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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Tirzepatide vs Wegovy — What's the Difference?
If you're exploring prescription weight loss options in the UK, you may have heard of both tirzepatide and Wegovy. Both are licensed weekly injection medications, but they work through different mechanisms and have different clinical trial results. This guide compares them factually to help you understand what each medication does and how a prescriber decides which might be suitable for you. Individual suitability is always determined through a clinical assessment, not by the medication alone.
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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 Tirzepatide and Wegovy Work — The Key Difference
Tirzepatide (Mounjaro) is a dual GIP and GLP-1 receptor agonist. It activates two hormone pathways simultaneously—GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual action may help regulate appetite, reduce cravings, and support a feeling of fullness after smaller meals.
Wegovy (semaglutide) is a GLP-1 receptor agonist only. It activates the single GLP-1 pathway, which also supports appetite control and fullness signals to the brain. Both slow gastric emptying (how quickly food leaves the stomach), though this effect typically reduces over time.
- Tirzepatide: dual mechanism (two hormone pathways)
- Wegovy: single mechanism (one hormone pathway)
- Both are weekly injections licensed in the UK
- Both require prescription and clinical assessment
The dual mechanism of tirzepatide may contribute to its slightly higher average weight loss in clinical trials, though individual results vary considerably and prescriber assessment determines suitability.
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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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Clinical Trial Results and Prescriber Decision-Making
In the SURMOUNT-1 trial, tirzepatide at the maximum dose (15 mg) showed average weight loss of approximately 20.9% of body weight over 72 weeks. In the STEP-1 trial, Wegovy at the maximum dose (2.4 mg) showed average weight loss of approximately 14.9% over 68 weeks. However, these are trial averages—individual results vary significantly, and both medications are used alongside reduced-calorie diet and increased physical activity.
Your prescriber will assess which is more suitable based on your personal health history, other medications, NHS eligibility criteria, and clinical judgment. Use our free GLP-1 weight loss calculator to explore potential outcomes at different doses, and our BMI calculator to check your current starting point. Both tools provide context for your prescriber discussion.
- 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 tirzepatide more effective than Wegovy?
Clinical trials show tirzepatide achieved higher average weight loss (20.9% vs 14.9%), but individual results vary significantly. "More effective" depends on your personal response—some people respond better to one than the other. Your prescriber decides which is most appropriate based on your health profile and NHS eligibility.
What's the difference between a GIP receptor and a GLP-1 receptor?
GLP-1 is a hormone that signals fullness and reduces appetite. GIP is a second hormone pathway that also affects appetite and nutrient processing. Tirzepatide activates both; Wegovy activates GLP-1 only. The dual action may explain tirzepatide's slightly higher trial results, but both are effective weight loss tools.
Can I switch from Wegovy to tirzepatide or vice versa?
Switching between medications requires prescriber guidance. There is no official established conversion chart—approximate equivalences exist (e.g., tirzepatide 10 mg ≈ Wegovy 1.7 mg), but any switch must be managed by a licensed prescriber who will assess your response and adjust accordingly.
Are tirzepatide and Wegovy available on the NHS in the UK?
Both are licensed in the UK for weight management. NHS access is through specialist tier 3 weight management services following GP referral—availability and waiting times vary by region (ICB). Private access is available through GPhC-registered pharmacies following an online clinical assessment.
Which medication has fewer side effects?
Both have similar side effect profiles—nausea, diarrhoea, constipation, and stomach pain are most common. Individual tolerance varies. Your prescriber will discuss side effects and help you understand what to expect based on your personal health factors.
Ready to take control of your weight loss journey?
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