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 — Which Weight Loss Injection Is Right for You?
If you're considering prescription weight loss injections in the UK, you've probably heard both Mounjaro and Wegovy mentioned. Both are licensed for weight management, but they work differently and produce different results in clinical trials. This guide compares the two head-to-head — mechanism of action, doses, expected weight loss, side effects, and how to decide which might suit you. Track your progress with our free progress tracker once you've started treatment.
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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 — Mechanism and Trial Data
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two separate hormone pathways simultaneously. This dual mechanism may explain why it achieved higher average weight loss in clinical trials. In the SURMOUNT-1 trial, participants on the maximum dose (15 mg) lost approximately 20.9% of their body weight over 72 weeks alongside a reduced-calorie diet.
Wegovy (semaglutide) is a GLP-1 receptor agonist only — it activates a single hormone pathway. In the STEP-1 clinical trial, participants on the maximum dose (2.4 mg) lost approximately 14.9% of body weight at 68 weeks with lifestyle changes. Both medications work by reducing appetite, increasing satiety, and slowing gastric emptying, but the dual mechanism of Mounjaro may offer an additional advantage.
Important context: these are average results from clinical trials. Individual outcomes vary significantly based on genetics, diet adherence, and lifestyle factors. Neither medication assures specific weight loss outcomes.
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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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Doses, Side Effects, and Which Medication Suits You?
Dosing differences: Mounjaro starts at 2.5 mg weekly and escalates to a maximum of 15 mg. Wegovy starts at 0.25 mg and reaches a maximum of 2.4 mg weekly. Both are once-weekly injections. Most people experience gastrointestinal side effects — nausea, diarrhoea, constipation — which often improve over time as the body adjusts.
Which is right for you? Your prescriber will assess your BMI, weight-related health conditions, medical history, and tolerance for side effects. Both require a clinical assessment and prescription. NHS access is through specialist tier 3 services; private access is available through UK-licensed prescribers. Your individual suitability depends on factors only a healthcare professional can evaluate. Use our GLP-1 weight loss calculator to explore potential outcomes at different doses, and our BMI calculator to understand your current health context.
- 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 more effective than Wegovy?
Clinical trials suggest Mounjaro may produce higher average weight loss (20.9% vs 14.9%), but this doesn't mean it's "better" for everyone. Individual results vary significantly. Some people respond better to one medication than the other. Your prescriber will recommend based on your health profile, not trial averages. Both are effective weight loss tools when used alongside lifestyle changes.
Can I switch from Wegovy to Mounjaro?
Yes, switching is possible and is managed by your prescriber. There's no official conversion chart, but approximate equivalence exists: Wegovy 0.5 mg ≈ Mounjaro 5 mg, and Wegovy 2.4 mg ≈ Mounjaro 15 mg. Any switch requires medical supervision to ensure safe transition and appropriate dose adjustment.
Which costs more in the UK?
Pricing varies by provider and dose. Both are available privately through GPhC-registered pharmacies following online clinical assessment. Costs typically range from £129–£320 per month depending on the dose and pharmacy. NHS access is free but involves longer waiting times and is available through specialist services only.
Do Mounjaro and Wegovy have different side effects?
Both commonly cause nausea, diarrhoea, and constipation, especially in the first weeks. These side effects often reduce over time. Serious side effects are rare but can include pancreatitis and gallbladder issues. Your GP or prescriber should monitor you for any unusual symptoms.
What does "dual GIP and GLP-1" mean?
GIP and GLP-1 are natural gut hormones that regulate appetite and blood sugar. Mounjaro activates both pathways; Wegovy activates only GLP-1. This dual action in Mounjaro may help explain higher average weight loss in clinical trials, but it doesn't assures better results for you personally.
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.