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 Reviews — How Mounjaro Compares to Wegovy in the UK
If you're researching tirzepatide reviews online, you're likely comparing it with other weight loss injections available in the UK, particularly Wegovy (semaglutide). Both are prescription-only weekly injections approved for adults with obesity or overweight and related health conditions. The key difference is their mechanism: tirzepatide is a dual GIP and GLP-1 receptor agonist, whilst semaglutide activates only the GLP-1 pathway. Understanding these differences, clinical trial results, and what real users report can help you make an informed decision with your prescriber. This guide breaks down the evidence from both clinical trials and user experience to help you understand what to expect.
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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.
Clinical Trial Data — SURMOUNT vs STEP Results
Clinical evidence shows both tirzepatide and semaglutide (weekly weight loss injection uk options) produce meaningful weight loss in controlled trials, though results differ slightly. The SURMOUNT-1 trial (tirzepatide/Mounjaro) tracked 2,539 adults for 72 weeks at the maximum 15 mg dose, achieving an average weight loss of 20.9% of body weight alongside diet and exercise. The STEP-1 trial (semaglutide/Wegovy) tracked 1,961 adults for 68 weeks at the 2.4 mg maximum dose, showing average weight loss of 14.9% of body weight under similar conditions. Both trials included participants with BMI ≥30 or BMI 27–29.9 with weight-related health conditions. Important context: individual results vary significantly. Trial participants also followed reduced-calorie diets and increased physical activity — medication alone is not responsible for these outcomes. Use our free GLP-1 weight loss calculator above to estimate potential results at different doses based on your starting weight, though your actual outcomes will depend on your individual response, adherence, and lifestyle changes.
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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
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Mechanism Difference — Why This Matters
Tirzepatide (Mounjaro) activates two hormone pathways: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1. This dual mechanism may amplify appetite suppression and metabolic effects, which some reviewers and clinical data suggest contributes to slightly higher average weight loss. Semaglutide (Wegovy) activates GLP-1 only, a single, well-established pathway for appetite regulation and gastric slowdown.
Neither mechanism is inherently "better" — suitability depends on individual health, other medications, tolerance, and prescriber assessment. Some users report stronger appetite suppression with tirzepatide; others find semaglutide equally effective. Both are GLP-1 receptor agonists in practical terms, and both require clinical assessment to determine if either is appropriate for you. Your GP or licensed prescriber will help decide which suits your health profile best.
- 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 do real users say about tirzepatide vs semaglutide?
Online reviews are mixed and individual. Some report stronger appetite suppression with tirzepatide; others find semaglutide equally effective. Common feedback includes nausea in early weeks (both), sustained weight loss, and improved energy. However, reviews reflect individual experiences — clinical trials provide more reliable outcome data. Always prioritise prescriber guidance over anecdotal reviews.
Is tirzepatide more effective than semaglutide based on trials?
Clinical trials show tirzepatide achieved ~20.9% average weight loss vs semaglutide's ~14.9% at maximum doses. However, both are effective. Tirzepatide's dual GIP+GLP-1 mechanism may provide slightly greater appetite suppression for some users, but individual response varies. Your prescriber will assess which is more suitable based on your health.
What are the main side effects users report?
Both injections commonly cause nausea, diarrhoea, vomiting, and stomach pain — typically improving after 4–8 weeks as the body adjusts. Less common: injection site reactions, fatigue, headache, and hair loss. Serious side effects are rare but include pancreatitis symptoms (severe stomach pain) and gallbladder issues. Report any concerning symptoms to your GP immediately.
How quickly do tirzepatide and semaglutide work?
Both medications start working within the first week, but appetite suppression gradually increases over 4–12 weeks as you escalate doses. Most users notice reduced hunger and increased fullness in weeks 2–4. Sustained weight loss typically appears by weeks 8–12, provided you maintain a calorie deficit and exercise regularly.
Can I switch from semaglutide to tirzepatide if one doesn't work?
Yes, switching is possible and managed by your prescriber. There is no official conversion chart, but approximate equivalences exist (5 mg tirzepatide ≈ 0.5 mg semaglutide, 15 mg tirzepatide ≈ 2.4 mg semaglutide). Any switch must be supervised by your GP or licensed prescriber to ensure safe transition and appropriate dosing.
Ready to take control of your weight loss journey?
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