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 Deficit and Weight Loss — UK Prescription Options
A calorie deficit — consuming fewer calories than you burn — is the fundamental requirement for weight loss. In the UK, adults struggling to achieve weight loss through diet and exercise alone may be eligible for prescription weight management medication. This guide explains how calorie deficit works, the UK-licensed medications available, and how to find out if you qualify for treatment through the NHS or private prescribing.
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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 How They Work
Three prescription medications are currently licensed in the UK for weight management: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). These medications work by activating hormones that help regulate appetite and support feeling fuller after meals — they are used alongside a reduced-calorie diet, not instead of one.
Mounjaro is a once-weekly injection that activates two hormone pathways (GLP-1 and GIP). In clinical trials, participants on Mounjaro lost an average of 15–21% of their body weight over 72 weeks, alongside reduced-calorie diet and increased physical activity. Wegovy, another once-weekly injection, is a GLP-1 receptor agonist that showed average weight loss of approximately 15% in clinical trials. Saxenda is a daily injection that works through the same GLP-1 mechanism.
All three medications are prescription-only and require a clinical assessment to determine suitability. Results vary significantly between individuals — medication is most effective when combined with dietary changes and physical activity.
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 and How to Check Your Eligibility
In the UK, you can access prescription weight loss medication through two routes. NHS access is through specialist tier 3 weight management services — your GP can refer you, though waiting times vary significantly by region (ICB). Private access is available through GPhC-registered online pharmacies following a clinical assessment with a UK-licensed prescriber, without needing an NHS referral.
You may be eligible if you have a BMI of 30 or above, or a BMI of 27–29.9 with at least one weight-related health condition (such as type 2 diabetes, high blood pressure, or sleep apnoea). For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, lower BMI thresholds apply — eligibility may start at BMI 27.5 or above. Start by discussing weight management options with your GP, who can advise on NHS availability in your area and whether prescription medication might be appropriate for you.
- 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 a calorie deficit lead to weight loss?
Your body requires energy (calories) to function. When you consume fewer calories than you burn, your body uses stored energy (fat) for fuel, resulting in weight loss. A deficit of approximately 500 calories per day typically results in a loss of about 0.5 kg (1 lb) per week. The NHS recommends this sustainable rate. Use our free BMI calculator to assess your current weight category and determine your healthy weight range.
Can weight loss medication work without a calorie deficit?
No. Prescription weight loss medications like Mounjaro and Wegovy are designed to support a calorie deficit — they help manage appetite and reduce food cravings, making it easier to eat fewer calories. They are not effective on their own without dietary changes. Medication works best alongside a reduced-calorie diet and increased physical activity, as shown in clinical trials.
What's the difference between Mounjaro and Wegovy?
Mounjaro (tirzepatide) activates two hormone pathways (GLP-1 and GIP), while Wegovy (semaglutide) activates one (GLP-1 only). Both are once-weekly injections. In clinical trials, Mounjaro showed average weight loss of 15–21%, while Wegovy showed approximately 15%. Results vary between individuals. Try our GLP-1 weight loss calculator to see projected weight loss at different doses.
How do I find out if I'm eligible for weight loss medication on the NHS?
Ask your GP about referral to your local specialist weight management service. Eligibility is based on your BMI and any weight-related health conditions. The NHS uses a BMI of 30+ or 27+ with comorbidities. You can use our free BMI calculator to check your current BMI, then discuss the results with your GP.
What happens if I lose weight but then stop the medication?
Weight loss maintenance requires sustained calorie deficit through diet and exercise. Research suggests that stopping weight loss medication without maintaining lifestyle changes may result in weight regain. A prescriber will discuss long-term management with you, which typically involves ongoing dietary awareness and physical activity to maintain results achieved.
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.