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 vs Wegovy: UK Private Weight Loss Injections
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are prescription-only weight loss injections available privately in the UK, but they work differently and have different dose ranges. Understanding the differences can help you discuss with a prescriber which might suit your health needs. This guide compares the two side-by-side using clinical trial data, mechanism of action, and practical UK considerations — so you can make an informed decision with your healthcare professional.
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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 hormone pathways simultaneously. This dual mechanism may help suppress appetite more effectively and support faster weight loss in some individuals. Wegovy (semaglutide) works via GLP-1 receptor activation alone, a single mechanism that signals fullness and reduces food cravings.
Clinical trial data shows the difference: in the SURMOUNT-1 trial, Mounjaro 15mg achieved average weight loss of approximately 20.9% of body weight over 72 weeks. In the STEP-1 trial, Wegovy 2.4mg (maximum dose) achieved approximately clinically studied weight loss over 68 weeks — though individual results vary significantly.
Mounjaro dosing ranges from 2.5mg to 15mg weekly; Wegovy ranges from 0.25mg to 2.4mg weekly. The different dose ranges reflect their different potencies. Our free GLP-1 weight loss calculator can help you explore potential outcomes at different doses.
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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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Choosing Between Them: Individual Assessment Matters
Both medications require a clinical assessment from a UK-licensed prescriber — they're not interchangeable, and suitability depends on your individual health profile. Your prescriber will assess factors like your BMI, any weight-related health conditions (type 2 diabetes, high blood pressure, cardiovascular risk), medical history, and any medications you're taking.
Neither is inherently "better" — the right choice depends on you. Some people respond better to dual-mechanism therapy; others do well on single-mechanism GLP-1. Side effect tolerance also varies: both can cause nausea, stomach issues, and injection reactions, but individual experiences differ.
Mounjaro is typically used when stronger appetite suppression is desired; Wegovy is often a first-line GLP-1 choice. A prescriber following NICE guidance will discuss which aligns with your health goals.
- 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.
Which injection causes more side effects, Mounjaro or Wegovy?
Both have similar common side effects (nausea, diarrhoea, constipation, stomach pain). Mounjaro's dual mechanism may cause slightly more gastrointestinal effects in early weeks. Individual tolerance varies significantly — some people tolerate one better than the other. Your prescriber can discuss tolerability based on your health history.
Is Mounjaro more expensive than Wegovy in UK private prescribing?
Costs vary by provider and dose. Generally, Mounjaro 15mg and Wegovy 2.4mg (maximum doses) have comparable costs, approximately £200–£320 per month privately in the UK. Lower doses cost less. Always check current pricing with your chosen prescriber or pharmacy.
Can I switch from Wegovy to Mounjaro or vice versa?
Switching is possible but must be managed by your prescriber — there's no official conversion chart. Any switch requires a new clinical assessment and typically a washout period. Never change medications without prescriber guidance.
How long do I need to use Mounjaro or Wegovy for weight loss to work?
Both typically show noticeable effects within 4–8 weeks, though full benefits take 12–16 weeks. Most people use them long-term (months to years) for sustained weight loss. Stopping either medication often leads to weight regain. Discuss duration with your prescriber.
Will my NHS GP prescribe Mounjaro or Wegovy, or do I need private prescribing?
NHS access is through specialist tier 3/4 weight management services following GP referral — availability and waiting times vary significantly by region. Private prescribing via online consultations is faster but requires payment. Check your local ICB's policy and waiting times first.
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