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 for Weight Loss — What Works in the UK?
Intermittent fasting is a popular approach to weight management in the UK, involving periods of eating and fasting. While some people find fasting helpful for weight loss, the evidence shows it works primarily through creating a calorie deficit — similar to other dietary methods. For eligible adults seeking additional support, prescription weight loss medication is also available through the NHS or private prescribing. This guide explores both fasting approaches and medication options to help you understand what might work for your situation.
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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.
Fasting Methods and Evidence for Weight Loss
Intermittent fasting (IF) involves eating within specific time windows and fasting for the remainder. Common methods include 16:8 fasting (16 hours fasting, 8-hour eating window), 5:2 fasting (eating normally 5 days, reducing intake to around 500–600 calories on 2 non-consecutive days), and eat-stop-eat (24-hour fasts once or twice weekly).
Research suggests intermittent fasting can support weight loss, but the primary mechanism is calorie reduction—not the fasting itself. NHS weight management guidance indicates that sustainable weight loss of 0.5–1 kg per week works best with any method, including fasting. Success depends on creating a calorie deficit while maintaining adequate nutrition and physical activity.
- 16:8 fasting: modest weight loss, may suit some routines
- 5:2 fasting: effective for calorie restriction, requires discipline on fasting days
- Individual results vary significantly based on overall diet quality and adherence
Use our free weight loss calculator to explore potential weight loss timelines with different approaches, or try our BMI calculator to understand your current health baseline.
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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.
Prescription Weight Loss Medication — UK Options
For adults meeting eligibility criteria, prescription weight loss medication offers a different approach to support weight management. Three GLP-1 medications are licensed in the UK: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). These medications work by affecting appetite regulation and may support weight loss alongside diet and exercise.
Eligibility typically includes adults with a BMI of 30 or above, or BMI 27–29.9 with weight-related health conditions. Access is available through NHS specialist weight management services or private online prescribing following clinical assessment. Clinical trials show average weight loss of 14–21% of body weight—though individual results vary considerably. Medication is always combined with lifestyle changes, not used alone.
- Check your BMI using our free calculator to explore potential eligibility
- Discuss options with your GP to understand what suits your situation
- 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.
Does intermittent fasting help you lose weight faster than other diets?
Intermittent fasting can support weight loss, but the mechanism is calorie deficit—not the fasting pattern itself. Research shows results similar to regular calorie restriction. Success depends on maintaining a lower overall calorie intake and sticking with your chosen method consistently. The "best" diet is the one you can sustain long-term.
What's the difference between fasting and prescription weight loss medication?
Fasting is a dietary approach that creates calorie reduction through eating timing. Prescription medications like Mounjaro and Wegovy work via GLP-1 pathways to reduce appetite and support satiety. Both are tools that work alongside lifestyle changes. Medication requires a prescription and clinical assessment, while fasting is self-managed.
Can I combine fasting with weight loss medication?
Combining approaches should only be done under medical supervision. Your GP or prescriber can advise whether combining fasting with medication is appropriate for your health. Some people find that medication reduces the need for strict fasting patterns, as appetite naturally decreases.
How do I know if I'm eligible for NHS weight loss medication?
Eligibility typically includes a BMI of 30 or above, or 27–29.9 with weight-related health conditions (type 2 diabetes, high blood pressure, cardiovascular disease). Your GP can assess your eligibility and refer you to specialist weight management services if appropriate. Waiting lists vary significantly by region.
What's the safest rate of weight loss according to NHS guidance?
The NHS recommends losing 0.5–1 kg (1–2 lbs) per week for sustainable, healthy results. Faster weight loss may cause muscle loss and nutritional deficiencies. Whether using fasting, medication, or lifestyle changes, this rate supports long-term success and helps maintain muscle mass during weight loss.
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.