South Asian, Chinese, Black African, and Black Caribbean adults may have lower BMI eligibility thresholds. Ask your GP for an assessment.
How much weight can I lose on Mounjaro or Wegovy?
Clinical trials showed average weight loss of 15–21% of body weight with Mounjaro and around 15% with Wegovy — but individual results vary significantly. For a person starting at 100 kg, this could be 15–21 kg over 6–12 months. Results depend on dose, consistency, diet, and exercise. Use our weight loss calculator for a personalised projection.
What happens after stopping weight loss medication?
Weight regain is common if diet and exercise habits revert to pre-treatment patterns. Weight loss medications work best alongside sustained lifestyle changes. Many people remain on medication long-term under medical supervision. Your prescriber will help you develop a plan for maintaining weight loss beyond the initial treatment phase.
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.