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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Where Does Weight Loss Happen First When Using UK Medication?
When you start prescription weight loss medication in the UK, weight loss typically begins within the first few weeks, though the rate and pattern varies between individuals. Prescription medications like Mounjaro and Wegovy work by reducing appetite and helping you feel fuller on smaller portions. This guide explains how UK-licensed weight management medications work, who is eligible, and how to access them through the NHS or private prescribing services.
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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 weight loss medications are currently licensed in the UK for adults with a BMI of 30 or above, or 27 and above with weight-related health conditions:
- Mounjaro (tirzepatide) — a once-weekly injection that activates two natural hormone pathways (GIP and GLP-1) to regulate appetite and support feeling fuller after meals. Clinical trials showed average weight loss of 15–21% of body weight at the highest dose.
- Wegovy (semaglutide) — a once-weekly GLP-1 injection that reduces hunger and slows gastric emptying. Clinical evidence indicates average weight loss around 14.9% of body weight.
- Saxenda (liraglutide) — a daily GLP-1 injection, the oldest option, with average weight loss of approximately 5–10% of body weight.
All three are prescription-only medicines requiring clinical assessment. Individual results vary significantly, and medication works most effectively alongside a reduced-calorie diet and increased physical activity. Use our free GLP-1 weight loss calculator to estimate potential outcomes at different doses.
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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.
Accessing Weight Loss Medication: NHS vs Private Routes in the UK
NHS access: Mounjaro and Wegovy are approved by NICE for specialist NHS weight management services (tier 3). Your GP can refer you to your local specialist weight management service, though waiting lists vary significantly by region and availability is not uniform across England, Scotland, Wales, and Northern Ireland.
Private access: Both medications are available through GPhC-registered online prescribing services. You'll complete an online clinical assessment with a UK-licensed prescriber who will determine eligibility. Private prescriptions typically cost £129–£320 per month depending on dose and provider.
To find out if you're eligible for either route, start by calculating your BMI using our free BMI calculator, then discuss your weight management goals with your GP or a licensed prescriber.
- 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 quickly does weight loss start with prescription medication?
Most people notice reduced appetite within the first week, but visible weight loss typically appears within 2–4 weeks. However, individual responses vary significantly. Some people lose weight steadily from the start, while others may take several weeks to reach their starting dose. Consistency with the medication and lifestyle changes matters more than speed.
Is weight loss assured with Mounjaro or Wegovy?
No. While clinical trials showed average weight loss of 15–21% with Mounjaro and 14.9% with Wegovy, results vary widely between individuals. Some people lose more, some less. These medications work best alongside a reduced-calorie diet and increased physical activity — they support weight loss but don't assures a specific outcome.
What is the difference between Mounjaro and Wegovy for weight loss?
Mounjaro (tirzepatide) activates two hormone pathways (GIP and GLP-1), while Wegovy (semaglutide) activates one (GLP-1 only). Both are once-weekly injections. Clinical trials suggest Mounjaro may produce slightly higher average weight loss, but the difference between individuals is larger than the difference between medications. Your prescriber will advise which is more suitable for you.
Can I get weight loss medication through my NHS GP?
Not directly. GPs cannot prescribe Mounjaro or Wegovy for weight loss — prescription is only through specialist tier 3 NHS weight management services. Your GP can refer you to your local service, but availability and waiting lists vary significantly across the UK. Private prescribing is an alternative route that does not require an NHS referral.
How do I know if I'm eligible for prescription weight loss medication in the UK?
You're eligible if you have a BMI of 30 or above, or 27 and above with a weight-related health condition (like high blood pressure, type 2 diabetes, or sleep apnoea). Your GP or a licensed private prescriber will assess your full health history. Use our free BMI calculator to check your BMI, then discuss eligibility with your GP or book a private consultation.
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.