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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Ordering Mounjaro vs Wegovy in the UK
If you're considering prescription weight loss treatment in the UK, you'll likely encounter both Mounjaro (tirzepatide) and Wegovy (semaglutide). Both are prescription-only GLP-1 receptor agonist injections licensed for weight management, but they work differently and have different dosing schedules. Understanding the key differences will help you and your prescriber decide which might be suitable for you. This guide breaks down how they compare, what the clinical trial evidence shows, and how to order each one through the UK healthcare system.
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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, meaning it activates two natural hormone pathways in the body. This dual mechanism may help explain why it showed slightly stronger average weight loss in clinical trials. Wegovy (semaglutide) is a single GLP-1 receptor agonist, activating only one pathway.
In the SURMOUNT-1 clinical trial, Mounjaro at the maximum dose (15 mg) produced an average weight loss of approximately 20.9% of body weight over 72 weeks, alongside a reduced-calorie diet and increased physical activity. In the STEP-1 clinical trial, Wegovy at its maximum dose (2.4 mg) produced an average weight loss of approximately 14.9% over 68 weeks under similar conditions.
However, these are trial averages. Individual results vary significantly, and what works best depends on your personal health profile, medical history, and how your body responds to each medication.
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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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Dosing, Ordering, and Which Might Suit You
Mounjaro dosing: Starting at 2.5 mg once weekly, escalating to a maximum of 15 mg. Wegovy dosing: Starting at 0.25 mg, escalating to 2.4 mg once weekly. Both require a minimum of 4 weeks at each dose before increasing.
To order either medication in the UK, you need a prescription from a UK-licensed prescriber following a clinical assessment. This can happen through NHS specialist weight management services (tier 3) or through a private online prescriber and GPhC-registered pharmacy. Your prescriber will assess your BMI, medical history, and weight-related health conditions to determine eligibility and which medication is more appropriate for you. Use our free GLP-1 weight loss calculator to understand potential outcomes, and check your BMI with our free BMI calculator to see where you stand.
- 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 receptor agonist (dual mechanism), while Wegovy is a GLP-1 receptor agonist only (single mechanism). Clinical trial data suggests Mounjaro may produce slightly higher average weight loss, but individual results vary. Both are prescription-only and require clinical assessment before ordering.
Can I switch between Mounjaro and Wegovy?
Switching between these medications is possible but must be managed by your prescriber. There is no official established conversion chart between the two, as they have different dosing schedules and mechanisms. Any change should be discussed with your GP or prescriber, who will assess what's appropriate for your situation.
How do I order Mounjaro or Wegovy in the UK?
Both medications require a UK prescription. You can access them through NHS specialist weight management services (referral from your GP) or through a private online prescriber followed by dispensing via a GPhC-registered pharmacy. A clinical assessment is required in both cases to confirm eligibility based on your BMI and health.
Which one will help me lose more weight — Mounjaro or Wegovy?
Clinical trials show Mounjaro had a slightly higher average weight loss (approximately 20.9%) compared to Wegovy (approximately 14.9%). However, individual results vary significantly. Your prescriber will recommend based on your personal health profile, not on trial averages alone. Both work alongside a reduced-calorie diet and exercise.
Are there side effects differences between Mounjaro and Wegovy?
Both medications share common side effects: nausea, diarrhoea, constipation, and stomach pain. Mounjaro may also cause hair loss in some users. Most side effects tend to improve over time as your body adjusts. Always discuss potential side effects with your prescriber before ordering.
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