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 Weight Loss Injections: How They Compare to Mounjaro
Wegovy (semaglutide) is a prescription-only, once-weekly weight loss injection licensed in the UK for adults with excess weight. Many people considering this weekly weight loss injection also want to understand how Wegovy compares to Mounjaro, the newer alternative. Both are licensed in the UK, but they work differently and may suit different individuals. This guide explains the key differences, clinical trial evidence, and how your prescriber will decide which is right for you.
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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 Wegovy and Mounjaro Work Differently
Wegovy (semaglutide) is a GLP-1 receptor agonist — it activates one natural hormone pathway in the body that regulates appetite and blood sugar. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, activating two pathways simultaneously.
This difference in mechanism matters. In the STEP-1 clinical trial, Wegovy at maximum dose (2.4 mg) helped participants lose an average of approximately 14.9% of body weight over 68 weeks alongside a reduced-calorie diet and increased physical activity. In the SURMOUNT-1 trial, Mounjaro at maximum dose (15 mg) showed average weight loss of approximately 20.9% over 72 weeks under similar conditions.
- Wegovy: GLP-1 only, up to 2.4 mg per week, five-dose escalation schedule
- Mounjaro: GIP + GLP-1, up to 15 mg per week, six-dose escalation schedule
Both are injectable pens used once weekly. Individual results vary significantly — some people respond very well to one but not the other.
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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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Eligibility, Access, and Your Prescriber's Decision
Both Wegovy and Mounjaro are prescribed in the UK using the same eligibility criteria: adults with a BMI of 30 or above, or 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or sleep apnoea. South Asian, Chinese, Black African, or Black Caribbean adults may be eligible at lower BMI thresholds.
Both are available through NHS specialist weight management services (with variable waiting times by region) or private online prescribing following a clinical assessment. Your prescriber will consider your health history, any other medications, and individual suitability when deciding which is appropriate. There is no "better" option — the right injection is the one that works for your body and fits your health profile.
- 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 is the main difference between Wegovy and Mounjaro for weight loss?
Wegovy (semaglutide) activates the GLP-1 hormone pathway only, while Mounjaro (tirzepatide) activates both GIP and GLP-1 pathways. This dual action may contribute to Mounjaro's slightly higher average weight loss in clinical trials, though both are effective and individual responses vary significantly.
Which weight loss injection works faster, Wegovy or Mounjaro?
Both follow a gradual dose escalation schedule over several months. Wegovy reaches maximum dose (2.4 mg) at week 17, while Mounjaro reaches maximum (15 mg) at week 21. Weight loss typically continues throughout the escalation period and beyond. Results vary between individuals — some see effects at lower doses, others need higher doses.
Can I switch from Wegovy to Mounjaro or vice versa in the UK?
Yes, switching between these medications is possible and managed by your prescriber. However, there is no official NHS conversion chart — any switch should be discussed with your GP or the prescriber managing your treatment to ensure safe transition and appropriate dose.
How do I access Wegovy weight loss injections on the NHS in the UK?
Wegovy is available through NHS specialist tier 3 weight management services. You'll need a referral from your GP. Availability and waiting times vary significantly by region. Private prescription is also available through UK-licensed online prescribers if NHS waiting lists are long.
What are the most common side effects of Wegovy compared to Mounjaro?
Both commonly cause nausea, diarrhoea, vomiting, and constipation, particularly during dose escalation. Most side effects reduce over time. Mounjaro users report slightly higher rates of nausea initially. Serious side effects (severe stomach pain, allergic reactions) are rare but require immediate medical attention. Always report concerns to your prescriber.
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