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: How They Compare
If you're considering prescription weight loss treatment in the UK, you've likely heard of both tirzepatide (Mounjaro) and Wegovy (semaglutide). Both are licensed weekly injections for weight management, but they work differently and suit different people. This guide compares the two side-by-side — how they work, clinical trial results, dosing, and what determines which might be right for you. Ultimately, your GP or prescriber will assess your individual needs and medical history to recommend the most suitable option.
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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 Differently
Tirzepatide (Mounjaro) is a dual GIP and GLP-1 receptor agonist. This means it activates two natural hormone pathways — GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). This dual mechanism may help regulate appetite more intensively and support fat utilisation alongside appetite suppression.
Wegovy (semaglutide) is a GLP-1 receptor agonist only — it activates one pathway. Both medications slow gastric emptying and signal fullness to the brain, but tirzepatide's additional GIP action is what distinguishes it mechanistically.
In clinical trials, tirzepatide showed slightly higher average weight loss than semaglutide at equivalent dose milestones. However, individual results vary significantly based on genetics, lifestyle, adherence, and overall health. Your prescriber will consider which mechanism may suit your health profile best.
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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.
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Clinical Trial Data and Individual Suitability
The SURMOUNT-1 trial (tirzepatide) showed average weight loss of approximately 15–21% of body weight at the highest dose (15 mg) over 72 weeks. The STEP-1 trial (semaglutide) showed approximately clinically studied weight loss at the maximum dose (2.4 mg) over 68 weeks. Both trials combined medication with reduced-calorie diet and increased physical activity.
These are clinical averages — some people lose more, some less. Both medications require UK prescription and clinical assessment. Your GP or prescriber will evaluate your BMI, weight-related health conditions, current medications, and preferences to determine which is appropriate for you. Neither is universally "better" — suitability is individual. Use our free BMI calculator and GLP-1 weight loss calculator to understand your baseline and projected outcomes.
- 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.
What's the main difference between tirzepatide and Wegovy?
Tirzepatide (Mounjaro) activates two hormone pathways (GIP and GLP-1), while Wegovy (semaglutide) activates one (GLP-1 only). This dual mechanism in tirzepatide may contribute to slightly higher average weight loss in clinical trials, though individual results vary. Both are once-weekly injections licensed in the UK for weight management.
How do the doses compare between tirzepatide and Wegovy?
Tirzepatide starts at 2.5 mg and goes up to a maximum of 15 mg weekly. Wegovy starts at 0.25 mg and reaches a maximum of 2.4 mg weekly. The dose ranges are different because of their different potencies. Your prescriber will escalate you gradually on whichever medication is chosen, with minimum 4 weeks on each dose before increasing.
Which one causes fewer side effects?
Both have similar common side effects: nausea, diarrhoea, constipation, and stomach pain are very common with both. There's no clear evidence that one causes significantly fewer side effects than the other. Individual tolerance varies. Your GP will discuss side effect profiles and manage them with dietary adjustments and support.
Can I switch from tirzepatide to Wegovy or vice versa?
Switching between medications must be managed by your prescriber. There's no official established conversion chart, though approximate equivalences exist based on clinical practice (e.g. tirzepatide 5 mg ≈ semaglutide 0.5 mg). Any switch requires medical oversight and reassessment of your suitability and response.
How do I know which one my GP will prescribe?
Your prescriber will assess your BMI, weight-related health conditions, medical history, medications, and preferences. Both are NHS-available through specialist tier 3 services (with regional variation and waiting lists) and privately through UK-licensed online prescribers following clinical assessment. Your GP or prescriber makes the recommendation based on what's most suitable for you individually.
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