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 Does Weight Loss Medication Work in the UK?
Weight loss medication works by helping to regulate appetite and reduce food cravings, making sustainable weight management easier alongside diet and exercise. In the UK, prescription weight management options are available for eligible adults through both NHS specialist services and private prescribing. Understanding how these medications work, who can access them, and what results to expect can help you make an informed decision about whether weight management through medication might be right for you. This guide explains the science behind prescription weight loss, available options, and how to find out if you're eligible.
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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.
UK-Licensed Weight Loss Medications Explained
Three prescription weight loss injections are licensed in the UK:
- Mounjaro (tirzepatide) — A once-weekly injection that works as a dual GIP and GLP-1 receptor agonist, activating two natural hormone pathways to reduce appetite and support feeling fuller after smaller meals. Clinical trials showed average weight loss of 15–21% of body weight over 72 weeks when combined with a reduced-calorie diet and physical activity.
- Wegovy (semaglutide) — A once-weekly GLP-1 receptor agonist that signals fullness to the brain and slows gastric emptying. Average weight loss in clinical trials was around 15% of body weight at the maximum dose.
- Saxenda (liraglutide) — A once-daily GLP-1 injection, the longest-established weight loss medication in the UK. Results are typically slightly more modest than the once-weekly options.
All three work by mimicking natural hormones that regulate appetite and blood sugar. Individual results vary significantly, and these medications are prescribed alongside dietary changes and increased physical activity—not as replacements for lifestyle changes. Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses.
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 Prescribing in the UK
NHS access: Weight loss medications are prescribed through specialist tier 3 and tier 4 weight management services. You'll need a referral from your GP, and availability varies significantly by region. Waiting lists can be very long in many areas, sometimes spanning months or years.
Private access: Available through GPhC-registered online pharmacies following a clinical assessment with a UK-licensed prescriber. No NHS referral needed. Costs typically range from £129–£320 per month depending on the dose and provider.
Eligibility: Adults with a BMI of 30 or above, or 27 or above with weight-related health conditions (such as type 2 diabetes, high blood pressure, or sleep apnoea), may be eligible. People of South Asian, Chinese, Black African, or Black Caribbean heritage may qualify at slightly lower BMI thresholds due to higher health risks at lower weights. Your GP or a licensed prescriber can assess your individual eligibility.
- 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 much weight will I lose with prescription weight loss medication?
Results vary significantly between individuals. Clinical trials show average weight loss of 15–21% of body weight with Mounjaro and around 15% with Wegovy over 72 weeks. However, some people lose more and others less. Weight loss depends on the specific medication, dose, your adherence, and how consistently you follow a reduced-calorie diet and increase physical activity. Use our weight loss calculator to estimate potential results at different doses.
Are weight loss injections safe?
Prescription weight loss medications are licensed in the UK and have been through rigorous clinical trials. Like all medications, they can cause side effects—most commonly nausea, diarrhoea, and constipation, particularly when starting or increasing doses. Serious side effects are rare. However, they are not suitable for everyone. A licensed prescriber will assess your medical history and current health to determine if the medication is safe and appropriate for you.
How quickly will I lose weight?
Weight loss typically begins within the first few weeks, but the rate varies. Most people lose weight gradually over weeks and months rather than overnight. The NHS recommends a sustainable loss of 0.5–1 kg per week. gradual weight management can increase muscle loss and other health risks. Prescription medications are designed to support gradual, sustainable weight loss alongside lifestyle changes.
Do I need to change my diet and exercise if I take weight loss medication?
Yes. Weight loss medications work best alongside a reduced-calorie diet and increased physical activity. They are not a substitute for lifestyle changes. The medication helps reduce appetite and cravings, making it easier to stick to a healthier diet and move more. Your prescriber or a dietitian can help you plan sustainable dietary and exercise changes alongside medication.
Can I get weight loss medication on the NHS?
Yes, but access is through specialist weight management services (tier 3 or tier 4), not standard GP prescription. You'll need a referral from your GP. Availability and waiting times vary significantly by region. Some areas have longer waiting lists than others. Ask your GP about local weight management services and waiting times in your area. Private prescribing is an alternative if NHS waiting times are long.
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.