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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Fasting Diets For Weight Loss — UK Health Guide
Fasting diets, including intermittent fasting, can support weight management when combined with a healthy lifestyle. The evidence shows modest weight loss results comparable to standard calorie restriction. However, if you have a BMI of 30 or above, or 27 or above with weight-related health conditions, prescription weight management options are now available in the UK. This guide covers how fasting works, its effectiveness, and the medical alternatives that may be suitable for you. Your GP can help you explore which approach is best for your individual circumstances.
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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 Management Options
For adults struggling with weight management in the UK, prescription weight loss medication is available alongside lifestyle changes like fasting and diet. Three medications are currently licensed for weight management:
- Mounjaro (tirzepatide) — a dual GIP and GLP-1 receptor agonist, injected once weekly, with average weight loss of 15–21% body weight in clinical trials
- Wegovy (semaglutide) — a GLP-1 receptor agonist, injected once weekly, with average weight loss around 15% body weight
- Saxenda (liraglutide) — a GLP-1 receptor agonist, injected daily, suitable for some patients
You can check your personal BMI using our free BMI calculator above to see if you may be eligible. Eligibility criteria include a BMI of 30 kg/m² or above, or 27–29.9 kg/m² with weight-related health conditions such as type 2 diabetes, high blood pressure, or sleep apnoea.
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 is through specialist tier 3 weight management services, usually via GP referral, though waiting lists vary significantly by region. Private access is available through GPhC-registered online prescribers following a clinical assessment, with no NHS referral needed.
If you're interested in exploring medication alongside fasting or other lifestyle changes, start by speaking with your GP about your weight management goals and health history. They can assess whether prescription treatment is appropriate for you, or refer you to a specialist service. Our free GLP-1 weight loss calculator can help you understand potential outcomes if you become eligible for treatment.
- 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 intermittent fasting effective for weight loss?
Yes, intermittent fasting can support weight loss when it helps create a calorie deficit. Evidence suggests results are similar to standard calorie restriction — typically 0.5–1 kg per week loss. However, individual results vary. Some people find fasting easier to follow than traditional diets; others do not. It works best combined with regular physical activity and balanced nutrition on eating days.
How does intermittent fasting compare to prescription weight loss medication?
Fasting is a lifestyle approach requiring no medication; prescription GLP-1 medications work through appetite regulation and are used alongside lifestyle changes. Clinical trials show prescription medications produce larger average weight losses (15–21% body weight) than fasting alone. Both approaches work best when combined with diet and exercise. Your GP can advise which is suitable for you.
Can I combine fasting with prescription weight loss injections?
Potentially yes, but this must be discussed with your prescriber. Fasting may interact with GLP-1 medications, which already slow stomach emptying and reduce hunger. Your GP or prescriber can advise on safe combinations based on your individual health. Never start both simultaneously without medical guidance.
What is the NHS guidance on fasting diets for weight loss?
The NHS recognises intermittent fasting as one dietary approach to weight management. NHS Weight Loss Plan recommends losing 0.5–1 kg per week through a calorie deficit of around 500 kcal daily. Very low calorie diets (under 800 kcal) should only be undertaken with medical supervision. Your GP can discuss safe fasting approaches.
Am I eligible for prescription weight loss medication in the UK?
Adults with a BMI of 30 kg/m² or above, or 27–29.9 kg/m² with weight-related health conditions (diabetes, high blood pressure, sleep apnoea) may be eligible. Eligibility is assessed individually by your GP or a licensed prescriber. Use our free BMI calculator to check your BMI, then speak with your GP about options available in your area.
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.