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
Weight Loss On Fasting: UK Medication & Health Guide
Intermittent fasting is a popular weight loss approach where you restrict eating to specific time windows. While fasting can help create a calorie deficit, combining it with prescription weight management options may offer additional support for eligible UK adults. This guide explores fasting for weight loss, NHS-approved medication alternatives, and how to find the right approach for your health goals.
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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.
Prescription Weight Loss Options Available in the UK
If you're considering weight loss management in the UK, several prescription medications are available alongside lifestyle changes like fasting or calorie restriction. Three main options are currently licensed:
- Mounjaro (tirzepatide) — a once-weekly injection that acts as a dual GIP and GLP-1 receptor agonist, helping regulate appetite and support feeling fuller after meals
- Wegovy (semaglutide) — a once-weekly GLP-1 injection approved for weight management in adults meeting eligibility criteria
- Saxenda (liraglutide) — a daily GLP-1 injection, an alternative option for some patients
In clinical trials, these medications helped participants lose an average of 15–21% of their body weight when combined with reduced-calorie eating and increased physical activity. However, individual results vary significantly, and prescription weight loss medication is only available following a clinical assessment by a licensed prescriber or through NHS specialist weight management services.
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 Routes & Finding Out If You're Eligible
In the UK, weight loss medication access depends on your BMI and health profile. You may be eligible if: your BMI is 30 or above, or 27–29.9 with a weight-related health condition (such as type 2 diabetes, high blood pressure, or sleep apnoea). NHS access is through specialist tier 3 weight management services, though waiting times and availability vary significantly by region. Private prescription routes are also available following online clinical assessment. Your GP can discuss whether fasting combined with medication support is appropriate for you, or you can explore eligibility through private prescribing services. Use our free BMI calculator to check your current BMI and see where you fall on NHS thresholds.
- 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 safe for weight loss in the UK?
Intermittent fasting can be safe for most healthy adults, but it's not suitable for everyone. People with a history of eating disorders, those taking certain medications, pregnant or breastfeeding women, and those with blood sugar control issues should consult their GP first. The NHS recommends creating a calorie deficit of around 500 kcal per day for sustainable weight loss of 0.5–1 kg per week. Speak to your GP before starting any fasting regime.
Can I combine fasting with prescription weight loss medication?
Yes, prescription weight loss medications like Mounjaro and Wegovy are designed to be used alongside reduced-calorie eating and increased physical activity — which may include fasting approaches. However, any combination should be discussed with your prescriber, as some medications may affect how quickly food is digested or interact with meal timing. Your healthcare provider will give you personalised guidance based on your health.
How do I know if I'm eligible for prescription weight loss medication in the UK?
Eligibility depends on BMI and health conditions. Generally, you may qualify with a BMI of 30 or above, or 27–29.9 if you have weight-related conditions like diabetes or high blood pressure. Use our free BMI calculator to check your BMI, then discuss your weight management options with your GP or a private prescriber who can assess your individual circumstances.
What's the difference between NHS and private weight loss medication in the UK?
NHS access is through specialist weight management services following GP referral, though waiting times vary by region. Private options are available through GPhC-registered online prescribing services without NHS referral, typically at cost. Both require clinical assessment. NHS services are free but may have longer waits; private services are faster but involve out-of-pocket cost.
Will prescription weight loss medication work if I'm also fasting?
Weight loss medication can support your efforts, but results depend on consistent adherence to a reduced-calorie diet, physical activity, and the medication itself. Clinical trials showed average weight loss of 15–21% of body weight — but individual results vary. Medications work alongside lifestyle changes, not instead of them. Your prescriber will monitor your progress and adjust treatment as needed.
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.