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
Free tracker included with every download this week
Mounjaro vs Wegovy — How These Weight Loss Pens Compare
Both Mounjaro and Wegovy are prescription-only weight loss injections available in the UK, but they work in different ways and come from different manufacturers. This guide compares the two pens side-by-side so you understand the key differences. Your prescriber will assess which is more suitable for you based on your individual health, eligibility, and clinical assessment. Both require a prescription and are used alongside reduced-calorie diet and increased physical activity for best results.
10,000+ calculations today · NHS-aligned thresholds · 100% free
Calculate Your BMI Instantly
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 — The Key Difference
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two hormone pathways in your body. This dual mechanism may help regulate appetite and blood sugar in two ways, which some evidence suggests could lead to greater average weight loss in clinical trials.
Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating a single hormone pathway. It still helps reduce appetite and supports feeling fuller after meals, but uses one mechanism rather than two.
- Mounjaro: Dual mechanism (GIP + GLP-1), once-weekly KwikPen, doses 2.5–15 mg
- Wegovy: Single mechanism (GLP-1 only), once-weekly auto-injection pen, doses 0.25–2.4 mg
- Manufacturer: Mounjaro is made by Eli Lilly; Wegovy is made by Novo Nordisk
The tirzepatide formulation in Mounjaro and the semaglutide in Wegovy are entirely different molecules with different pharmacology. Use our free BMI calculator above to understand your current health baseline before discussing treatment options with your GP.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Clinical Trial Results — What the Evidence Shows
The SURMOUNT-1 trial tested Mounjaro and showed average weight loss of approximately 15% at 5 mg, 19.5% at 10 mg, and 20.9% at 15 mg over 72 weeks alongside diet and exercise.
The STEP-1 trial tested Wegovy and showed average weight loss of approximately 14.9% at 2.4 mg over 68 weeks alongside diet and exercise.
Important context: These are trial averages — individual results vary significantly. Your own weight loss depends on your starting weight, adherence to the medication, diet, exercise, metabolism, and other health factors. Neither medication assures specific outcomes. Trial participants also followed reduced-calorie diets, so weight loss results reflect medication plus lifestyle changes combined.
- 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 stronger than Wegovy?
Mounjaro and Wegovy are different medicines with different mechanisms. Clinical trials show slightly higher average weight loss with Mounjaro, but this doesn't mean it's "stronger" — individual responses vary widely. Your prescriber will choose based on your medical history, eligibility, and suitability, not on trial averages. Both are prescription-only and require clinical assessment.
Can I switch from Wegovy to Mounjaro or vice versa?
Yes, some people do switch between GLP-1 medications under prescriber guidance. If you switch, your prescriber will typically restart you at a low dose and titrate (gradually increase) over weeks. There's no official established conversion between the two — the switch is managed individually based on your response and tolerance.
Which costs more — Mounjaro or Wegovy?
Costs vary by provider and dose. Typically, both range from approximately £129–£320 per month on private prescription in the UK, depending on the dose. NHS access is limited to specialist tier 3/4 weight management services in some regions. Your prescriber or pharmacist can tell you the exact cost for your dose.
Do Mounjaro and Wegovy have the same side effects?
Both cause similar common side effects including nausea, diarrhoea, vomiting, constipation, and stomach pain — especially when starting or increasing doses. Serious side effects are rare but similar (pancreatitis, gallbladder issues, allergic reactions). Your prescriber will discuss individual risk based on your health history.
How does my GP or prescriber decide which one to prescribe?
Your prescriber considers your BMI, weight-related health conditions, medical history, allergies, other medications, and which medication is available or preferred for your circumstances. Both require clinical assessment via a consultation. If one isn't suitable, the other may be. Your prescriber will explain their recommendation based on your individual case.
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.