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
3 Day Water Fast Weight Loss — Safety and Prescription Alternatives
A 3 day water fast may result in rapid initial weight loss, but most of this is water and muscle rather than fat. In the UK, safer and more sustainable weight management options are available — including prescription medications like Mounjaro and Wegovy for eligible adults. This guide explains the risks of extreme fasting, how prescription weight loss medication works, and how to find out if you qualify for treatment through the NHS or private providers. Check your BMI using our free BMI calculator to understand your starting point for any weight loss programme.
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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 Options
If you're considering gradual weight management methods like water fasting, prescription weight management options offer safer, evidence-based alternatives. Mounjaro (tirzepatide) and Wegovy (semaglutide) are UK-licensed GLP-1 and dual GIP/GLP-1 receptor agonist medications designed to support weight loss through appetite regulation and improved satiety. Clinical trials show Mounjaro helps participants lose an average of 15–21% of body weight over time, while Wegovy supports approximately 15% average weight loss — though individual results vary. Saxenda (liraglutide) is also available as a daily injection option. These prescription weight loss medications work best alongside reduced-calorie diet and physical activity, not as replacements for lifestyle changes. Eligibility requires assessment by a prescriber, and treatment is adjusted gradually over weeks with close monitoring.
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 Prescription Routes
In the UK, prescription weight loss medication is available through two main routes: NHS specialist tier 3 weight management services (requires GP referral, but availability and waiting times vary significantly by region) and private online prescribing (available through GPhC-registered pharmacies after an online clinical assessment). Your GP can advise on NHS eligibility and local services. Use our GLP-1 weight loss calculator to explore potential outcomes with prescription medication, and our free progress tracker to monitor your journey if you're approved for treatment. Adults with a BMI of 30 or above, or 27 or above with weight-related health conditions like type 2 diabetes or high blood pressure, may be eligible for prescription weight loss support in the UK. Always consult your GP to discuss whether medication is appropriate 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.
Why is a 3 day water fast risky compared to prescription weight loss medication?
A 3 day water fast causes rapid fluid loss and muscle breakdown, not sustainable fat loss. Most weight is regained immediately. Prescription weight loss medication like Mounjaro and Wegovy supports gradual fat loss (averaging 15–21% body weight) through appetite control, without the extreme calorie restriction or muscle loss associated with fasting. Medication works alongside diet and exercise, not in isolation.
What's the difference between Mounjaro and Wegovy in the UK?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — once weekly at up to 15 mg. Wegovy (semaglutide) is a GLP-1 agonist only — once weekly at up to 2.4 mg. Clinical trials show Mounjaro may support slightly greater average weight loss (20.9% vs 14.9%), but individual results vary. Both require prescription and clinical assessment. Your GP or prescriber can advise which suits you.
How do I know if I'm eligible for prescription weight loss treatment in the UK?
You may be eligible if your BMI is 30 or above, or 27–29.9 with a weight-related health condition (type 2 diabetes, hypertension, high cholesterol, sleep apnoea, or cardiovascular disease). Lower thresholds apply if you're of South Asian, Chinese, Black African, or Black Caribbean heritage. Check your BMI using our free calculator, then speak to your GP about eligibility and available options.
Can I get Mounjaro or Wegovy through the NHS?
Yes, both are available on the NHS through tier 3 specialist weight management services. You need a GP referral. However, NHS availability and waiting times vary significantly by region — some areas have limited capacity. Private online prescribing is also available. Ask your GP about local services and waiting times.
Is water fasting ever recommended by UK health services?
Very low calorie diets (under 800 kcal/day, including water fasting) are only recommended under strict medical supervision in specialist services, and for very limited periods. The NHS supports sustainable approaches: 0.5–1 kg weight loss per week through calorie deficit, physical activity, and behavioural support. Prescription medication can support this approach for eligible adults without the risks of extreme fasting.
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.