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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The Difference Between Mounjaro and Wegovy in the UK
Mounjaro (tirzepatide) and Wegovy (semaglutide) are both prescription-only weight loss injections available in the UK, but they work in different ways. Mounjaro is a dual GIP and GLP-1 receptor agonist, while Wegovy is a GLP-1 agonist only. Both are licensed for weight management in adults, but they differ in mechanism, dosing, clinical trial results, and NHS access routes. This guide explains the key differences to help you understand how they compare.
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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 Mounjaro and Wegovy Work Differently
Mounjaro (tirzepatide) activates two hormone pathways: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual mechanism may help regulate appetite, reduce food cravings, and support feeling fuller after meals more effectively than single-pathway medications.
Wegovy (semaglutide) activates the GLP-1 pathway only, signalling to the brain that you are full and slowing gastric emptying. This single mechanism is still effective, but evidence suggests the dual approach of Mounjaro may produce slightly greater average weight loss in clinical trials.
In the SURMOUNT-1 trial, Mounjaro at 15mg achieved approximately 20.9% average body weight loss, whilst Wegovy's STEP-1 trial showed approximately 14.9% average weight loss — though individual results vary significantly. Both medications are used alongside a reduced-calorie diet and increased physical activity.
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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 Access and Eligibility in the UK
Both Mounjaro and Wegovy are available through NHS specialist tier 3 weight management services, though regional availability varies significantly. NICE has approved both for use in NHS services. Eligibility typically requires a BMI of 30 or above, or 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or cardiovascular disease.
However, waiting times for NHS specialist services can be considerable in many areas. Both medications are also available privately through UK-licensed online prescribing services following a clinical assessment. Your GP can refer you to NHS tier 3 services, or you can pursue private assessment. Either route requires a prescription from a qualified UK prescriber who will assess whether the medication is appropriate for your individual health.
- 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 main difference between Mounjaro and Wegovy?
Mounjaro is a dual GIP and GLP-1 agonist, activating two hormone pathways. Wegovy is a GLP-1 agonist only. This dual mechanism may contribute to Mounjaro's slightly higher average weight loss in clinical trials, though individual results vary.
Which is more effective for weight loss?
Clinical trials showed Mounjaro at 15mg achieved approximately 20.9% average body weight loss versus Wegovy's 14.9% average. However, results vary significantly between individuals. A prescriber will assess which suits your circumstances.
Can I switch between Mounjaro and Wegovy on the NHS?
Switching between medications is possible but must be managed by your prescriber. An approximate equivalence exists (Mounjaro 5mg ≈ Wegovy 0.5mg), but any switch requires clinical review. NHS prescribing routes vary by region.
What's the starting dose for each?
Mounjaro starts at 2.5mg weekly, escalating every 4 weeks. Wegovy starts at 0.25mg weekly, escalating over 16 weeks. Maximum doses are Mounjaro 15mg and Wegovy 2.4mg weekly. Dosing is personalised based on tolerance and response.
Are side effects different between them?
Both share common side effects: nausea, diarrhoea, constipation, and stomach pain. Side effect profiles are similar because both activate the GLP-1 pathway. Individual tolerance varies. Discuss specific concerns with your prescriber.
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