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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What Causes Weight Loss — And UK Treatment Options
Weight loss happens when your body burns more calories than you consume, but the causes and solutions vary widely. In the UK, weight management is supported by lifestyle changes, NHS programmes, and for eligible adults, prescription medications like GLP-1 injections. Understanding what drives weight loss—and what options are available to you—is the first step towards sustainable health. This guide covers the main causes of weight loss and the prescription weight management options available through NHS and private routes in the UK.
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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 Prescription Weight Management Options
For eligible UK adults, three prescription medications are licensed for weight management: Mounjaro (tirzepatide), a dual GLP-1 and GIP receptor agonist; Wegovy (semaglutide), a GLP-1 receptor agonist; and Saxenda (liraglutide), another GLP-1 option. These work by helping regulate appetite, reduce cravings, and support feeling fuller after smaller meals.
- Mounjaro: Clinical trials showed average weight loss of 15–21% of body weight over 72 weeks alongside diet and exercise
- Wegovy: Average weight loss of approximately 15% body weight in clinical trials
- Saxenda: Daily injection option; weight loss averages around 5–10% body weight
All are prescription-only and require assessment by a UK-licensed prescriber. Individual results vary, and these medications work best alongside reduced-calorie diet and increased physical activity. Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses, and check your BMI using our BMI calculator to understand whether you may be eligible.
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 — How to Get Started
NHS access: Available through tier 3 specialist weight management services following GP referral. Availability and waiting times vary significantly by region. Private access: Available through GPhC-registered online pharmacies following an online clinical assessment with a UK-licensed prescriber—no NHS referral required.
To explore your options, speak with your GP first—they can assess whether prescription weight loss medication is appropriate for you, refer you to NHS services if eligible, or advise on private routes. Your GP can also discuss your BMI, health conditions, and any medications you take. Even if you're not eligible for prescription medication, NHS weight management programmes, dietitian support, and exercise referrals are available to all adults—ask your practice about local provision.
- 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.
What causes weight loss in the body?
Weight loss occurs when your body burns more calories than it consumes, creating a calorie deficit. This can happen through reduced food intake, increased physical activity, or both. Prescription weight loss medications like GLP-1 injections reduce appetite and increase fullness, helping create this deficit more easily alongside lifestyle changes.
Who is eligible for prescription weight loss medication in the UK?
UK adults with a BMI of 30 or above are typically eligible; those with a BMI of 27–29.9 and weight-related health conditions (such as type 2 diabetes, high blood pressure, or cardiovascular disease) may also qualify. Lower thresholds apply for people of South Asian, Chinese, Black African, or Black Caribbean heritage. A prescriber will assess your individual eligibility.
What's the difference between Mounjaro and Wegovy?
Both are prescription-only GLP-1 injectables. Mounjaro is a dual GIP and GLP-1 agonist (maximum 15 mg weekly); Wegovy is a GLP-1 agonist alone (maximum 2.4 mg weekly). Clinical trials showed Mounjaro produced slightly higher average weight loss (15–21%) compared to Wegovy (around 15%), but individual results vary significantly.
How long do I need to take prescription weight loss medication?
Weight loss medication is typically continued as long as it's helping and well-tolerated. Many people remain on treatment long-term, as stopping can lead to weight regain. Your prescriber will review this with you regularly and adjust as needed based on your progress and health.
Can I get weight loss medication on the NHS?
Yes, through specialist tier 3 weight management services, but availability varies by region and waiting lists can be lengthy. Private access is faster—available through online prescribing services. Either way, a clinical assessment is required. Speak to your GP about local NHS options or to discuss private alternatives.
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.