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
Diet for Fast Weight Loss — UK Prescription Options
While fast weight loss diets can work in the short term, sustainable results combine dietary changes with professional support. For eligible adults in the UK, prescription weight management medications are available alongside diet. These include GLP-1 weight loss injections like Mounjaro and Wegovy, approved for use within the NHS and through private prescribing. This guide explains how diet works with medication, who qualifies, and how to start the conversation with your GP about prescription weight loss options available to you.
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 licensed in the UK for weight management:
- Mounjaro (tirzepatide) — A once-weekly injection with dual GIP and GLP-1 action. Clinical trials showed an average weight loss of 15–21% of body weight at 72 weeks, though individual results vary significantly.
- Wegovy (semaglutide) — A once-weekly GLP-1 injection. STEP trial data showed average weight loss of approximately 15% body weight alongside reduced-calorie diet and increased physical activity.
- Saxenda (liraglutide) — A once-daily GLP-1 injection, useful for those who prefer daily dosing.
All three are prescription-only medicines requiring clinical assessment. They work by reducing appetite and supporting you to feel fuller after smaller meals, making it easier to maintain a reduced-calorie diet. For weight loss injections in the UK, eligibility typically starts at BMI 30 or above, or BMI 27+ with weight-related health conditions. Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses.
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 — How to Get Started
In the UK, prescription weight loss medication is available through two routes. NHS access involves referral to a specialist tier 3 weight management service through your GP — this is free but may involve waiting lists. Private access is through GPhC-registered online pharmacies following a clinical assessment with a UK-licensed prescriber, usually faster but at personal cost. Either way, your prescriber will assess your BMI, health conditions, and medications to determine eligibility. Start by speaking with your GP about whether prescription weight management is appropriate for you. They can discuss your nhs weight loss options or support you in exploring private routes. Combined with a structured diet plan and the support of our free progress tracker, prescription medication can significantly improve your weight loss results.
- 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.
Can I lose weight fast with diet alone?
gradual weight management is possible with very low-calorie diets (under 800 kcal/day), but the NHS recommends sustainable loss of 0.5–1 kg per week. Faster loss risks muscle loss and nutritional deficiency. Prescription weight loss medication combined with a calorie-controlled diet can help you lose weight faster while preserving muscle and maintaining nutrition.
What's the difference between Mounjaro and Wegovy?
Both are once-weekly injections for weight management. Mounjaro (tirzepatide) is a dual GIP + GLP-1 agonist; Wegovy (semaglutide) is a GLP-1-only agonist. Clinical trial data shows Mounjaro achieved slightly higher average weight loss (15–21% vs approximately 15%), but individual results vary. Your prescriber will help determine which suits you best.
Am I eligible for prescription weight loss medication in the UK?
You may be eligible if you have a BMI of 30 or above, or a BMI of 27–29 with a weight-related health condition such as type 2 diabetes, high blood pressure, or sleep apnoea. For South Asian, Chinese, Black African, or Black Caribbean adults, lower BMI thresholds apply. Your GP or a licensed prescriber will assess your individual eligibility.
How much weight can I expect to lose with medication?
Clinical trials show average weight loss of 15–21% of body weight over 72 weeks with Mounjaro, and approximately 15% with Wegovy — though individual results vary significantly. Weight loss occurs alongside a reduced-calorie diet and increased physical activity. Use our GLP-1 weight loss calculator to explore potential outcomes at different doses.
How do I start prescription weight loss treatment on the NHS?
Speak with your GP about your weight management goals. They can refer you to a specialist tier 3 weight management service if appropriate. Waiting lists vary by region, but the service is free. Alternatively, you can access private prescription weight loss through online assessments with a UK-licensed prescriber — typically faster but at personal cost.
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.