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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Wegovy Drug vs Mounjaro — What's the Difference?
Wegovy and Mounjaro are both prescription-only weight management injections available in the UK, but they work in different ways. Wegovy (semaglutide) is a GLP-1 receptor agonist, whilst Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist. Both are administered as once-weekly injections and may help adults with obesity or overweight with weight-related health conditions. Understanding how they differ — including their mechanisms, clinical trial data, doses, and side effects — can help inform conversations with your prescriber about which might be suitable for your individual circumstances.
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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 Wegovy and Mounjaro Work — Mechanism and Clinical Evidence
Wegovy (semaglutide) activates the GLP-1 hormone pathway, which may help reduce appetite, support feeling fuller after smaller meals, and slow how quickly food leaves the stomach. Clinical trials (STEP-1) showed that adults taking Wegovy 2.4 mg lost an average of approximately 14.9% of their body weight over 68 weeks, alongside a reduced-calorie diet and increased physical activity.
Mounjaro (tirzepatide) activates both GIP and GLP-1 pathways — a dual mechanism that may provide additional appetite regulation. The SURMOUNT-1 trial showed average weight loss of approximately 15% at 5 mg, 19.5% at 10 mg, and 20.9% at 15 mg over 72 weeks. These are clinical trial averages; individual results vary significantly. Both medications require a clinical assessment and prescription from a UK-licensed prescriber before use.
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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.
Doses, Side Effects, and Individual Suitability
Wegovy dosing starts at 0.25 mg weekly and escalates to a maximum of 2.4 mg. Mounjaro starts at 2.5 mg and escalates to 15 mg. Both can cause nausea, diarrhoea, constipation, and stomach pain — particularly during dose escalation. These side effects often improve over time. Your GP or prescriber will assess your medical history, current medications, and health conditions to determine which medication (if either) is appropriate for you. Neither is suitable during pregnancy, and eligibility depends on BMI and weight-related health factors. Try our GLP-1 weight loss calculator to understand potential outcomes at different doses, and use our BMI calculator to check your starting BMI.
- 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 Wegovy a GLP-1 drug?
Yes. Wegovy (semaglutide) is a GLP-1 receptor agonist — it activates the natural GLP-1 hormone pathway to reduce appetite and support weight loss. It's prescription-only in the UK and requires a clinical assessment before being prescribed.
What's the main difference between Wegovy and Mounjaro?
Wegovy is a single GLP-1 agonist, whilst Mounjaro is a dual GIP and GLP-1 agonist. This dual mechanism in Mounjaro may provide slightly different appetite regulation, and clinical trials suggest it may achieve slightly higher average weight loss — though individual results vary considerably.
Which drug causes fewer side effects — Wegovy or Mounjaro?
Both have similar side effect profiles: nausea, diarrhoea, constipation, and stomach pain are common, especially during dose increases. Side effects often reduce over time. Neither is inherently "easier" — individual tolerance varies. Your prescriber will help choose based on your health and preferences.
Can I switch from Wegovy to Mounjaro?
Switching between medications is possible but must be managed by your prescriber. There is no official conversion chart — any switch requires a clinical assessment and appropriate timing between injections. Never stop one medication and start another without guidance.
How do I access Wegovy or Mounjaro in the UK?
Both are available through NHS specialist weight management services (with significant regional variation and waiting lists) or through private online consultations with UK-licensed prescribers. Either route requires a clinical assessment to confirm eligibility based on BMI and health factors.
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.