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
Free tracker included with every download this week
What's the Best Injection for Weight Loss in the UK?
There are currently three prescription-only weight loss injections licensed in the UK, and the "best" one depends on your individual health, preferences, and medical assessment. The most commonly discussed options are Wegovy (semaglutide), Mounjaro (tirzepatide), and Saxenda (liraglutide). All work as GLP-1 receptor agonists — or in Mounjaro's case, a dual GIP/GLP-1 agonist — to help reduce appetite and support weight management alongside a reduced-calorie diet and increased physical activity. This guide explains how each works, what results to expect, and how to access them through the NHS or private prescribing 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.
How Weight Loss Injections Work and What the Evidence Shows
Weight loss injections work by mimicking natural hormones that regulate hunger and blood sugar. GLP-1 receptor agonists activate the same hormones your gut naturally produces after eating, signalling to your brain that you're full, slowing how quickly food leaves your stomach, and helping control cravings. Mounjaro adds a second mechanism — GIP activation — which may enhance fat utilisation and appetite suppression.
Clinical trial evidence shows:
- Wegovy (semaglutide): Average weight loss of approximately 15% of body weight over 68 weeks at the maximum dose (2.4 mg once weekly)
- Mounjaro (tirzepatide): Average weight loss of 15–21% of body weight over 72 weeks, depending on dose (maximum 15 mg once weekly)
- Saxenda (liraglutide): Average weight loss of approximately 8% of body weight (daily injection, less commonly prescribed)
Important: These are clinical trial averages. Individual results vary significantly, and medication is most effective alongside diet and exercise changes.
Clinically BackedWeight Loss
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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.
NHS vs Private Access and How to Get Started
In the UK, weight loss injections are prescription-only and available through two routes. NHS access is through tier 3 specialist weight management services — you'll need a GP referral and assessment. Waiting lists vary by region but can be lengthy in many areas. Private access is through UK-licensed online prescribing services following a clinical consultation — faster but involves a cost, typically £129–£320 per month depending on dose.
Both routes require a clinical assessment to confirm eligibility: usually a BMI of 30 or above, or 27+ with weight-related health conditions like type 2 diabetes, high blood pressure, or cardiovascular disease. Your prescriber will also check for contraindications. Use our free GLP-1 weight loss calculator to estimate potential results, and our BMI calculator to check your current status before consulting 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.
Is Mounjaro or Wegovy better for weight loss?
Clinical data suggests Mounjaro may produce slightly higher average weight loss (15–21% vs 15%), but individual results vary. The "best" option depends on your health profile, tolerance to side effects, and prescriber assessment. Neither is universally superior — both are effective prescription medications requiring clinical evaluation.
Can I get weight loss injections on the NHS?
Yes, but access is limited. NICE has approved tirzepatide and semaglutide for NHS use within specialist tier 3 weight management services. You'll need a GP referral and may face significant waiting times. Private prescribing is an alternative if you want faster access.
What are the main side effects of weight loss injections?
Very common side effects include nausea, diarrhoea, constipation, and stomach pain — usually temporary. Less common are gallbladder issues, pancreatitis symptoms (severe stomach pain), and changes in heart rate. Side effects often improve over time as your body adjusts.
How much weight can I expect to lose with these injections?
Based on clinical trials, expect an average of 15–clinically studied weight loss over 12–18 months, but results vary widely. Some people lose more, others less. Results depend on starting weight, dose, lifestyle changes, and individual metabolism. Track your progress with our free progress tracker.
Do I need to diet and exercise while taking weight loss injections?
Yes. Weight loss injections work best alongside a reduced-calorie diet and increased physical activity. They're not standalone solutions — they support, not replace, lifestyle changes. Your prescriber will discuss diet and exercise expectations with you.
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.