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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Water Fasting and Weight Loss Medication in the UK
Water fasting—consuming only water for a set period—is sometimes used for gradual weight management, but it's not sustainable long-term and carries health risks. If you're interested in weight management in the UK, prescription medications like Mounjaro and Wegovy offer evidence-based alternatives alongside lifestyle changes. These are available through NHS specialist services or private online prescribing for eligible adults. This guide explains UK-licensed weight loss options and how to find out if you qualify for prescription support.
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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-only weight loss medications are currently licensed in the UK for eligible adults: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). These work differently from restrictive fasting approaches—they reduce appetite and support feeling fuller on smaller portions, making calorie deficit achievable without extreme restriction.
Eligibility typically includes:
- BMI of 30 kg/m² or above, OR
- BMI of 27–29.9 with weight-related health conditions (type 2 diabetes, high blood pressure, sleep apnoea, cardiovascular disease)
- For South Asian, Chinese, Black African, or Black Caribbean adults: lower BMI thresholds may apply
Clinical trials showed Mounjaro helped participants lose an average of 15–21% of body weight, while Wegovy averaged around clinically studied weight loss—though individual results vary significantly. These prescription options work best alongside a reduced-calorie diet and increased physical activity, not as standalone solutions. A prescriber will assess your individual suitability through a clinical assessment before issuing a prescription.
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 in the UK
NHS route: Weight loss medications are available through tier 3 specialist weight management services following a GP referral. Availability and waiting times vary significantly by region (ICB). Some areas have lengthy waiting lists. Ask your GP about referral options in your area.
Private route: GPhC-registered online pharmacies dispense these medications following a consultation with a UK-licensed prescriber. No NHS referral needed. Costs range from approximately £129–£320 per month depending on dose and provider.
Whichever route you choose, use our free BMI calculator to check your personal BMI and understand your starting point. Track progress with our progress tracker. If you're considering medication, our GLP-1 weight loss calculator can help estimate potential weight loss at different doses based on clinical trial data.
- 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.
Is water fasting safe for weight loss?
Water fasting can cause muscle loss, nutritional deficiencies, dehydration, and metabolic slowdown. The NHS recommends sustainable approaches—losing 0.5–1 kg per week through balanced eating and activity. If gradual weight management is medically necessary, your GP can supervise safe methods. Prescription weight loss medication offers evidence-based support with medical oversight.
Who is eligible for prescription weight loss medication in the UK?
Adults with a BMI of 30 or above, or 27–29.9 with weight-related health conditions (diabetes, high blood pressure, sleep apnoea, heart disease), may be eligible. South Asian, Chinese, Black African, and Black Caribbean adults may qualify at lower BMI thresholds due to higher health risks at those levels. A prescriber will assess your individual situation.
How much weight can I expect to lose with Mounjaro or Wegovy?
Clinical trials showed Mounjaro helped participants lose an average of 15–21% body weight, and Wegovy around 15%—but individual results vary significantly. Weight loss depends on starting weight, adherence to diet and activity changes, dose tolerance, and personal metabolism. Results are not assured. A prescriber can discuss realistic expectations based on your circumstances.
Can I get weight loss medication through the NHS?
Yes, through specialist tier 3 weight management services. You'll need a GP referral. Availability and waiting times vary by region. Contact your GP surgery to ask about referral eligibility and local services. Some areas have long waiting lists; private prescribing is an alternative if you meet eligibility criteria.
Are weight loss injections a replacement for diet and exercise?
No. Prescription medications like Mounjaro and Wegovy work best alongside reduced-calorie diet and increased physical activity. They support appetite control and help create a sustainable calorie deficit, but they don't replace lifestyle changes. A balanced approach—medication plus behaviour change—gives the best long-term results.
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.