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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Mounjaro 15mg vs Wegovy — What's the Difference?
If you're looking for the cheapest Mounjaro 15mg option in the UK, you might also be considering Wegovy as an alternative. Both are prescription-only weight loss injections with strong clinical evidence, but they work differently and have different costs. This guide compares the two medications factually, so you can understand the key differences and discuss options with your prescriber. Both require a clinical assessment before being prescribed, whether through the NHS or a private online prescriber.
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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 Mounjaro and Wegovy Work Differently
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two natural hormone pathways simultaneously. This dual action may help regulate appetite and support feeling fuller after meals more effectively than a single mechanism.
Wegovy (semaglutide) is a GLP-1 receptor agonist only — it activates one hormone pathway. Both medications slow gastric emptying, but Mounjaro's additional GIP action means it works on two separate systems.
In the SURMOUNT-1 clinical trial, Mounjaro 15mg produced average weight loss of approximately 20.9% of body weight over 72 weeks. In the STEP-1 trial, Wegovy 2.4mg achieved approximately clinically studied weight loss over 68 weeks. However, individual results vary significantly, and the studies used different protocols.
Use our GLP-1 weight loss calculator above to estimate potential weight loss outcomes at different doses of either medication based on your starting weight.
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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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Cost, Eligibility, and Choosing Between Them
Both medications are prescription-only and require clinical assessment. Private costs typically range from £129–£320 per month depending on dose and provider. At maximum dose, Mounjaro 15mg and Wegovy 2.4mg may have similar monthly costs — the "cheapest" option depends on your specific prescriber and dose escalation path.
On the NHS, access is through specialist tier 3 weight management services with significant regional variation. Waiting lists can be lengthy, and availability varies by integrated care board (ICB).
Your prescriber will assess which medication suits you based on your medical history, BMI, and health conditions. If you have a BMI of 27–29.9 with weight-related conditions, or BMI 30+, you may be eligible. A licensed prescriber will determine the best option after a clinical assessment. Both medications work alongside reduced-calorie diet and increased physical activity — neither replaces lifestyle changes.
- 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 15mg cheaper than Wegovy 2.4mg in the UK?
Private costs are similar at maximum dose (£129–£320/month range), varying by provider and dose progression. "Cheapest" depends on your starting dose and how quickly you escalate. Mounjaro starts at 2.5mg; Wegovy starts at 0.25mg. NHS costs vary by region — ask your GP about local availability and waiting times.
Which medication causes more weight loss — Mounjaro or Wegovy?
Clinical trials showed Mounjaro 15mg averaged clinically studied weight loss versus Wegovy 2.4mg at 14.9%. However, studies used different protocols and timeframes. Individual results vary significantly. Your prescriber can discuss which mechanism might suit your health profile better.
Can I switch from Mounjaro to Wegovy to save money?
Approximate conversions exist (Mounjaro 10mg ≈ Wegovy 1.7mg), but any medication switch must be managed by your prescriber. Switching involves stopping one medication and titrating another, which isn't simply a cost decision. Discuss cost concerns with your prescriber — they may adjust your treatment plan.
How quickly do I reach the maximum Mounjaro 15mg dose?
Mounjaro requires a minimum of 4 weeks on each dose before escalation. Starting at 2.5mg, reaching 15mg takes approximately 20 weeks (5 months) of weekly injections. Wegovy takes longer — approximately 16 weeks to reach 2.4mg maximum dose.
Do I need a GP referral to get Mounjaro or Wegovy privately?
No. Private prescribing through a UK-licensed prescriber requires an online clinical assessment but no NHS referral. NHS access requires GP referral to specialist weight management services. Private and NHS routes are separate pathways with different timescales and costs.
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.