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 Weight Loss Expectations vs Wegovy
If you're considering Mounjaro or Wegovy for weight management in the UK, understanding what weight loss to realistically expect is essential. Both are prescription-only weekly weight loss injections that work through different mechanisms, and clinical trials show different average outcomes. This guide compares the two based on clinical evidence, helping you understand what each medication may achieve when used alongside diet and exercise changes. Individual results vary significantly, and a prescriber will assess which option suits your circumstances following a clinical assessment.
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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.
Clinical Trial Results — SURMOUNT vs STEP Data
Mounjaro (tirzepatide) and Wegovy (semaglutide) showed different weight loss outcomes in their respective clinical trials. In the SURMOUNT-1 trial, Mounjaro at the maximum 15 mg dose resulted in an average weight loss of approximately 20.9% of body weight over 72 weeks, alongside a reduced-calorie diet and physical activity. At the 10 mg dose, average loss was around 19.5%. Wegovy's STEP-1 trial showed an average weight loss of approximately 14.9% of body weight at the maximum 2.4 mg dose over 68 weeks, also with lifestyle changes.
These averages mask significant individual variation—some participants lost considerably more, others less. Neither medication assures specific weight loss, and results depend on adherence, diet quality, exercise, and individual factors like metabolism and genetics.
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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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How They Work Differently — Mechanism and Titration
The key difference is mechanism: Mounjaro is a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways. Wegovy is a GLP-1 receptor agonist only, activating a single pathway. This dual action may explain Mounjaro's slightly higher average weight loss in clinical trials, though prescriber preference and individual suitability vary. Mounjaro starts at 2.5 mg and escalates to a maximum of 15 mg over 20+ weeks. Wegovy starts at 0.25 mg and reaches 2.4 mg over 16+ weeks. Both are once-weekly injections delivered via KwikPen. Your prescriber will assess which weekly weight loss injection best suits your health profile, eligibility, and previous responses to GLP-1 medications if applicable.
The key difference is mechanism: Mounjaro is a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways. Wegovy is a GLP-1 receptor agonist only, activating a single pathway. This dual action may explain Mounjaro's slightly higher average weight loss in clinical trials, though prescriber preference and individual suitability vary. Mounjaro starts at 2.5 mg and escalates to a maximum of 15 mg over 20+ weeks. Wegovy starts at 0.25 mg and reaches 2.4 mg over 16+ weeks. Both are once-weekly injections delivered via KwikPen. Your prescriber will assess which prescription weight loss option best suits your health profile, eligibility, and previous responses to GLP-1 medications if applicable. Use our free GLP-1 weight loss calculator to explore projected outcomes at different doses for both medications.
- 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 causes more weight loss — Mounjaro or Wegovy?
Clinical trials suggest Mounjaro may result in slightly higher average weight loss (around 20.9% vs 14.9% at maximum doses), but individual results vary considerably. Some people lose more on Wegovy, others on Mounjaro. A prescriber assesses which is suitable for you based on your health history, not on predicted outcomes alone.
Can I switch from Wegovy to Mounjaro if results are disappointing?
Possibly—prescribers may consider switching between GLP-1 medications, though there is no official conversion chart. Any switch must be managed by your prescriber, not self-directed. Discuss concerns about weight loss progress with your GP or private prescriber.
How long does it take to see weight loss results?
Weight loss typically becomes noticeable after 4–8 weeks on Mounjaro or Wegovy, though this varies. Most clinical trial weight loss occurred between weeks 8–24. Patience is important—sustainable weight loss is gradual, usually 0.5–1 kg per week on these medications alongside lifestyle changes.
Will I keep the weight off after stopping Mounjaro or Wegovy?
Clinical data shows weight regain occurs if medication stops without sustained lifestyle changes. These medications work alongside, not instead of, diet and exercise. Use our free progress tracker to monitor trends and maintain habits formed during treatment.
Are Mounjaro and Wegovy available on the NHS for weight loss?
NICE has approved both for NHS specialist tier 3 weight management services, but access varies by region and availability is limited. Many people access these medications privately through UK-licensed online prescribers following clinical assessment, which is faster than NHS waiting lists in many areas.
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.