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 Injection Comparison — What's Right for You?
If you're exploring weight loss injection options in the UK, you'll likely come across Mounjaro and Wegovy — the two most commonly discussed prescription weight management injections. Both are licensed GLP-1 and GLP-1/GIP receptor agonist medications that may help support weight loss alongside diet and exercise. However, they work differently, have different dosing schedules, and eligibility varies. This guide compares the main weight loss injections available in the UK, explaining how they work, what clinical trials showed, and how to access them — either through the NHS or private prescribing.
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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.
How Weight Loss Injections Work and Compare
Weight loss injections licensed in the UK work by activating natural hormone pathways that regulate appetite and blood sugar. Wegovy (semaglutide) is a GLP-1 receptor agonist that mimics glucagon-like peptide-1, signalling to your brain that you're full and slowing how quickly food leaves your stomach. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — activating two hormone pathways simultaneously — which may offer additional appetite-suppressing effects.
In clinical trials, Mounjaro showed average weight loss of approximately 15–21% of body weight, while Wegovy showed average weight loss of around 14.9% at the maximum dose. A third option, Saxenda (liraglutide), is a daily GLP-1 injection with slightly lower average weight loss. All three are prescription-only medicines and individual results vary significantly. Each requires a clinical assessment before prescription and works best alongside reduced-calorie diet and increased physical activity.
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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.
Eligibility, Dosing and Access in the UK
To access weight loss injection treatment in the UK, you'll typically need a BMI of 30 or above, or 27 or above with weight-related health conditions such as type 2 diabetes, high blood pressure, or cardiovascular disease. People of South Asian, Chinese, Black African, or Black Caribbean heritage may be eligible at lower BMI thresholds due to higher health risks at lower weights. Calculate your BMI using our free BMI calculator above to see where you fall.
Dosing differs between medications: Wegovy ranges from 0.25 mg to 2.4 mg weekly, while Mounjaro ranges from 2.5 mg to 15 mg weekly. Saxenda is dosed daily from 0.6 mg to 3 mg. Access is available through NHS specialist weight management services (with regional variation and potential waiting lists) or private prescribing via GPhC-registered pharmacies following an online clinical assessment. Our GLP-1 weight loss calculator can help you estimate potential outcomes at different doses.
- 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's the difference between Mounjaro and Wegovy for weight loss?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, while Wegovy (semaglutide) is a GLP-1 agonist only. Mounjaro showed average weight loss of 15–21% in clinical trials compared to Wegovy's 14.9%, but individual results vary. Both are once-weekly injections and prescription-only. The right choice depends on individual health factors and clinical assessment by your prescriber.
Can I get weight loss injections on the NHS in the UK?
Yes, weight loss injections are available through NHS tier 3 specialist weight management services. Your GP can refer you, but eligibility and availability vary significantly by region. Waiting times can be lengthy in some areas. Private access is available through online prescribing services without NHS referral, though this involves a cost.
How much weight can I expect to lose with these injections?
Clinical trials showed average weight loss of 15–21% of body weight with Mounjaro and 14.9% with Wegovy. However, this is achieved alongside a reduced-calorie diet and increased physical activity. Individual results vary — some people lose less, some lose more. Weight loss typically occurs gradually over weeks and months of treatment.
Are weight loss injections safe?
Prescription weight loss injections are licensed and undergo rigorous safety testing. Common side effects include nausea, diarrhoea, and constipation — typically mild and temporary. Serious side effects are rare but can include pancreatitis and gallbladder problems. Always discuss risks and benefits with your GP or prescriber before starting treatment.
What's the difference between Wegovy and Ozempic?
Both Wegovy and Ozempic contain semaglutide, but Ozempic is licensed for type 2 diabetes management (up to 2 mg) and is NOT licensed for weight loss in the UK. Wegovy is the licensed weight management version at 2.4 mg maximum dose. Using Ozempic for weight loss is off-label use — Wegovy is the correct prescription for weight management.
Ready to take control of your weight loss journey?
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