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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Weight Loss Without Any Reason — When to Seek Help
If you're losing weight without trying, it's important to take it seriously and speak to your GP. Unexplained weight loss can sometimes signal an underlying health condition that needs attention. However, if you've decided you want to lose weight intentionally and are looking for support, the UK now offers several prescription medication options alongside lifestyle changes. This guide explains what might cause unintended weight loss, when to seek help, and what prescription weight loss options are available 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 Prescription Weight Loss Medication Options
If you're overweight or obese and want to lose weight intentionally, prescription medication is available in the UK for eligible adults. Three medications are currently licensed for weight management:
- Mounjaro (tirzepatide) — A dual GIP and GLP-1 receptor agonist, given as a once-weekly injection. Clinical trials showed participants lost an average of 15–21% of their body weight over 72 weeks alongside reduced-calorie diet and increased physical activity.
- Wegovy (semaglutide) — A GLP-1 receptor agonist, given as a once-weekly injection. Clinical evidence demonstrated average weight loss of around 15% of body weight at the maximum dose.
- Saxenda (liraglutide) — A GLP-1 receptor agonist, given as a daily injection. An established option with evidence of supporting weight loss when combined with lifestyle changes.
All three are prescription-only and require a clinical assessment before being prescribed. Individual results vary significantly. Try our free GLP-1 weight loss calculator to explore potential outcomes at different medication 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 vs Private Weight Loss Treatment in the UK
You can access prescription weight loss medication through two routes in the UK. NHS access is through specialist tier 3 weight management services — you'll need a GP referral and acceptance into your local NHS integrated care board (ICB) programme. Waiting lists vary significantly by region. Private access is available through online consultations with UK-licensed prescribers, without NHS referral. To find out if you're eligible for either route, start by discussing your health and weight goals with your GP. They can assess whether medication is appropriate for you, refer you to NHS services if suitable, or advise on private options. Use our free BMI calculator to check your starting BMI and understand your health category.
- 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.
Should I be concerned if I'm losing weight without trying?
Yes. Unexplained weight loss can indicate an underlying health condition such as thyroid problems, diabetes, cancer, depression, or digestive disorders. If you're losing weight without intentional changes to diet or exercise, speak to your GP promptly. They can run tests to identify any medical causes and advise appropriate next steps.
What's the difference between Mounjaro and Wegovy for weight loss?
Both are once-weekly injections for weight management. Mounjaro works on two hormone pathways (GIP and GLP-1), whilst Wegovy works on one (GLP-1 only). Clinical trials show Mounjaro may produce slightly higher average weight loss, but individual results vary. Your prescriber will recommend which is more suitable for you based on your health profile.
How much weight can I expect to lose with prescription medication?
Results vary significantly between individuals. Clinical trials show average weight loss of 15–21% of body weight over several months, but some people lose more and some less. Weight loss is most effective when combined with reduced-calorie diet, increased physical activity, and behaviour change. Your individual result depends on dose, adherence, lifestyle changes, and personal factors.
Can my GP prescribe weight loss injections like Mounjaro or Wegovy on the NHS?
Not typically through standard GP prescribing. NHS prescription weight loss medication is usually available through specialist tier 3 weight management services after GP referral. Access varies by region and waiting lists can be long. Alternatively, you can access these medications privately through online consultations with UK-licensed prescribers without an NHS referral.
What eligibility criteria apply for prescription weight loss medication in the UK?
You may be eligible if you have a BMI of 30 kg/m² 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 cardiovascular disease). For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, eligibility thresholds may be lower. Your GP or prescriber will assess individual eligibility following a clinical assessment.
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.