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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Cheapest 12.5mg Mounjaro vs Wegovy — What's the Difference?
If you're comparing Mounjaro at the 12.5mg dose to Wegovy, cost is often the first consideration — but mechanism, trial data, and individual suitability matter just as much. Both are prescription-only GLP-1-based weight management injections available in the UK, yet they work differently and produce different results in clinical trials. This guide compares the two directly to help you understand which might suit your situation better, and why a clinical assessment is essential before either can be prescribed.
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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) is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways simultaneously. This dual mechanism may explain why clinical trial data shows stronger average weight loss compared to single-mechanism medications.
Wegovy (semaglutide) is a GLP-1-only receptor agonist. It activates one hormone pathway, which is still effective but operates through a single mechanism. Wegovy is also available at a lower maximum dose (2.4mg) compared to Mounjaro's 15mg maximum.
- Mounjaro 12.5mg: Dual mechanism; often used mid-to-upper dose range during titration
- Wegovy maximum: 2.4mg (single mechanism); reaches plateau at lower dose level
- Injection frequency: Both are once-weekly injections delivered via KwikPen (Mounjaro) or auto-injection pen (Wegovy)
Both require a UK prescription and clinical assessment before use. Your prescriber will consider your individual health profile, medical history, and weight loss goals when deciding which, if either, is appropriate for you.
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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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Trial Data and Cost — Key Practical Differences
Clinical trial results: In the SURMOUNT-1 trial, Mounjaro at 15mg showed average weight loss of approximately 20.9% of body weight over 72 weeks. Wegovy in the STEP-1 trial showed approximately 14.9% average weight loss over 68 weeks at its maximum 2.4mg dose. Individual results vary significantly. Both medications work alongside reduced-calorie diet and increased physical activity — neither produces results without lifestyle changes.
Cost in the UK: Private pricing varies by provider and dose. At 12.5mg, Mounjaro is often slightly less expensive per month than Wegovy, but costs depend on your prescriber and pharmacy. NHS access is region-dependent and typically through tier 3 specialist weight management services, where costs are covered but waiting lists can be lengthy. Always check current pricing and NHS availability in your area.
- 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 12.5mg cheaper than Wegovy in the UK?
Pricing varies by private prescriber and pharmacy, but Mounjaro at 12.5mg is often marginally cheaper than Wegovy per month. NHS costs are covered but availability is region-dependent with potential waiting lists. Always compare current prices from GPhC-registered pharmacies, as costs fluctuate.
Why do Mounjaro and Wegovy produce different weight loss results?
Mounjaro uses a dual GIP+GLP-1 mechanism, whilst Wegovy uses GLP-1 only. This difference in mechanism likely explains why SURMOUNT trial data showed higher average weight loss with Mounjaro (~20.9%) versus STEP trial data for Wegovy (~14.9%). Individual outcomes vary based on genetics, adherence, and lifestyle factors.
Can I switch from Wegovy to Mounjaro, or vice versa?
Switching between medications must be managed by your prescriber. There is no official NHS conversion chart, though approximate dose equivalences exist (Wegovy 2.4mg roughly corresponds to Mounjaro 10–12.5mg). Any change requires a new clinical assessment and prescription.
Which one will my GP prescribe — Mounjaro or Wegovy?
Your GP cannot prescribe either directly. Both require referral to an NHS tier 3 specialist weight management service, or a private prescription following online clinical assessment with a UK-licensed prescriber. Availability and choice depend on your NHS region and private provider.
Do I still need diet and exercise changes if I take Mounjaro 12.5mg or Wegovy?
Yes. Both medications are most effective when combined with a reduced-calorie diet and increased physical activity. Clinical trial results were achieved with these lifestyle changes in place. Neither medication alone assures weight loss without dietary and activity modifications.
Ready to take control of your weight loss journey?
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