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
Rapid Water Weight Loss: What You Need to Know
Rapid water weight loss refers to quick fluctuations in weight caused by changes in fluid retention, which is why many people see significant results in the first week of dieting. While initial gradual weight management can be encouraging, sustainable weight management in the UK typically involves prescription-backed approaches like GLP-1 weight loss injections, combined with dietary changes and physical activity. This guide explains the difference between water weight and fat loss, and explores prescription weight loss medication options available to eligible UK adults through the NHS and private prescribing services.
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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
For adults struggling with weight management in the UK, prescription medications can provide structured support alongside lifestyle changes. The main GLP-1 weight loss medications currently licensed for weight management are Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). Mounjaro uses a dual GIP and GLP-1 mechanism and is injected once weekly at doses from 2.5 mg to 15 mg. Clinical trials showed adults on Mounjaro lost an average of 15–21% of their body weight over 72 weeks, alongside a reduced-calorie diet and increased physical activity. Wegovy is a once-weekly GLP-1 injection with a maximum dose of 2.4 mg, producing average weight loss of approximately 15% in clinical trials. Both medications require a prescription and are suitable only for adults with a BMI of 30 or above, or 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or cardiovascular disease. Individual results vary significantly, and these medications are tools to support behavioural change, not replacements for dietary modifications. Our free GLP-1 weight loss calculator can help you estimate potential outcomes at different doses if you meet eligibility criteria.
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 Getting Started
Prescription weight loss in the UK is available through two routes. The NHS weight loss pathway involves referral from your GP to a tier 3 specialist weight management service, where eligibility is assessed and medication prescribed if appropriate—though waiting lists vary considerably by region and can extend months or years in some areas. Private access is available through GPhC-registered online prescribing services, where a UK-licensed prescriber conducts a clinical assessment and issues a prescription if suitable. To find out if prescription medication is right for you, start by speaking with your GP, who can discuss your individual health context, conduct relevant blood tests, and either initiate NHS referral or advise on private options. Use our free BMI calculator above to check your starting point, and remember that rapid initial weight loss (often water weight) typically slows after the first 2–4 weeks as your body reaches a sustainable fat loss rate of around 0.5–1 kg per week.
- 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 water weight and fat loss?
Water weight fluctuates daily due to hydration, sodium intake, hormones, and glycogen storage—it can change by 2–3 kg in 24 hours. Fat loss is slower and more stable, typically 0.5–1 kg per week. gradual weight management in the first week is mostly water; sustainable loss requires calorie deficit and time.
Can prescription weight loss injections help me lose weight faster?
GLP-1 medications like Mounjaro and Wegovy support weight loss by reducing appetite and promoting feeling fuller, but they work alongside diet and exercise, not instead of them. Clinical trials showed average weight loss of 15–21% over several months—significant but not overnight. Results vary by individual.
Am I eligible for prescription weight loss medication on the NHS?
You may be eligible if your BMI is 30 or above, or 27–29.9 with weight-related conditions like type 2 diabetes or high blood pressure. Ask your GP for referral to a specialist weight management service to be assessed. Eligibility and waiting times vary by region.
What's the safest rate of weight loss?
The NHS recommends losing 0.5–1 kg per week for sustainable results and to preserve muscle mass. Faster loss increases risks of nutrient deficiency, muscle loss, and fatigue. Prescription medication supports this rate when combined with a calorie deficit and physical activity.
How long does it take to see results with weight loss medication?
Many people notice appetite changes within days, but meaningful weight loss typically becomes apparent after 2–4 weeks. Maximum benefit is usually seen after 12–16 weeks at the target dose. Track progress using our free progress tracker to see patterns beyond daily fluctuations.
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.