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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How to do intermittent fasting for weight loss
Intermittent fasting is a popular approach to weight management where you eat during set time windows and fast during others. Many people in the UK use intermittent fasting alongside other lifestyle changes to support sustainable weight loss. Whether you're considering this method alone or combining it with prescription weight management options, it's important to understand how it works and whether it's right for you. Calculate your BMI and current weight loss potential using our free BMI calculator and GLP-1 weight loss calculator to see where you stand today.
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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.
Intermittent fasting methods and how they compare to prescription weight loss options
Intermittent fasting involves eating during specific time windows and fasting for the rest of the day or week. The most popular methods include 16:8 fasting (eating within an 8-hour window), 5:2 fasting (eating normally five days, restricting calories two days), and eat-stop-eat (24-hour fasts once or twice weekly). Evidence suggests intermittent fasting can support modest weight loss by creating a calorie deficit, though results are similar to standard calorie restriction when calories consumed are equal.
For people seeking more structured support, prescription weight loss medication is available in the UK through NHS weight loss services or private prescribing. These include Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide)—all GLP-1 receptor agonists that may help reduce appetite and support weight management alongside dietary changes. Clinical trials show these medications help participants lose an average of 15–21% body weight, though individual results vary significantly.
Intermittent fasting and prescription medications work differently: fasting controls when you eat, while GLP-1 injections help regulate appetite and hunger signals. Many people combine both approaches for enhanced weight management outcomes.
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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 weight loss support and how to find prescription options in the UK
The NHS offers structured weight management support through specialist services (tier 3 and tier 4). To access prescription weight loss medication on the NHS, you'll need a referral from your GP to a specialist weight management service in your area. However, NHS waiting lists can be lengthy, and availability varies significantly by region.
If you're interested in exploring prescription options more quickly, private online prescribing is available through UK-licensed prescribers. You'll complete an online health assessment, and a prescriber will determine whether you're eligible based on your BMI, health history, and any weight-related conditions. Eligibility typically requires a BMI of 30 or above, or 27 or above if you have weight-related health conditions like type 2 diabetes or high blood pressure. Start a conversation with your GP about which approach suits you best.
- 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 intermittent fasting and GLP-1 weight loss medication?
Intermittent fasting controls your eating schedule to create a calorie deficit. GLP-1 medications like Mounjaro and Wegovy work by reducing appetite and hunger signals. Both can support weight loss, but they work through different mechanisms. Many people combine them for better results alongside diet and exercise.
Can I do intermittent fasting while taking prescription weight loss medication?
Yes, you can combine intermittent fasting with GLP-1 medications, and some people find this combination helpful. However, you should discuss this with your prescriber or GP first. GLP-1 medications already reduce appetite, so very restricted eating patterns may not be necessary and could lead to inadequate nutrition.
How quickly will I lose weight with intermittent fasting?
The NHS recommends losing 0.5–1 kg per week for sustainable results. Intermittent fasting effectiveness depends on your total calorie intake, not just when you eat. You can use our free progress tracker to monitor your weight loss journey and see what works best for you.
How do I know if I'm eligible for prescription weight loss medication in the UK?
UK eligibility typically requires a BMI of 30 or above, or 27 or above with weight-related health conditions like prediabetes, high blood pressure, or heart disease. Check your BMI using our free BMI calculator, then speak to your GP or a private prescriber about whether you might be suitable.
Is intermittent fasting safe for everyone?
Intermittent fasting isn't suitable for everyone, particularly pregnant women, those with a history of eating disorders, people taking certain medications, or those with unstable diabetes. Always consult your GP before starting intermittent fasting, especially if you have existing health conditions or take medications regularly.
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.