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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How Does Mounjaro Work Compared to Wegovy?
Mounjaro and Wegovy are both prescription-only weight loss injections licensed in the UK, but they work in different ways. Mounjaro (tirzepatide) uses a dual mechanism targeting two hormone pathways, whilst Wegovy (semaglutide) activates a single pathway. Understanding how each medication works can help you and your GP or prescriber assess which might be suitable for your individual circumstances. Both require clinical assessment and are used alongside diet and lifestyle changes.
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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.
How Mounjaro and Wegovy Work Differently
Mounjaro is a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways simultaneously. GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1) are both produced in the gut after eating. By stimulating both pathways, Mounjaro may help regulate appetite more effectively than single-pathway medications.
Wegovy, by contrast, is a GLP-1 receptor agonist only—it targets the GLP-1 pathway alone. This single mechanism still supports appetite reduction and helps you feel fuller after smaller meals, but without the additional GIP activation that tirzepatide provides.
Clinical trial data (SURMOUNT-1 for Mounjaro, STEP-1 for Wegovy) showed average weight loss of around 15–21% with Mounjaro and approximately 15% with Wegovy at maximum doses. However, individual results vary significantly, and the choice between them depends on clinical assessment, not just trial averages. Your prescriber will assess which is appropriate for you based on your health profile, other medications, and medical history.
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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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Which Medication Might Be Right for You?
Both Mounjaro and Wegovy require a prescription-only assessment from a UK-licensed prescriber before they can be provided. Eligibility criteria are broadly similar: a BMI of 30 or above, or 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or cardiovascular disease.
Your prescriber will consider factors including your medical history, other medications (particularly oral contraceptives, where timing adjustments apply), and any contraindications. Neither medication is suitable during pregnancy, and both work best alongside a reduced-calorie diet and increased physical activity. You can use our free GLP-1 weight loss calculator to explore potential outcomes at different doses, and our BMI calculator to understand your current health context—both tools can support conversations with your healthcare provider about weight management options.
- 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 Mounjaro's dual mechanism definitely better than Wegovy's single mechanism?
Not necessarily. Clinical trials suggest Mounjaro may achieve slightly higher average weight loss, but individual results vary considerably. Some people respond well to Wegovy; others may benefit more from Mounjaro's dual action. Your prescriber will assess which is more suitable for you based on your individual health circumstances, not just theoretical superiority.
How quickly do Mounjaro and Wegovy work?
Both are used for ongoing weight management over months. Initial appetite suppression may begin within days, but meaningful weight loss typically becomes visible over 4–8 weeks. Maximum doses are reached over 16–20 weeks (Mounjaro) or 17 weeks (Wegovy) through gradual escalation. Patience and consistency with diet and exercise are essential.
Can I switch from Wegovy to Mounjaro or vice versa?
Switching is possible but must be managed by your prescriber. There is no official established conversion formula—approximate equivalences exist (e.g., Wegovy 2.4 mg roughly equates to Mounjaro 10–15 mg), but any change requires clinical guidance and may involve a transition period.
What's the difference between Mounjaro and Ozempic if they're both made by the same manufacturers?
Mounjaro and Ozempic are actually made by different manufacturers (Eli Lilly vs Novo Nordisk). Additionally, Ozempic is approved for type 2 diabetes up to 2 mg—using it for weight loss is off-label in the UK. Wegovy, also from Novo Nordisk, is the licensed weight management version of semaglutide at the higher 2.4 mg dose.
Do I need to stay on Mounjaro or Wegovy forever?
Weight loss medications are typically prescribed for as long as they remain effective and you continue to benefit. Some people eventually stabilise at a lower dose, whilst others discontinue after achieving weight loss goals and maintain results through lifestyle changes. Your prescriber will review this with you regularly—it's not a lifelong commitment unless you choose to continue.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.