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 vs Mounjaro: How They Compare
Both Wegovy (semaglutide) and Mounjaro (tirzepatide) are prescription-only weight loss injections licensed in the UK, but they work differently and deliver different clinical results. This guide compares the two medications factually — mechanism, dosing, trial data, side effects, and eligibility — to help you understand what your prescriber might recommend. Individual suitability depends on your personal health profile and requires a clinical assessment.
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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 They Work: Key Mechanism Differences
Wegovy (semaglutide) is a GLP-1 receptor agonist — it activates one natural hormone pathway in your body. It signals fullness to your brain, slows how quickly food leaves your stomach, and helps regulate blood sugar.
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two hormone pathways simultaneously. This dual mechanism may provide additional appetite suppression and fat utilisation support compared to single-pathway activation.
- Wegovy: Single mechanism (GLP-1 only)
- Mounjaro: Dual mechanism (GIP + GLP-1)
In clinical trials, this dual approach in Mounjaro appeared to deliver slightly greater average weight loss than Wegovy, though individual results vary significantly and a prescriber will assess which is more suitable for you.
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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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Doses, Trial Results, and Side Effects Compared
Wegovy: Doses range from 0.25 mg to 2.4 mg, once weekly. In the STEP-1 trial, participants lost an average of approximately 14.9% of body weight over 68 weeks.
Mounjaro: Doses range from 2.5 mg to 15 mg, once weekly. In the SURMOUNT-1 trial, participants lost an average of approximately 15–20.9% of body weight over 72 weeks, depending on dose.
Both medications commonly cause nausea, diarrhoea, vomiting, and constipation — especially during dose escalation. Side effect profiles are similar. Both require 4 weeks minimum at each dose before increasing. Your prescriber will discuss which suits your medical history best. Use our GLP-1 weight loss calculator to explore potential outcomes at different doses.
Eligibility and how to access both in the UK: Both are available to UK adults with BMI 30+, or 27+ with weight-related health conditions (type 2 diabetes, high blood pressure, cardiovascular disease, sleep apnoea). Access routes are identical: NHS specialist tier 3 weight management services (with potentially long waiting lists) or private prescription through a UK-licensed prescriber following online clinical assessment. Neither is available on standard GP prescription in primary care. Your prescriber will assess your personal suitability for either medication — they are not interchangeable, and individual health factors determine the right choice.
- 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 better than Wegovy for weight loss?
In clinical trials, Mounjaro showed slightly higher average weight loss (15–20.9%) than Wegovy (14.9%), but results vary significantly between individuals. Neither is universally "better" — suitability depends on your medical history, other medications, and how your body responds. A prescriber will assess which is more appropriate for you.
Which injection has fewer side effects?
Side effect profiles are very similar — both commonly cause nausea, diarrhoea, vomiting, and constipation, especially during dose escalation. Individual tolerance varies. Some people experience fewer side effects on one medication than the other, but this cannot be predicted in advance. Your prescriber may recommend one based on your specific health context.
Can you switch from Wegovy to Mounjaro or vice versa?
A prescriber can manage a switch between medications if needed, but there is no official conversion chart because the mechanisms differ. Any switch requires medical supervision. Approximate equivalence is sometimes used in clinical practice (e.g., Mounjaro 10 mg is roughly equivalent to Wegovy 1.7 mg), but dosing must be individualised.
How do I access Wegovy or Mounjaro in the UK?
Both are available through NHS specialist weight management services (tier 3) via GP referral, or privately through a UK-licensed prescriber following an online clinical assessment. Neither is available on standard GP prescription. Private access is typically faster but costs vary (£130–£320 per month depending on dose and provider).
Which one should I ask my prescriber for?
Do not request a specific medication — instead, discuss your weight loss goals, medical history, and any concerns with a qualified prescriber. They will assess your eligibility and suitability for either Wegovy or Mounjaro based on clinical evidence and your individual health profile. Use our BMI calculator to understand your starting point and our GLP-1 weight loss calculator to explore potential outcomes.
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