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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Mounjaro vs Wegovy Through Your GP — What's the Difference?
If you're considering weight management medication through your GP, you may have heard of both Mounjaro (tirzepatide) and Wegovy (semaglutide). Both are prescription-only once-weekly injections licensed for weight loss in the UK, but they work slightly differently and show different results in clinical trials. This guide compares both medications to help you understand the key differences so you can discuss options with your healthcare team.
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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 Mounjaro and Wegovy Work — Mechanism Differences
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two separate hormone pathways in the body. The GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) dual action may help regulate appetite more effectively by working on multiple hunger and fullness signals simultaneously.
Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating a single hormone pathway. It reduces appetite and increases feelings of fullness, though through a single mechanism rather than dual.
- Mounjaro: dual mechanism (GIP + GLP-1)
- Wegovy: single mechanism (GLP-1 only)
- Both slow gastric emptying and reduce food cravings
- Both require a reduced-calorie diet and physical activity for best results
This mechanistic difference is why clinical trials show Mounjaro achieving slightly higher average weight loss, though individual results vary considerably.
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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 — SURMOUNT vs STEP Data
The SURMOUNT-1 trial (Mounjaro) showed average weight loss of approximately 20.9% of body weight at the maximum 15 mg dose over 72 weeks. The STEP-1 trial (Wegovy) showed average weight loss of approximately 14.9% of body weight at the 2.4 mg dose over 68 weeks. Both studies included participants following reduced-calorie diets and increasing physical activity. Try our GLP-1 weight loss calculator to estimate potential results at different doses. It's crucial to remember that these are clinical trial averages — individual results vary significantly based on adherence, lifestyle, baseline weight, metabolism, and other health factors. Your GP will discuss what realistic results might look like for you personally.
- 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.
Can my GP prescribe Mounjaro directly, or do I need a referral to specialist services?
In the UK, Mounjaro access through the NHS typically requires referral to a specialist tier 3 weight management service, not direct GP prescription. However, private prescribing is available through online consultations with UK-licensed prescribers. Your GP can advise on NHS pathways in your area and may refer you to specialist services.
What's the main advantage of Mounjaro over Wegovy?
Mounjaro's dual GIP + GLP-1 mechanism often produces higher average weight loss in clinical trials (20.9% vs 14.9%). However, this doesn't assures better individual results — some people respond equally well to Wegovy, and personal suitability depends on your health profile and how you respond to treatment.
Are the side effects of Mounjaro and Wegovy similar?
Yes, both share common side effects including nausea, diarrhoea, vomiting, constipation, and stomach pain. Nausea tends to be most common when starting or increasing doses. Side effects often settle after a few weeks on the same dose. Your GP or prescriber will discuss management strategies.
Which medication should I choose — Mounjaro or Wegovy?
The choice depends on your individual health factors, eligibility, availability, and how you respond to treatment. Only a prescriber can assess this through clinical consultation. Both medications require a prescription and clinical assessment before you can start. Discuss options with your GP or specialist.
How does my GP assess if I'm eligible for either Mounjaro or Wegovy?
Your GP considers your BMI (30 or above, or 27+ with weight-related health conditions like type 2 diabetes or high blood pressure), your medical history, current medications, and any contraindications. Ethnicity-specific thresholds apply for South Asian, Chinese, Black African, and Black Caribbean adults. A clinical assessment is essential before either medication can be prescribed.
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