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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Semaglutide Versus Mounjaro — What's the Difference?
If you're exploring prescription weight loss options in the UK, you've likely encountered both semaglutide (Wegovy) and Mounjaro (tirzepatide). Both are once-weekly injectable medications approved for weight management in UK adults, but they work slightly differently and have distinct eligibility pathways. This guide explains how they compare, what NHS and NICE guidance says about each, and how to access them through NHS specialist services or private prescribing. Use our free GLP-1 weight loss calculator to explore potential outcomes with either medication.
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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 Semaglutide and Mounjaro Work Differently
Semaglutide (Wegovy) is a GLP-1 receptor agonist — it activates one natural hormone pathway to reduce appetite and support feeling fuller after meals. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways simultaneously, offering a different mechanism of action.
- Semaglutide: Doses escalate from 0.25 mg to 2.4 mg over 17 weeks. Average weight loss in clinical trials (STEP-1) was approximately 15% of body weight over 68 weeks, alongside a reduced-calorie diet and increased physical activity.
- Mounjaro: Doses escalate from 2.5 mg to 15 mg over 20+ weeks. Average weight loss in clinical trials (SURMOUNT-1) ranged from 15% to 21% of body weight depending on dose, also with dietary and lifestyle changes.
Neither medication is insulin. Both are prescription-only and require clinical assessment before being prescribed. Individual results vary significantly between users.
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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.
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NHS Eligibility and Accessing Both Medications in the UK
For NHS access, both semaglutide and Mounjaro are available through tier 3 specialist weight management services — not standard GP prescription. You'll typically need GP referral to a local NHS specialist service. NICE has approved both medications, but availability varies significantly by region (ICB) and waiting lists can be several months to over a year in many areas.
For private access, both are available through online consultations with UK-licensed prescribers and dispensed by GPhC-registered pharmacies. Private access doesn't require NHS referral and costs typically range from £130–£320 per month depending on dose. A licensed prescriber will assess your eligibility based on BMI (30 or above, or 27+ with weight-related health conditions) and medical history. Use our BMI calculator to check your eligibility threshold instantly.
- 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 semaglutide the same as Mounjaro?
No. Both are GLP-1 type medications, but semaglutide (Wegovy) is a single-mechanism GLP-1 agonist, while Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist. They have different dosing schedules, escalation protocols, and slightly different weight loss profiles in clinical trials. Your prescriber can help decide which suits your situation.
Which causes fewer side effects — semaglutide or Mounjaro?
Both have similar common side effects: nausea, diarrhoea, constipation, and stomach pain. These typically ease as your body adjusts. Individual tolerance varies significantly. Semaglutide has a longer clinical history in the UK market, while Mounjaro is newer. Your GP or prescriber can discuss which may be better tolerated based on your medical history.
Can I switch from semaglutide to Mounjaro on the NHS?
Yes, you can request to switch medications through your NHS specialist weight management service. There is no official conversion chart between the two — your prescriber will assess and recommend appropriate dosing if you switch. Any switch requires clinical review and should be managed by your NHS team or private prescriber.
How long do I need to take semaglutide or Mounjaro?
Both are typically used long-term. Clinical trials lasted 68 weeks (STEP-1 for semaglutide) and 72 weeks (SURMOUNT-1 for Mounjaro), but weight management is often ongoing. Your prescriber will discuss duration based on your weight loss goals and health. Stopping either medication may lead to weight regain without continued lifestyle changes.
Which medication is available faster on the NHS?
Availability depends entirely on your local NHS ICB (integrated care board) and tier 3 service capacity. Some regions have established Mounjaro pathways, others prioritise semaglutide based on stock or commissioning decisions. Ask your GP which medications your local NHS service currently offers and typical waiting times for assessment and initiation.
Ready to take control of your weight loss journey?
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