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 — What the Reviews and Evidence Show
If you're researching weight loss injections in the UK, you've probably read reviews comparing Mounjaro and Wegovy. Both are prescription-only once-weekly jabs licensed for weight management in UK adults, but they work differently. Mounjaro (tirzepatide) uses a dual mechanism targeting two hormone pathways, while Wegovy (semaglutide) targets one. This guide compares both based on clinical trial data and eligibility, not hype. Your GP or a licensed prescriber will assess which is right for you individually.
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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 Results
Wegovy (semaglutide) is a GLP-1 receptor agonist that activates a single hormone pathway to reduce appetite and support feeling fuller after smaller meals. In the STEP-1 clinical trial, participants on the maximum 2.4 mg dose lost an average of 14.9% of their body weight over 68 weeks alongside diet and exercise.
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways simultaneously. The SURMOUNT-1 trial showed participants on 15 mg lost an average of 20.9% of body weight over 72 weeks. The dual mechanism may explain why some reviews report stronger appetite suppression, though individual results vary considerably.
- Wegovy: single GLP-1 pathway, maximum 2.4 mg weekly
- Mounjaro: dual GIP+GLP-1 pathways, maximum 15 mg weekly
- Both require reduced-calorie diet and increased physical activity to work effectively
- Both are injected once weekly under the skin
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
Choosing Between Them — Eligibility, Access, and Prescriber Decision
Eligibility for both medications is broadly the same in the UK: adults with BMI 30 or above, or BMI 27–29.9 with weight-related health conditions like type 2 diabetes or high blood pressure. Access is available through NHS specialist weight management services (with possible waiting lists) or private prescribing following an online clinical assessment. Your prescriber will assess your medical history, current medications, and individual health needs — neither jab is suitable for everyone.
You can use our free GLP-1 weight loss calculator to explore potential outcomes at different doses for context, but this does not replace professional guidance. Try the MyWeightCheck BMI calculator to see where you currently fall on NHS guidelines. Both medications require a prescription; neither is available over-the-counter.
- 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.
Why do some reviews say Mounjaro works better than Wegovy?
Mounjaro's dual mechanism (GIP + GLP-1) produced higher average weight loss in trials (20.9% vs 14.9%). However, trial averages don't predict individual outcomes — some people respond better to Wegovy. Factors like genetics, lifestyle, dose tolerability, and adherence matter more than which drug is "better" in theory.
Can I switch from Wegovy to Mounjaro or vice versa?
A prescriber can switch you between medications if needed, but there's no official conversion chart. Approximate dose equivalence exists (e.g. Wegovy 2.4 mg ≈ Mounjaro 15 mg), but any switch requires medical supervision. Switching should only happen under prescriber guidance based on your individual response.
What's the difference between Mounjaro KwikPen and Wegovy pens?
Both use pre-filled injection pens. Mounjaro KwikPen delivers tirzepatide; Wegovy delivers semaglutide. Both store in the fridge unopened, can be kept at room temperature for 30 days after first use, and are injected once weekly. Injection technique is similar for both.
Do Mounjaro and Wegovy have different side effects?
Both cause nausea, vomiting, diarrhoea, and constipation as very common side effects. Mounjaro users report slightly different GI tolerability profiles in some reviews, but differences are individual. Hair loss (alopecia) appears more frequently in Mounjaro reports, though it's uncommon. Always read the patient information leaflet and discuss side effect concerns with your prescriber.
Which one is available on the NHS?
Both Mounjaro and Wegovy are approved by NICE for NHS use, but availability varies by region and specialist tier 3 services. Waiting times differ significantly across the UK. Private prescribing offers faster access but involves costs. Your GP can refer you to NHS services or you can pursue private prescribing independently — your prescriber will clarify options.
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.