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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Calorie Intake and Weight Loss — UK Prescription Options
Calorie intake is fundamental to weight loss, but prescription medications available in the UK can now support this process alongside dietary changes. If you're struggling with weight management, understanding both the calorie basics and the medication options available to eligible adults is essential. The NHS recommends a calorie deficit of around 500 kcal per day for sustainable weight loss of approximately 0.5 kg per week. However, for many people, achieving and maintaining this deficit is challenging—which is where prescription weight loss medications may help. This guide covers UK-licensed options and eligibility criteria.
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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 Weight Loss Medications and Calorie Control
Three prescription weight loss injections are currently licensed in the UK to support weight management alongside reduced-calorie diet and increased physical activity:
- Mounjaro (tirzepatide) — A dual GIP and GLP-1 receptor agonist that may help reduce appetite and support calorie control. Clinical trials showed participants lost an average of 15–21% of their body weight.
- Wegovy (semaglutide) — A GLP-1 receptor agonist injection. STEP trials demonstrated average weight loss of approximately 15% of body weight on the highest dose.
- Saxenda (liraglutide) — A daily GLP-1 injection approved for weight management, though less commonly prescribed than weekly options.
These medications work by affecting appetite signals in the brain, helping you feel fuller on smaller portions—making calorie control more achievable. Importantly, they are prescription-only medicines requiring assessment by a licensed prescriber. Eligibility typically includes adults with a BMI of 30 or above, or 27 or above with weight-related health conditions such as type 2 diabetes or high blood pressure. Individual results vary significantly.
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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 vs Private Access and Next Steps
In the UK, prescription weight loss medications are available through two routes:
- NHS access — Via referral to a specialist tier 3 or tier 4 weight management service. Availability varies by region and waiting times can be lengthy.
- Private access — Through GPhC-registered online prescribing services following a clinical consultation. Available without NHS referral.
To find out if you're eligible, speak to your GP about your weight management goals. They can assess your BMI, health conditions, and suitability for prescription treatment. Use our free BMI calculator above to check your current BMI, and our GLP-1 weight loss calculator to explore potential outcomes at different doses. Your GP can also discuss whether medication is appropriate for you or whether other support—such as structured dietary intervention or NHS weight loss programmes—may be more suitable initially.
- 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.
How does calorie deficit work with weight loss medication?
A calorie deficit remains essential even with medication. Prescription weight loss medications like Mounjaro and Wegovy support the process by reducing appetite, making it easier to maintain a lower calorie intake. The medication helps control cravings and supports feeling fuller faster, but successful weight loss still requires dietary changes. Aim for a deficit of around 500 kcal per day for sustainable results.
What is the difference between Mounjaro and Wegovy for calorie control?
Mounjaro is a dual GIP and GLP-1 agonist, whereas Wegovy is a GLP-1 agonist only. In clinical trials, Mounjaro achieved slightly higher average weight loss (up to 21%) compared to Wegovy (around 15%), though individual responses vary. Your prescriber can advise which is most suitable based on your health profile and needs.
Can I get weight loss medication on the NHS in the UK?
Yes. Prescription weight loss medications are available through NHS specialist weight management services (tier 3 and 4) for eligible adults. Access requires GP referral and assessment, though availability and waiting times vary significantly by region. Private prescribing is also available through GPhC-registered online services without NHS referral.
What BMI do I need to be eligible for prescription weight loss medication?
Eligibility typically includes adults with 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). For South Asian, Chinese, Black African, or Black Caribbean adults, lower thresholds may apply. Use our free BMI calculator to check your current BMI and discuss eligibility with your GP.
How quickly will I lose weight with these medications?
Clinical trials show that participants lose weight gradually over time, with maximum results typically seen around 12–16 weeks after reaching the highest dose. Average weight loss ranges from 15–21% of body weight depending on the medication and dose. However, individual results vary significantly based on diet, activity level, metabolism, and adherence. Weight loss happens alongside calorie reduction and increased physical activity, not instead of them.
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.